PARTICLE PHYSICS AND COSMOLOGY THE QUEST FOR PHYSICS BEYOND THE STANDARD MODEL(S) Editors
Howard E. Haber Ann E. Nelson
PARTICLE PHYSICS AND COSMOLOGY THE QUEST FOR PHYSICS BEYOND THE STANDARD MODEL(S)
TASI 2002
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PARTICLE PHYSICS AND COSMOLOGY THE QUEST FOR PHYSICS BEYOND THE STANDARD MODEL(S)
TASI2002 Boulder, Colorado, USA
3 - 2 8 June 2002
Editors
Howard E. Haber University of California, Santa Cruz, USA
Ann E. Nelson University of Washington, USA
\^p World Scientific NEW JERSEY • LONDON • SINGAPORE • BEIJING • SHANGHAI • HONGKONG • TAIPEI • CHENNAI
Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401^02, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE
British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.
The editors and the publisher would like to thank Professor John Terning for permission to reproduce his lectures in this book.
PARTICLE PHYSICS AND COSMOLOGY The Quest for Physics Beyond the Standard Model(s) (TASI 2002) Copyright © 2004 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.
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ISBN 981-238-892-3
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PREFACE
The Theoretical Advanced Study Institute (TASI) in elementary particle physics has been held each summer since 1984. TASI was first organized in Ann Arbor, and was held in subsequent years in New Haven, Santa Cruz, Santa Fe and Providence. Since 1989, TASI has been permanently hosted by the University of Colorado in Boulder. Each year TASI attracts about fifty of the most promising advanced theory graduate students in the United States, along with some graduate students from abroad, and a few experimental graduate students and beginning postdoctoral fellows. The emphasis of the topics treated at TASI has shifted from year to year, but typically there have been courses of lectures in phenomenology, field theory, string theory, mathematical physics, and particle-astrophysics, as well as seminars on related theoretical and experimental topics. TASI has been highly successful, especially in introducing the students to a much broader range of ideas than they normally experience at their home institutions while working on their (often narrow) dissertation topics. The title of TASI-2002 was Particle Physics and Cosmology: The Quest for Physics Beyond the Standard Model(s). There were three parallel lecture series given. The Phenomenology lecture series aimed to cover a broad spectrum of standard research techniques used to interpret current and future collider data. This series included the following lecture courses: • • • • • • •
Neutrino Mixing and Masses, by Yuval Grossman Precision Electroweak Physics, by Konstantin Matchev Higgs Theory and Phenomenology, by Marcela Carena Techniques in Effective Field Theory, by Ira Rothstein Bottom Quark Physics, by Michael Luke Top Quark Physics, by Sally Dawson Tevatron Physics, by John Womersley
Complementing the phenomenology lecture series, the TeV-Scale Physics lecture series focused on modern speculations about physics beyond the Standard Model, with an emphasis on supersymmetry and extraV
vi
dimensional theories. This series included the following lecture courses: • • • • • • • •
Introduction to Super symmetry, by Carlos Wagner Non-perturbative Techniques in Supersymmetry, by John Terning The $upersymmetric Flavor Problem, by Ann Nelson Kaluza-Klein Theory and Large Extra Dimensions, by Keith Dienes New Ideas in Symmetry Breaking, by Mariano Quiros Extra Dimensions and Branes, by Csaba Csaki Deconstruction, by Andrew Cohen Strings for Pedestrians, by Mirjam Cvetic
Finally, in recognition of the increasingly important role of astrophysics and cosmology in modern particle physics as we seek to find a single description of physics that can be used at all distance and energy scales, the traditional particle physics lecture series was supplemented by an Astroparticle Physics lecture series. In this series of courses, after an introduction to cosmology, we learned about the current dark matter paradigms; the recent exciting discovery of dark energy and its implications; the implications of the observation of the cosmic microwave background, its acoustic peaks and polarization; and the upcoming era of gravitational wave astronomy. The Astroparticle Physics lecture series included the following lecture courses: • • • • •
Introduction to Cosmology, by Sean Carroll Dark Matter, by Keith Olive Dark Energy, by Joshua Frieman The Cosmic Microwave Background, by Mathias Zaldariagga Gravitational Waves, by Alessandra Buonanno
The Scientific Program was organized by Howard Haber and Ann Nelson, with invaluable assistance provided by the local director, K.T. Mahanthappa. In addition, we would especially like to acknowledge Kathy Oliver for efficient secretarial help; Mu-Chun Chen and Alex Flournoy for help in the day-to-day running of TASI; Tom DeGrand for assisting participants with computer related matters and also leading the weekend hikes; Dan Hooper and Bjorn O. Lange for organizing student seminars; and the National Science Foundation, U.S. Department of Energy and University of Colorado for providing the facilities and financial support. Howard E. Haber and Ann Nelson TASI-02 Program Co-directors January 30, 2004
CONTENTS Preface
v
Lecturers, local organizing committee and directors Student seminars
ix xii
P A R T I: P h e n o m e n o l o g y L e c t u r e S e r i e s Neutrinos Yuval
5 Grossman
Precision Electroweak Physics Konstantin
Matchev
Effective Field Theories Ira Z.
Michael
193
Luke
T h e Top Quark, QCD and New Physics
245
Dawson
Tevatron Physics John
101
Rothstein
B o t t o m Quark Physics and the Heavy Quark Expansion
Sally
51
303
Womersley
P A R T II: T e V - S c a l e P h y s i c s L e c t u r e S e r i e s Non-perturbative Supersymmetry John
343
Terning
New Directions for New Dimensions: Kaluza-Klein Theory, Large E x t r a Dimensions and the Brane World • Keith R.
Dienes
447
viii
New Ideas in Symmetry Breaking
549
Mariano Quiros Extra Dimensions and Branes
605
Csaba Csaki PART III: Astroparticle Physics Lecture Series Introduction to Cosmology
703
Mark Trodden and Sean M. Carroll Dark Matter
797
Keith A. Olive Gravitational Waves from the Early Universe Alessandra Buonanno
855
Lecturers
Name
(Institution)
Alessandra Buonanno Marcela Carena Sean Carroll Andrew Cohen Csaba Csaki Mirjam Cvetic Sally Dawson Keith Dienes Joshua Prieman Yuval Grossman Michael Luke Konstantin Matchev Ann Nelson Keith Olive Mariano Quiros Ira Rothstein John Terning Carlos Wagner John Womersley Matias Zaldarriaga
California Institute of Technology Fermilab Enrico Fermi Institute Boston University Cornell University University of Pennsylvania Brookhaven National Laboratory University of Arizona Fermilab Israel Institute of Technology, Technion University of Toronto University of Florida University of Washington University of Minnesota Universidad Autonoma de Barcelona Carnegie Mellon University Los Alamos National Laboratory Argonne National Laboratory Fermilab New York University
Local Organizing Committee
Name
(Institution)
Shanta DeAlwis Tom DeGrand Anna Hasenfratz K.T. Mahanthappa
University University University University
of of of of
Colorado Colorado Colorado Colorado
Directors Name
(Institution)
Howard Haber K.T. Mahanthappa A n n Nelson
University of California, Santa Cruz University of Colorado University of Washington
Participants Name
(Institution)
Christopher Aubin Sandor Benczik HuBo Elena Brewer Qinghong Cao Spencer Chang Jennifer Chen Mu-Chun Chen Ana Maria Curiel Samik Dasgupta Rob Fardon Bryan Field Terrance Figy Alex Flournoy Michael Forbes Hock-Seng Goh Roni Harnik Daniel Hooper Kyuwan Hwang Alberto Iglesias Christopher Jackson Abu K h a n Vishesh Khemani Savvas Koushiappas Tadas Krupovnickas Andrei Kryjevski
Washington University Virginia Tech Texas A&M University SUNY at Buffalo Michigan State University Harvard University University of Chicago University of Colorado CSIC, Madrid University of Colorado University of Washington SUNY at Stony Brook University of Wisconsin University of Colorado MIT University of Maryland UC Berkeley University of Wisconsin Seoul National University SUNY at Stony Brook Florida State University University of Florida MIT Ohio State University Florida State University University of Washington
Joydip Kundu Herry Kwee Bjorn Lange Christopher Lee Jason Lennon Nuno Leonardo Eugene Lim Zhaofeng Liu Mattew Martin Michael May David Maybury Robert McElrath Stavros Mouslopoulos Brandon Murakami Siew-Phang Ng Takemichi Okui Steven Oliver Vakif Onemli John Phillips Martin Puchwein Saswat Sarangi Liguo Song Marc Soussa Keith Thomas Manuel Toharia Dionysis Triantafyllopoulos Michael Trott Mithat Unsal Haibin Wang Ho-Ung Yee Andrew Zentner Xiaomin Zu
MIP College of William & Mary Cornell University Caltech University of Notre Dame Fermilab University of Chicago University of Colorado Carnegie Mellon University Johns Hopkins University University of Alberta University of Wisconsin UC Berkeley UC Davis University of Maryland Lawrence Berkeley National Lab UC Berkeley University of Florida Ohio State University ITP, ETH-Zurich Cornell University Vanderbilt University University of Florida University of Wisconsin UC Davis Columbia University University of Toronto University of Washington University of Michigan Yale University Ohio State University Pennsylvania State University
xii
Student Seminars* (In Alphabetic Order)
1. Sandor Benczik, "The classical limit of the minimal-length uncertainty relation." 2. Spencer Chang, "SU(3) Symmetry at a TeV and the Weak Mixing Angle." 3. Jennifer Chen, "Quintessential time variation of the fine structure constant?" 4. Ana Maria Curiel, "Optimal Observables to search for indirect SUSY-QCD signals." 5. Rob Fardon, "The littlest Higgs." 6. Michael McNeil Forbes, "Topological Defects in QCD." 7. Roni Harnik, "Viable Anomaly Mediation in 4-D." 8. Dan Hooper, "Microscopic Black Holes and TeV Gravity in Neutrino Telescopes." 9. Bo Hu, "Yukawa texture, neutrinos, \i —> e + 7 and M-theory." 10. Kyuwan Hwang, "Orbifolded SU(n) and Unification of families." 11. Vishesh Khemani, "Decoupling Fermion doublets from the SM." 12. Savvas Koushiappas, "Gamma rays from neutralino annihilations." 13. Joydip Kundu, "Crystaline Color Superconductivity." 14. Bjorn O. Lange, "QCD Sum Rules (and the heavy meson decay constant / B ) . " 15. Jason E. Lennon, "Higgs mediated Bs —> /Lt+/x~ in SUSY models." 16. Eugene Lim, "How to make Stuff (or: Generating cosmological perturbations)." 17. Matthew R. Martin, "Radion Induced Brane Preheating." 18. Mike May, "Brane running in RS 1." 19. David Maybury, "Model building with SO(10)." 20. Robert McElrath, "Black Holes at Colliders." 21. Stavros Mouslopoulos, "Multigravity from Brane worlds." 22. Brandon Murakami, "Lepton Flavor Violation through Z' Bosons." 23. Takemichi Okui, "Gauge Coupling Unification from Particle in a Box Physics." 24. John Phillips, "Halo substructure from lensing." 25. Saswat Sarangi, "Brane-Antibrane Inflation." 'Organized by Dan Hooper and Bjorn O. Lange.
Xlll
26. 27. 28. 29. 30.
Manuel Toharia, "On the Radion." Mithat Unsal, "Supersymmetry on a spatial lattice." Ho-Ung Yee, "K K contributions to S,T,U." Andrew Zentner, "Small-scale Cold Dark Matter and Inflation. Xiaomin Zu, "Factorization Theorems and perturbative QCD."
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PART I: Phenomenology Lecture Series
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illpiiMMlii
YUVAL GROSSMAN
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NEUTRINOS
YUVAL GROSSMAN Department of Physics, Technion-Israel Institute of Technology, Technion City, 32000 Haifa, Israel We present a pedagogical review of neutrino physics. In the first lecture we describe the theoretical motivation for neutrino masses, and explain how neutrino flavor oscillation experiments can probe neutrino masses. In the second lecture we review the experimental data, and show that it is best explained if neutrinos are massive. In the third lecture we explain what are the theoretical implications of the data, in particular, what are the challenges they impose on models of physics beyond the SM. We give examples of theoretical models that cope with some of these challenges.
Contents 1
Introduction
6
2
Neutrino masses 2.1 Fermion masses 2.2 Neutrino masses in the SM 2.3 Neutrino masses beyond the SM 2.4 Neutrino mixing
7 7 9 10 13
3
M e t h o d s for probing neutrino masses 3.1 Kinematic tests 3.2 Neutrinoless double (3 decay 3.3 Neutrino vacuum oscillation 3.4 Matter effects 3.5 Non-uniform density 3.6 Neutrino oscillation experiments
14 15 15 16 18 19 22
4
Solar neutrinos 4.1 Solar neutrinos production 4.2 Solar neutrinos propagation 4.3 Solar neutrinos detections
23 23 24 25 5
6
4.4 4.5 4.6 4.7 4.8
Chlorine: Homestake Gallium: SAGE, GALLEX and GNO Water Cerenkov: Kamiokande and SuperKamiokande Heavy Water: SNO Solar neutrinos: fits
26 27 27 28 29
5
Atmospheric neutrinos 5.1 Atmospheric neutrino production 5.2 Atmospheric neutrino propagation 5.3 Atmospheric neutrino detection 5.4 Atmospheric neutrino data 5.5 The atmospheric neutrino problem
32 32 32 33 33 35
6
Terrestrial neutrinos 6.1 K2K 6.2 KamLAND 6.3 LSND
35 36 36 37
7
Theoretical implications 7.1 Two neutrino mixing 7.2 Three neutrino mixing 7.3 Four neutrino mixing
38 38 40 41
8
Models for neutrino masses 8.1 Grand Unified Theories (GUTs) 8.2 Flavor physics
42 42 43
9
Conclusions
45
1. I n t r o d u c t i o n T h e success of the S t a n d a r d Model (SM) can be seen as a proof t h a t it is an effective low energy description of Nature. We are therefore interested in probing the more fundamental theory. One way to go is to search for new particles t h a t can be produced in yet unreached energies. Another way to look for new physics is to search for indirect effects of heavy unknown particles. In this set of lectures we explain how neutrino physics is used to probe such indirect signals of physics beyond the SM. In the SM the neutrinos are exactly massless. This prediction, however, is rather specific to the SM. In almost all of the SM extensions the neutrinos are massive and they mix. T h e search for neutrino flavor oscillation, a phenomenon which is possible only for massive neutrinos, is a search for new physics beyond the SM. T h e recent experimental indications for neutrino oscillations are indirect evidences for new physics, most likely, at distances much shorter t h a n the weak scale.
7
In the first lecture the basic mechanisms for generating neutrino masses are described and the ingredients of the SM that ensure massless neutrinos are explained. Then, the neutrino oscillation formalism is developed. In the second lecture the current experimental situation is summarized. In particular, we describe the oscillation signals observed by solar neutrino experiments, atmospheric neutrino experiments and long baseline terrestrial neutrino experiments. Each of these results separately can be accounted for by a rather simple modification to the SM. Trying to accommodate all of them simultaneously, however, is not trivial. In the third lecture we explain what are the theoretical challenges in trying to combine all the experimental indications for neutrino masses, and give several examples of models that cope with some of these challenges. These lecture notes are aimed to provide an introduction to the topic of neutrino physics. They are not meant to be a review. Therefore, many details are not given and many references are omitted. There are many textbooks [1] and reviews [2-4] about neutrinos. There is also a lot of information about neutrinos on the web [5,6]. All these sources provide more detailed discussions with complete set of references on the topics covered in these lectures. Moreover, they also cover many subjects that are not mentioned here. In preparing the lectures I used mainly the recent review by GonzalezGarcia and Nir [4]. This review is a very good starting point to anyone who wants to learn more about neutrino physics. 2. Neutrino masses 2.1. Fermion
masses
In general, there are two possible mass terms for fermions: Dirac and Majorana mass terms. All fermions can have Dirac mass terms, but only neutral fermions can have Majorana mass terms. Indeed, all the massive fermions in the SM, the quarks and charged leptons, have Dirac mass terms. The neutrinos, however, while massless in the SM, can have both Dirac and Majorana mass terms. Dirac masses couple left and right handed fields mo^L^R
+ h.c,
(1)
where m_o is the Dirac mass and ipL and ipn are left and right handed Weyl spinor fields, respectively. Note the following points regarding eq. (1): • Consider a theory with one or more exact U(l) symmetries. The
charges of 4>L and ipR under these symmetries must be opposite. In particular, the two fields can carry electric charge as long as
QhPi) = QWR)• Since ipL and ipR are different fields, there are four degrees of freedom with the same mass, mo• When there are several fields with the same quantum numbers, we define the Dirac mass matrix, (rriD)ij ( " i D l . j f f L J . W j + /l.C,
(2)
where i(j) runs from one to the number of left (right) handed fields with the same quantum numbers. In the SM, the fermion fields are present in three copies, and the Dirac mass matrices are 3 x 3 matrices. In general, however, mo does not have to be a square matrix. A Majorana mass couples a left handed or a right handed field to itself. Consider ipR, a SM singlet right handed field. Its Majorana mass term is mMipcRipR,
ipc = Cip ,
(3)
where TUM is the Majorana mass and C is the charge conjugation matrix [7]. A similar expression holds for left handed fields. Note the following points regarding eq. (3): • Since only one Weyl fermion field is needed in order to generate a Majorana mass term, there are only two degrees of freedom that have the same mass, TOM• When there are several neutral fields, TOM is promoted to be a Majorana mass matrix
(mMkW^OM,-.
(4)
Here, i and j runs from one to the number of neutral fields. A Majorana mass matrix is symmetric. • The additive quantum numbers of ipR and tpn are the same. Thus, a fermion field can have a Majorana mass only if it is neutral under all unbroken local and global U(l) symmetries. In particular, fields that carry electric charges cannot acquire Majorana masses. • We usually work with theories where only local symmetries are imposed. In such theories global symmetries can only be accidental. A Majorana mass term for a fermion field ip breaks all the global U(l) symmetries under which the fermion is charged.
9
2.2. Neutrino
masses
in the SM
The SM is a renormalizable four dimensional quantum field theory where • The gauge group is SU(3)C x SU(2)L x £7(l)y. • There are three generations of fermions QL(3,2)+1/6,
UR(3, l)+2/3,
LL(1,2)_1/2)
E*(1,1)-I.
DR(3,
1)_I/3, (5)
We use the (qc,qL)qY notation, such that qc is the representation under SU(3)c, 1L is the representation under SU{2)iJ and qy is the U(l)y charge. • The vev of the scalar Higgs field, H(l, 2) + 1 / 2 , leads to the Spontaneous Symmetry Breaking (SSB) SU(2)LXU(1)Y^U(1)EM-
(6)
The SM has four accidental global symmetries: baryon number (B) and lepton flavor numbers (Le, L^ and LT). It is also convenient to define total lepton number as L = Le + LM + LT, which is also conserved in the SM. An accidental symmetry is a symmetry that is not imposed on the action. It is there only due to the field content of the theory and by the requirement of renormalizability. If we would not require the theory to be renormalizable, accidental symmetries would not be present. For example, in the SM dimension five and six operators break lepton and baryon numbers. The SM implies that the neutrinos are exactly massless. There are several ingredients that combine to ensure it: • The SM does not include fields that are singlets under the gauge group, NR(1, 1)O- This implies that there are no Dirac mass terms {H)uZvR.a (We define H = ir2H*.) • There are no scalar triplets, A(l,3)i, in the SM. Therefore, Majorana mass terms of the form ( A ) I / £ I / £ cannot be written. • The SM is renormalizable. This implies that no dimension five Majorana mass terms of the form {H)(H)V1VL are possible. a
O u r notation is such that we use capital letters to denote fields before SSB and lowercase Greek or Roman letters to denote the fields after SSB. While such differentiation is meaningful only to field that are charged under the SM gauge group, we use this convention also for SM singlets.
10
•
is an accidental non-anomalous global symmetry of the SM. Thus, quantum corrections cannot generate Majorana (i?)(ii")^£^L mass terms. b U(1)B-L
Both the neutrinos and the 5i7(3)c x U(1)EM gauge bosons are massless in the SM. There is, however, a fundamental difference between these two cases. The symmetry that protects the neutrinos from acquiring masses is lepton number, which is an accidental symmetry. Namely, one does not impose it on the SM. In contrast, the photon and gluon are massless due to local gauge invariance, which is a symmetry that one imposes on the theory. This difference is significant. An imposed symmetry is exact also when one considers possible new physics at very short distances. Accidental symmetries, however, are likely to be broken by new heavy fields. One can understand the result that the neutrinos are massless in the SM in simple terms as follows. For a massive particle one can always find a reference frame where the particle is left handed and another reference frame where it is right handed. Thus, in order to have massive neutrinos, the SM left handed neutrino fields should couple to right handed fields. There are two options for such couplings. First, the SM left handed neutrinos can couple to right handed SM singlet fermions. This is not allowed in the SM just because there are no such fields in the SM. The second option is to have couplings between the left handed neutrinos and the right handed anti-neutrinos. Such couplings break lepton number. The fact that lepton number is an accidental symmetry of the SM forbids also this possibility. 2.3. Neutrino
masses
beyond the SM
Once the SM is embedded into a more fundamental theory, some of its properties mentioned above are lost. Therefore, in many SM extensions the neutrinos are massive. Neutrino masses can be generated by adding light or heavy fields to the SM. (By light we refer to fields that have weak scale or smaller masses, while heavy means much above the weak scale.) When light fields are added the resulting neutrino masses are generally too large and some mechanism is needed in order to suppress them. New heavy fields, on the contrary, generate very small masses to the neutrinos. We are now going to elaborate on these two possibilities. b
Note that U(1)B-L has gravitational anomalies. Namely, once gravity effects are included the symmetry is broken and neutrino masses can be generated. The SM, however, does not include gravity. What we learn is that it is likely that the neutrinos are massive in any SM extension that include gravity, in particular, in the real world.
11
There are two kinds of light fields that can be used to generate neutrino masses. First, right handed neutrino fields, NR(1, 1) 0 , couple to the SM left handed lepton fields via the Yukawa couplings and generate Dirac masses for the neutrinos yNHLLNR
+ h.c.
=>
mDvlvR,
(7)
where %JN is a dimensionless Yukawa coupling and the symbol "=>" indicates electroweak SSB. This mechanism offers no explanation why the neutrinos are so light and why the right handed neutrinos do not acquire large Majorana masses. Another way to generate neutrino masses is to add to the SM a scalar triplet, A(l, 3)i. Assuming that the neutral component of this triplet acquires a vev, a Majorana mass term is generated XNALCLLL
=>
mNvcLvL,
(8)
where \M is a dimensionless coupling. In that case it remains to be explained why the triplet vev is much smaller than the SM Higgs doublet vev. Note that the triplet vev is required to be small not only to ensure light neutrinos but also from electroweak precision data, in particular, from the measurement of the p parameter [8]. The other way to generate neutrino masses is to add new heavy fields to the SM. In that case the low energy theory is the SM, but the full high energy theory includes new fields. Using an effective field theory approach, the effects of these heavy fields are described by adding non-renormalizable terms to the SM action. All such terms are suppressed by powers of the small parameter v/M. Here v is the SM Higgs vev, which characterizes the weak scale, and M is the unknown scale of the new physics, which can be, for example, the Planck scale or the GUT scale. Neutrino masses are generated by the following dimension five operator ^HHLILL
=*
m„ = A ^ ,
(9)
where A is a dimensionless coupling. Note the following points regarding eq. (9): • Taking the SM to be an effective field theory implies mu ^ 0. Since we know that new physics must exist at, or below, the Planck scale, it is rather likely that the neutrinos are massive. • m„ is small since it arises from non-renormalizable terms. • Neutrino masses probe the high energy physics. • The neutrino acquires Majorana mass. Thus, total lepton number is broken by two units.
12
Figure 1. The seesaw mechanism acquire its name from analogy with a children seesaw. The lower one child takes its arm of the seesaw, the higher the other child goes. For neutrinos, the larger MN is, the lighter the neutrino becomes. (I Thank Oleg Khasanov for the figure.)
• The situation can be generalized to the case of several generations. Then, generically, both total lepton number and family lepton numbers are broken and lepton mixing and CP violation are expected. A famous example of a realization of this effective field theory description is the seesaw mechanism, see fig. 1. (Historically, it was invented within 50(10) GUT theory, and later implemented in many other models.) Consider one generation SM with an additional fermion singlet NR(1, 1)OThe following two terms in the Lagrangian are relevant for neutrino masses ^MNN%NR
+ YvHTENR + h.c,
(10)
where MN ~3> v is the Majorana mass of the right handed neutrino. After electroweak symmetry breaking the second term leads to a Dirac mass of the neutrino. In the (VL,VR) basis the neutrino mass matrix is
\mD
MNJ
where mp = Yvv. Using Tr(m„) = MN ,
| det(m„)| = m2D ,
(12)
13
to first order in V/MN we find 2
mUR =MN
mPL=^-.
(13)
Comparing this result with eq. (9) one identifies the new physics scale M with MM and the new physics coupling A with Y^. We learn that the seesaw mechanism is indeed a realization of the effective field theory approach for neutrino masses. 2.4.
Neutrino
mixing
Massive neutrinos generally mix. The neutrino mass terms break the accidental family lepton number symmetries. (Total lepton number is also violated if the neutrinos have Majorana masses.) This phenomenon is very similar to quark mixing in the SM. It is therefore instructive to describe both lepton and quark mixing in parallel. For quarks, the Cabibbo-Kobayashi-Maskawa (CKM) matrix, V, corresponds to non-diagonal charged current interactions between quark mass eigenstates
^=(ul)iVl3r(dL)JW+
••u,c,t,
j = d,s,b.
(14)
For leptons, it is common to use two different bases. The flavor basis is defined to be the one where the charged lepton mass matrix and the W interactions are diagonal. In the mass basis both the charged lepton and the neutrino mass matrices are diagonal, but the W interaction is not. In that basis the leptonic mixing matrix U is the analogue of the CKM matrix. 0 Namely, it shows up in the charged current interactions A=TlUtirf{vL)iW-,
£ = e,n,T,
i = 1,2,3.
(15)
When working in the mass basis, the formalism of quark and lepton flavor mixings are very similar. The difference between these two phenomena arises due to the way neutrino experiments are done. While quarks and c
Recently, in many papers the matrix U is called the Maki-Nakagawa-Sakata (MNS) matrix, UMNSI o r the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix, L^PMNSI since these authors were the first to discuss leptonic flavor mixing. Others call it the CKM or KM analogue for the lepton sector since Kobayashi and Maskawa were the first to discuss CP violation from such matrices. Here we adopt the notation of [4] and call U the leptonic mixing matrix. The fact that the CKM matrix is denoted by V helps in avoiding confusion.
14
charged leptons are identified as mass eigenstates, neutrinos are identified as flavor eigenstates. Indeed, there are two equivalent ways to think about fermion mixing. For quarks, mixing is best understood as the fact that the W interaction is not diagonal in the mass basis. For leptons, we usually refer to the rotation between the neutrino mass and flavor bases as neutrino mixing. Note that the indices of the two matrices are of different order. In V the first index corresponds to the up type component of the doublet and the second one to the down type. In U it is the other way around. In general, U is described by three mixing angles and one Dirac CP violating phase. If the neutrinos are Majorana particles there are in addition two more CP violating phases called Majorana phases. The Majorana phases, however, do not affect neutrino flavor oscillations, and we do not discuss them any further. SM singlet states can mix with the three SM neutrinos. When the masses of the singlet states are small they can participate in neutrino oscillation. Then, the singlet states are usually called sterile neutrinos while the standard model neutrinos are called active neutrinos. d The mixings between all neutrinos are taken into account by enlarging the mixing matrix U to a 3 x (n + 3) matrix, where n is the number of sterile states. When the SM singlet states are very heavy (with mass M 3> v), they cannot participate in neutrino oscillations. Their couplings to the light neutrino is important since it gives them their masses. Thus, the heavy states are usually referred to as right handed neutrinos. The mixings relevant for neutrino oscillations is described by the matrix U that refers only to the 3 x 3 sub-matrix that corresponds to the mixing between the mostly active states. The presence of the heavy states affects the theory since in that case the matrix U does not describe the full mixing and, therefore, it is not unitary. This deviation from unitarity, however, is very small, of order v/M, and it is usually neglected. 3. Methods for probing neutrino masses The conclusion of the last section is that it is very likely that the neutrinos have very small masses and that they mix. There are several ways to probe Oscillation occurs only between left handed states, that is, both active and sterile states are left handed. One should not get confused by the fact that the sterile states are left handed; the identification of left handed states as SM doublets and right handed states as SM singlets does not have to be true in theories that extend the SM.
15
neutrino masses and mixing angles. Discussions about astrophysical and cosmological probes of neutrino masses can b e found, for example, in [1,3]. Here we briefly discuss kinematic tests for neutrino masses and neutrinoless double b e t a decay. Then we develop the formalism of neutrino oscillation, which is the most promising way t o probe the neutrino masses and mixing angles. 3 . 1 . Kinematic
tests
In decays t h a t produce neutrinos the decay spectra are sensitive to neutrino masses. For example, in TT —> \iv the muon m o m e n t u m is fixed (up to tiny width effects), and it is determined only by t h e masses of the pion, t h e muon and the neutrino. To first order in m%/m%, the muon m o m e n t u m in the pion rest frame is given by l
I-I \P = a
( r
2
2
m
\ K- u,
m
l + ml 5
M
2m7r \
2\
/1fix
T mt
ml - ml
•
v
J
(16) v
'
Since the correction to the massless neutrino limit is proportional to m ^ , the kinematic tests are not very sensitive to small neutrino masses. T h e current best bounds obtained using kinematic tests are [8] m „ < 18.2 MeV
from r -> 5TT + v ,
mu < 190 KeV
from TT -^ fit/,
mv < 2.2 eV from 3 H ^ 3 H e + e + 17. (17) T h e sensitivities of neutrino oscillation experiments are much better t h a n these bounds. Note t h a t while oscillation experiments are sensitive to the neutrino mass-squared differences (see below) the kinematic tests are sensitive to the neutrino masses themselves. 3 . 2 . Neutrinoless
double
f3
decay
Neutrino Majorana masses violate lepton number by two units. Therefore, if neutrinos have Majorana masses we expect there there are also AL = 2 processes. T h e smallness of the neutrino masses indicates t h a t such processes have very small rates. Therefore, the only practical way to look for AL = 2 processes is in places where the lepton number conserving ones are forbidden or highly suppressed. Neutrinoless double (3 decay where the single b e t a decay is forbidden is such a process. An example for such processes is ?§Ge - •
3
7t
Se + 2 e - .
(18)
16
The only physical background to neutrinoless double (3 decay is from double (3 decay with two neutrinos. Note the following points: • Since neutrinoless double (3 decay is a AL = 2 process, it is sensitive only to Majorana neutrino masses. Dirac masses conserve lepton number and do not contribute to this decay. • The neutrinoless double (3 decay rate is related to the neutrino mass-squared. It is also proportional to some nuclear matrix elements. Those matrix elements introduce theoretical uncertainties in extracting the neutrino mass from the signal, or in deriving a bound if no signal is seen. • Neutrinoless double (3 decay is sensitive to any AL = 2 operator, not only to the neutrino Majorana masses. Thus, the relation between the Neutrinoless double (3 decay rate and the neutrino mass is model dependent. • The best bound derived from neutrinoless double (3 decay is mv < 0.34 eV [9]. 3.3. Neutrino
vacuum
oscillation
We now turn to the derivation of the neutrino oscillation formalism. We use several assumptions in this derivation. We emphasize, however, that in all practical situations it is correct to use these assumptions and that more sophisticated derivations give the same results (see, for example, [10]). In an ideal neutrino oscillation experiment, a neutrino beam is generated with known flavor and energy spectrum. The flavor and the energy spectrum is then measured at some distance away. If the flavor composition of the beam changed during the propagation it is a signal of neutrino oscillation, which indicates neutrino masses and mixings. The flavor of the neutrino is identified via its charged current interaction. It is convenient to express a flavor state in terms of mass states
ka)=E^>*)>
(19)
i
where Greek indices runs over the flavor states (a = e, /i, r) and Roman ones over the mass states (i = 1,2,3). An initially produced flavor state \va) evolves in time according to
Wa(t))=^2e~^tU*mWi),
(20)
17
where we assume that all the components in the neutrino wave packet have the same momenta (pi = p). For ultra relativistic neutrinos we can use the following approximation o '171-
Bk*P+-£.
(21)
Then, the probability to observe oscillation between flavor a and /3 is given by Pa/3 = \{vp\Va(t))\2
= 5a0 ~ ^^UaiUfjiUajUpj
s i n 2 Xtj,
(22)
where in the last step we assumed CP conservation and Xij
_ Am?,-* ~~ 4p '
'
In many cases the three flavor oscillation is well approximated by considering only two flavors. Then, °
/ cos0 \-sm6
sin£\ costfy*'
[Z
'
and for a ^ (3 we obtain Pap = sin2 261 sin2 x .
(25)
Since the neutrinos are relativistic we usually use L = t, E = p and write _ TTL __ AnE . x = 7 > -^osc — -7 5" j (2o) L osc Am 2 such that L osc is the oscillation length. The following formula is also convenient
»-(£)GI)(^)The oscillation master formula, eq. (25), teaches us the following: • Longer baseline, L, or smaller energy, E, is needed in order to probe smaller Am 2 . • When L <€. Losc we can approximate sin x ~ x2. In that case the signal is usually too small to be detected. • When L » Losc the energy spread of the beam and decoherence effects cause the oscillations to average out. That is, in our formula we should take the average value: (sin x) = 0.5. In that case only a lower bound on the mass-squared difference between the two neutrinos can be obtained.
18
3.4. Matter
effects
When neutrinos travel through matter the oscillation formalism is modified. While neutrinos can scatter off the medium, in almost all relevant cases the scattering cross sections are very small, and the effect of scattering is negligible. The effect we are concentrating on is that of the forward scattering of the neutrinos. Like photons, when neutrinos travel inside a medium they acquire effective masses. There is no energy or momentum exchange between the neutrinos and the medium. The effect of the medium is that it induces effective masses for the neutrinos. First we discuss the case of a medium with constant density. Moreover, we consider only matter at low temperature, which implies that it consists of electrons, protons and neutrons. Note that there are no muons, taus and anti-leptons in that case. The charged current interaction between the electron neutrinos and the electrons in the medium induces an effective potential for the neutrinos [11] Vc = V2GFNe « 7.6 Ye ( ^ — j )
eV,
(28)
where Ne (Np, Nn) is the number density of electrons (protons, neutrons) and Ye = Ne/(Np + Nn) is the relative electron number density. To get a feeling for the size of this potential, note that at the Earth core p ~ 10 g/cm 3 , which gives rise to Vc ~ 1 0 - 1 3 eV. As we discuss below, the current data indicates that m„ > 10~ 3 eV, and thus mu ^> Vc- This seems to suggest that matter effects are irrelevant. This is, however, not the case since the matter effects arise from vector interactions while masses are scalar operators. The right comparison to make is between m2, and EVc where E is the neutrino energy. Since E » mv matter effects are enhanced and can be important. To explain this enhancement, we consider a uniform, unpolarized medium at rest. (Discussion about the general case which includes all types of interactions and arbitrary polarization can be found, for example, in [12].) In that case the neutrino feels the four-vector interaction VM = (Vc, 0,0,0). Due to Vc the vacuum dispersion relation of the neutrino, p^p1* = m2, is modified as follows (pM-^)(PM-^)=m2
777
=»
E*p
2
+ Vc + —,
(29)
where the approximation holds for ultra relativistic neutrinos. We learn that the effective mass-squared in matter, m ^ , is given by m2m=m2+A,
A = 2EVC,
(30)
19
where we used p w E. It is the vector nature of the weak interaction that makes the matter effects practically relevant. Interactions that are flavor universal only shift the neutrino energy by a negligibly small amount and do not affect the oscillation. Non-universal interactions, however, are important since in their presence the values of the effective masses and mixing angles are different from their vacuum ones. While the weak neutral current is flavor universal, the charged current is not. In normal matter, where there are electrons but not muons and taus, only electron neutrinos interact via the charged current with the medium. The mixing matrix is modified by this non-universal matter effect such that the effective squared mass difference and mixing angle are given by AmJ, = ^ / ( A T O 2 cos 29 - A)2 + (Am2 sin 2<9)2, (31) 2 _ Am sin 29 t&nWm ~ Am2cos20-A' where the subindex m stand for matter. The oscillation probability is then obtained from (25) by replacing the vacuum masses and mixing angles by the corresponding parameters in matter Pap = sin2 29m sin2 xm,
xm =
™ .
(32)
The following points are worth mentioning regarding eq. (31): • The vacuum result is reproduced for A — 0, as it should. • Vacuum mixing is needed in order to get mixing in matter. • To first order in xm the matter effects cancel in the oscillation probability. To see this note that xsin28 = xmsm.29m + 0(x^n). Therefore, when approximating smxm ~ xm eq. (32) reduces to the oscillation probability in vacuum, eq. (25). • For Am 2 cos 29 3> \A\, the matter effect is a small perturbation to the vacuum result. • For Am 2 cos 29 <€. \A\, the neutrino mass is a small perturbation to the matter effect. In that case the oscillations are highly suppressed since the effective mixing angle is very small. • For Am 2 cos 29 = A, the mixing is maximal, namely it is on resonance.
3.5. Non-uniform
density
When the matter density is not constant there are further modifications to the oscillation formalism. Density variation results in changing the effective
20
neutrino masses and their mixing angles. Then, the flavor composition of the neutrinos along their path is a function of the medium density profile. At any point r on the neutrino path we define the derivative of the density
^ W - ^ l .
(33)
For constant density, A' = 0, the flavor conversion probability is controlled by the effective masses and mixing angles. For varying density, A' ^ 0, there are extra parameters that affect the flavor conversion probability. Most important is the adiabatic parameter Q ( r ) =
Am 2 sin2 26 A(r) £ cos 20 1 ^ ) -
(34)
In the adiabatic limit, Q ^> 1, the density variation is slow. In this case the transition between effective mass eigenstates is highly suppressed, and the constant density formalism can be applied locally. In the non-adiabatic limit, Q < 1, the density variation is fast. Then, transition between effective mass eigenstates is possible, and the constant density formalism cannot be used. Both limits can be of interest in reality. The best known example of the effect of the density variation is the MSW effect [11,f 3]. It can account for a very efficient flavor conversion in the case of small mixing angle. Consider a two generation model where m,2 > rrii with small vacuum mixing angle, 6
zve(r = 0) « z,2m,
v^r = 0) « v™.
(35)
(By writing va « v™ we mean that a very small rotation in flavor space is needed in order to move from the flavor eigenstate va to the effective mass eigenstate v™.) In particular, the produced electron neutrino is almost a pure V2- Also at the edge of the Sun, r = RQ, the flavor eigenstates are almost pure mass eigenstates 9m(r = RQ) -> 9 < 1,
ve{r = R®) & Vl,
Vy.{r
=
RQ)
W V2-
(36)
When the adiabatic limit applies, neutrinos propagate in the Sun as effective mass eigenstates. Since the neutrinos are produced as almost pure V2, they
21
e
sin2 2i?=0.001:
80 60 40 20
A Figure 2. A demonstration of an MSW resonance conversion. 9m is the effective mixing angle in matter, A is the effective mass due to the matter effect defined in (30) and AR is the resonance point. The neutrinos are produced at a region where A > An. They propagate as effective mass eigenstates, which is shown in the figure as solid lines. In the plot the effect is demonstrated for two different values of the vacuum mixing angles. (The plot is taken from [4].)
leave the Sun as v-i- Since in vacuum J/2 is almost a pure i/M, we learn that there is almost full conversion from ve to v^. This phenomenon, which is called MSW resonance conversion, is demonstrated in fig. 2. In the more general case, corrections to the adiabatic limit are taken into account. In particular, transitions between the effective mass eigenstates are important in the vicinity of the resonance. Such level crossing is described by the Landau-Zener probability, PLZ- Then, the flavor conversion probability is given by the Parke formula [14] Pee = \ [1 + (1 " 2 P L Z ) COS 20m
COS 26?] ,
(37)
where 9m is the effective mixing angle at the production point. In this general case, the flavor transition probabilities become a rather involved function of the neutrino and medium parameters. It is this richness that makes the flavor composition of the solar neutrinos a complicated and not a monotonic function of their energies. As it is shown below this is important in order to be able to explain the solar neutrino data.
22
3.6. Neutrino
oscillation
experiments
Ideally, neutrino oscillation experiments measure the neutrino flavor transition probabilities as a function of the neutrino energies and their path. In practice, however, there are many experimental complications. The basic setup of any neutrino oscillation experiment is as follows: Neutrinos are produced and then propagate until few of them are detected at another location, far away from their production point. Therefore, in order to be able to probe the fundamental physics parameters we would like to know and control the following: • The total flux, the flavor composition and the energy spectrum of the beam. • The distance traveled by the neutrinos and the matter profile that they passed. • The total flux, the flavor composition and the energy spectrum of the detected neutrinos. In reality, it is impossible to know and control all of these parameters. Therefore, a lot of work is involved in order to get the maximum out of each experimental setup. In general we distinguish between two kinds of experiments. Disappearance experiments search for reduction in the flux of a specific neutrino flavor N[va(L)} < JV[i/a(0)],
(38)
where N[i/a(L)] is the number of neutrinos of flavor a in the neutrino beam at distance L. This would imply Paa(L)
(39)
Appearance experiments, on the other hand, look for enhancement of a flux of a specific flavor N[vf,(L)] > N[Vf3(0)},
(40)
Pap(L) > 0.
(41)
which would imply
In particular, in many cases -/V[z/g(0)] = 0. Then, any observation of neutrinos with flavor (3 is an appearance signal. Oscillation experiments are usually named after their production place and mechanism. In the following we discuss solar neutrinos, atmospheric neutrinos and terrestrial neutrinos.
23
4. Solar neutrinos Solar neutrinos receive a lot of attention since there are indications for disappearance of electron neutrinos. These indications are collectively referred to as the solar neutrino problem or the solar neutrino anomaly. The best explanation for the solar neutrino problem is neutrino flavor oscillation. The use of the words "problem" or "anomaly" should not worry us. They are used only to indicate the fact that solar neutrinos pose a problem to the SM. We think, however, that the SM is an effective theory of Nature, and thus, the fact that it faces problems is not surprising. Actually, we expect that neutrinos have masses and that they mix, and consequently, that the solar neutrino data cannot be explained by the SM. 4.1. Solar neutrinos
production
Based on our understanding of stellar evolution we know that nuclear reactions fuel the Sun. Nuclear processes involve only the first generation of fermions and thus only electron neutrinos are generated in the Sun. Moreover, since the Sun is burn by fusion reactions it produces only neutrinos and not anti-neutrinos. The neutrino energies are relatively small, Ev < 10 MeV. Therefore, only disappearance experiments can be done; the neutrinos energies are too small to produce muons or taus in any target at rest. There are many processes that produce solar neutrinos. The main reaction chain, which produces about 98.5% of the solar energy (and most of the solar neutrinos) is the pp cycle Ap - • 4 He + 2e+ + 2ve + 2 7 .
(42)
The neutrinos emitted in this cycle have continuum spectrum which goes up to Eu < 0.42 MeV. Assuming that the Sun is in equilibrium, one can use its current luminosity to calculate the neutrino flux from the pp reaction chain. The other nuclear reactions in the Sun are not very important for its luminosity, but they are important for neutrino physics. The difference originates from the fact that photons are thermalized almost immediately after they are produced. Neutrinos, on the other hand, do not interact in the Sun, and leave the Sun with their original energies. Several subdominant reactions produce neutrinos with energies above the threshold of the pp chain. Since it is easier to detect high energy neutrinos, in many cases the experimental signals consist only of neutrinos that were produced by sub-dominant reactions.
24
SuperK, SNO
X
JE! E o c 2
0.3
1
3
10
Neutrino Energy (MeV) Figure 3. The Bahcall and Pinsonneau.lt 2000 solar model prediction of the neutrino flux as function of the neutrino energy. This figure was taken from [15], where many more details on solar neutrinos can be found.
While the spectra of the different reactions are known, the sub-dominant reaction chains fluxes are not known in a model independent way. Solar models, namely, theoretical arguments, are used to obtain the total neutrino spectrum. As an example, the Bahcall and Pinsonneault 2000 (BP00) solar model prediction for the neutrino flux is given in fig. 3. Since solar models involve theoretical inputs, one would like to perform experiments where the interpretation of the data can be done with minimum dependence on a specific solar model. 4.2. Solar neutrinos
propagation
Solar neutrinos are produced, in the vicinity 'of the solar core. Then, they freely propagate out of the Sun and some of them travel to our detectors. During the night, neutrinos reach the detector after they have passed
25
through the Earth. It is convenient to separate the solar neutrino propagation into three regions: inside the Sun, between the Sun and the Earth, and inside the Earth. We discuss each of these below. As already mentioned before, matter effects in the Sun can be important for flavor conversion. To calculate these effects we need to know the matter profile in the Sun and the rates of the different reactions in the different parts of the Sun. While these parameters are roughly known, there are uncertainties in their values. Therefore, we have to count on solar models to provide the needed inputs. The distance to the Sun is known to high precision. It varies by about 7% during the year. If vacuum oscillation is relevant to solar neutrinos then this distance variation can be used in order to get better control on the neutrino parameters. During the night neutrinos pass through the Earth on their way to the detector, while during the day they do not. Therefore, the Earth matter effect, which shows up as a day-night difference of the detected neutrino flux, provides further sensitivity to the neutrino parameters. Moreover, depending on the location of the detector and the time of the year the neutrino may pass through both the Earth mantle and its core or only through the mantle. This is important since the matter density in these two regions is different, and therefore can affect the oscillations [16]. 4.3. Solar neutrinos
detections
There are three reactions that are used to detect solar neutrinos. Charged Current (CC) interaction ve + n —> p + e~ ,
The (43)
where n is a neutron, can be used to detect only electron neutrinos. All the neutrinos can undergo Elastic Scattering (ES) vi + e~ —+ vt + e~ .
(44)
This reaction is mediated by neutral current Z exchange for all neutrino flavors. For electron neutrinos there is also contribution from a W exchange amplitude. Due to this difference, the elastic scattering cross section for electron neutrinos is about six times larger than for muon and tau neutrinos. Finally, the cross section for the Neutral Current (NC) interaction vi + n —> vi + n , is the same for all neutrino flavors (here n is a neutron).
(45)
26
When presenting results of neutrino oscillation experiment it is convenient to define R = Nobs/NMc
,
(46)
such that N0bs is the number of detected neutrinos and NMC is the number of events predicted from a solar model based Monte Carlo (MC) simulation assuming no flavor oscillation. For detectors that use neutral current reactions R should be one regardless if there are active neutrino flavor oscillations or not. For charged current and elastic scattering based detectors, however, R = 1 is expected only if there are no neutrino flavor oscillations, and R < 1 is expected if there are oscillations. There are several solar neutrino experiments that use the above mentioned basic reactions. They have different experimental setups that us various reactions with different sensitivities and thresholds. Next we describe these experiments. 4.4. Chlorine:
Homestake
The first detection of solar neutrinos was announced 35 years ago. The neutrinos were detected in the Homestake mine in South Dakota [17]. The experiment uses the ue + 37C1 -> 37Ar + e~ ,
(47)
charged current interaction in order to capture the electron neutrinos. We note the following points: • The signal is extracted off-line. Namely, only integrated rates are measured. Therefore, no day-night and almost no spectral information are available. (The only information about the neutrino spectrum is that the energy of the neutrino is above the threshold.) • The energy threshold is 0.814 MeV. Therefore, the experiment is sensitive mainly to the 7 Be and 8 B neutrinos, pp neutrinos cannot be detected in this experiment. • After many years of data collection the published result is [18] i?«0.3.
(48)
Of course, in order to perform a fit to the neutrino parameters the above crude approximation should not be used. Yet, quoting the order of magnitude, as we did in eq. (48) is sufficient to understand the main ingredients of the solar neutrino problem.
27
The fact that the Homestake experiment found R < 1 was the first indication for solar electron neutrino disappearance. This result gave the motivation to build different types of detectors in order to further study the solar neutrinos. 4.5. Gallium:
SAGE,
GALLEX
and
GNO
The Homestake experiment had relatively high threshold. The SAGE, GALLEX [19] and GNO [20] Gallium experiments were built in order to detect neutrinos with much smaller energies. In particular, their threshold is low enough to detect pp neutrinos. In the Gallium experiments the neutrinos are detected using the i/e +
71
G a ^ 7 1 G e + e-
(49)
charged current reaction. We note the following points: • Like the Chlorine experiment, the signal is extracted off-line. Thus, also here no day-night and spectral information are available. • The energy threshold is 0.233 MeV. Thus, Gallium detectors are sensitive to pp neutrinos. The 7 Be and 8 B neutrinos also contribute significantly to the neutrino signal. Therefore, the total predicted flux is not solar model independent. • Combining the results of the three experiments, one obtains R w 0.6.
(50)
We learn that like in the Chlorine experiment, also here, the number of observed electron neutrinos is less than predicted. Moreover, the measured R is different for the two type of experiments. Since these two types are sensitive to different energy ranges, this fact indicates that the electron neutrino flux reduction must be energy dependent. 4.6. Water
Cerenkov:
Kamiokande
and
SuperKamiokande
Both the Chlorine and Gallium experiments extract their signal off-line. The Kamiokande and SuperKamiokande [21] water Cerenkov detectors can extract their signal on-line. Therefore, in these experiments day-night and spectral information can be extracted. The price to pay is that the energy threshold is much higher compared to the thresholds of the Chlorine and Gallium experiments.
28
In the water Cerenkov detectors the neutrinos are detected by elastic scattering of the neutrinos off the electrons in the water va + e~ —> va + e~ .
(51)
We note the following points: • Since the detection is done on-line, the time of each event is known, and also some information about the direction and energy of the neutrino in each event can be extracted. These features enable the experiments to clearly show that the detected neutrinos are coming from the Sun. • The water Cerenkov detectors threshold is about 6 MeV. Thus, the Kamiokande and SuperKamiokande experiments are sensitive mainly to 8 B neutrinos. • Since elastic scattering is used for the detection, all neutrino flavors contribute to the signal. This fact is important if there are active flavor oscillations. Then, the converted neutrinos also contribute to the signal. This is in contrast to detectors that use charged current interactions where only electron neutrinos generate the signal. • The integrated suppression is found to be R « 0.5.
(52)
Spectrum and day-night information were also reported. We learn again that there is an energy dependent suppression of the electron neutrino flux. 4.7. Heavy
Water:
SNO
We already mentioned that neutrino flavor conversion is the most plausible explanation to the observed energy dependent electron neutrino flux suppression. Thus, it is interesting to measure solar neutrinos using neutral current reactions since in that case all neutrino flavors contribute with the same strength. Namely, neutral current based experiments measure the solar neutrino flux independent of whether there are active flavor oscillations or not. The SNO detector [22] was built for this purpose. Its apparatus contains an inner tank filled with heavy water and an outer tank filled with regular water. Neutrino can be detected by all the three basic reaction types CC(Eth = 2.23 MeV):
ve + 2 H -> p + p + e~ ,
29
ES(Eth ~ 6 MeV) : NC(£ t / l ~ 6MeV) :
Va
+e~ -+va +e~ ,
^ a + 2 H - • n + p + va ,
(53)
where Eth is the threshold energy. Note that the presence of Deuterium (2H) in the heavy water is crucial for the charged current and neutral current measurements. Note the following points: • The signal is extracted on-line. Thus day-night and spectrum information is available. • We define the ratios
RB = %£,
^f52.
-KES
( 54 )
-KNC
where RQC {RES, -RNC) is the ratio between the observed number of charged current (elastic scattering, neutral current) events and the predicted number assuming no oscillation. Extracting RE and RN has several advantages. First, many systematic uncertainties cancel in their measurements. Second, their predicted values are almost solar model independent and depend only on the neutrino parameters. In particular, RE ^ 1 or RN J^ 1 can occur only if electron neutrinos are converted to muon or tau neutrinos. • SNO found RE ^ 1 and i?j\r 7^ 1- This result confirms the existence of Up or vT component in the solar neutrino flux at 5.3a. In addition, the BPOO solar model prediction is confirmed by the neutral current measurement. The SNO result [23] is illustrated in fig. 4.
4.8. Solar neutrinos:
fits
The general picture emerging by combining all the solar neutrino data is as follows. All charged current and elastic scattering based measurements found less events than predicted for massless neutrinos. This suppression is energy dependent. The BPOO solar model prediction for the neutral current based measurement, which holds also if there are active neutrino flavor conversions, was confirmed by SNO. The first and most robust conclusion is that the SM, which predicts massless neutrinos, fails to accommodate the data. While there are several exotic explanations that may be able to explain the data [24], the simplest and most plausible one is that of active neutrino flavor oscillation ve —> i/x,
x = fj,,r.
(55)
30
E
.SNO
,SNO
Figure 4. The SNO data is plotted as three independent measurements of the muon or tau neutrino flux (<^MT) as a function of the electron neutrino i u x (e)- The three bands represent the neutral current, charged current and elastic scattering based measurements. The dotted line is the BP00 solar model prediction for the neutrino flux. It is plotted only in conjunction with the neutral current measurement since only this measurement is insensitive to electron to muon or tau flavor conversion. (This plot is taken from [23].)
In that case, experiments that use reactions that are flavor dependent, namely charged current interactions and elastic scattering, are expected to detect fewer events than anticipated. On the other hand, experiments that use neutral current interactions, which are flavor blind, should not observe such reduction. Indeed, this is what the data shows. Assuming that the neutrinos are massive, the data can be used to determine the neutrino masses and mixing angles. While three flavor oscillation fits are available [25], it is a good approximation to fit the data assuming only two neutrino mixing. An example of such fit is shown in fig. 5. (Recent fits can be found in [25]. Note, however, that whenever new data is published, new fits are performed and published usually within days.) The regions where a good fit is found are indicated in the figure. The best fit is obtained in the Large Mixing Angle (LMA) region. The recent SNO data actually exclude the Small Mixing Angle (SMA) region. The LOW and the
31
10""
•fUH'I'l
t i l l i>M;t|;
i H Hill
I -I-,' t i \nW'"~
1^
i I Ml±
io4fe ID*
SMA 10
S
LOW ,'i'rV', 10'
E <
io r
I 1 i \
10
vo 10
10
10
li I
10
•'*»* i
i i MIII
10"
i
I i 11 m l
i
i
i l mil
I 11 n i l
-i
10"
.-J3T =
10
10
10
tan"© Figure 5. An example of a neutrino parameters fit to some solar neutrino data. The names of the regions where good fit is found are indicated in the figure. Note that by now this fit is outdated, and it is shown here for illustration only. (This plot is based on a plot taken from [26].)
Vacuum Oscillation (VO) regions, are still allowed by all solar data. They are excluded, however, by the recent KamLAND result, see below. To conclude, there are strong indications for active ue —> vx oscillation. This conclusion is robust since it has practically no dependence on the solar model. The best fit to the neutrino parameters is in the LMA region with A m 2 = 6 x 10" 5 eV 2 ,
tan 2 0 = 0.4.
(56)
Future data and new experiments will teach us more about solar neutrinos.
32
Besides having more statistics, future experiments will measure observables where we have only very little information at present. In particular, we expect to have more information about the day-night effect and the low energy neutrino spectrum. 5. Atmospheric neutrinos In the reaction chain initiated by cosmic ray collisions off nuclei in the upper atmosphere neutrinos are produced. These neutrinos are searched for by atmospheric neutrinos experiments. There are indications for muon neutrinos disappearance. These indications are called the atmospheric neutrino problem or the atmospheric neutrino anomaly. The likely explanation of this phenomenon is active neutrino oscillation. 5.1. Atmospheric
neutrino
production
Cosmic ray collisions off nuclei in the upper atmosphere produce hadrons, mainly pions. In their decay chain / i + -> e+veVli,
Tr+^/i+z^,
(57)
(and the charge conjugated decay chain) neutrinos are produced. Note that almost exclusively electron and muon neutrinos are produced. Based on (57) we expect NiyJ/Niye)
« 2,
(58)
where N{vz) is the number of neutrinos of flavor I. There are corrections to this prediction. Other mesons, mainly kaons, are also produced by cosmic rays. Their decay chains produce different neutrino flavor ratio. Another effect is that at high energies the muon lifetime in the lab frame is much longer than in its rest frame. In fact, it is long enough such that not all the muons decay before they arrive to the detector. All in all, the neutrino production rates and spectra are known to an accuracy of about 20%. The ratio N(y!j)/N(ve) is known better, to an accuracy of about 5%. The atmospheric neutrino energies are relatively large, Eu > 100 MeV. Therefore, both disappearance and appearance experiments are possible. In particular, since the initial tau neutrino flux is tiny, searching for tau appearance is an attractive option. 5.2. Atmospheric
neutrino
propagation
Since neutrinos are produced isotropically in the upper atmosphere, they reach the detector from all directions. The neutrino zenith angle in the
33
detector is correlated with the distance it traveled. Neutrinos with small zenith angle, namely those coming from above, travel short distances (of order 102 km). Neutrinos with large zenith angle, namely those coming from below, are neutrinos that cross the Earth and travel much longer distances (of order 104 km). Since the production is isotropic, the neutrino flux in the detector is expected to be isotropic in the absence of oscillation. (There are small known corrections due to geo-magnetic effects.) Matter effects can be important for neutrinos that cross the Earth. Oscillations that involve electron neutrinos or oscillations between active and sterile neutrinos are modified in the presence of matter. In contrast, the matter has almost no effect effect on fM —> vT oscillation. 5.3. Atmospheric
neutrino
detection
There are two types of atmospheric neutrino detectors. The water Cerenkov detectors, 1MB, Kamiokande and SuperKamiokande [21], use large tanks of water as targets. The water is surrounded by photomultipliers which detect the Cerenkov light emitted by charged leptons that are produced in charged current interactions between the neutrinos and the water. Compared to electrons, muons are likely to create sharper Cerenkov rings. Thus, the shape of the ring helps in determining the flavor of the incoming neutrino. Iron calorimeter detectors are based on a set of layers of iron which act as a target, and some tracing elements that are used to reconstruct muon tracks or showers produced by electrons. Both water Cerenkov and Iron calorimeter detectors identify the neutrino flavor via its charged current interactions. They have some ability to look for neutral current interactions. Information on the neutrino energy and direction can also be extracted by both methods. 5.4. Atmospheric
neutrino
data
All atmospheric neutrino experiments measure the double ratio TVt INe, r> _
ly
obs/obs
JV
MC7JVMC
(KQ\
where NlMC is the expected number of flavor I type events and N*bs are the observed ones. The advantage of using this double ratio is that many systematic and theoretical errors cancel in this ratio. Almost all of the experiments found R < 1; see fig. 6. The observation of R < 1 can be explained by muon neutrino disappearance, electron neutrino appearance, or both. The SuperKamiokande
34
1.5
a:
~
FREJUS
- " t 1
.
1MB
t
0.5 - NUSEX
S0UDAN2
\
KAM KAM multi sub
+
•
SK SK multi sub
Figure 6. Summary of the experimental measurements of R defined in eq. (59). (This plot is taken from [4].)
450 Snh-fipV e-like 400 350 300 *-3 Z±&**J±^+_ 250 200 150 100 50 0 -0.5 0.5 cosG
.+
i
i i i i i i i i i i i i i
i
450 fiuh-GpV[i-likP 400 350 300 ^y¥ i ^ t o * * 250 * • 200 150 100 50 0 111 1 1 1 1 1 1 1 1 1 1 1 1 1 1
-0.5
0rt
0.5
f*
i
1
cosG
Figure 7. The low energy electron and muon neutrino fluxes compared to the MC predictions as observed by SuperKamiokande. The solid lines are the expected fluxes assuming no oscillations. The dashed lines are the best fits to neutrino oscillation. The solid circles are the data points. (This plot is taken from [27].)
experiment also measured the zenith angle dependence of the neutrino flux. The data indicate that the preferred explanation is that of muon neutrino disappearance. For example, fig. 7 shows that the low energy ve flux agrees while the v^ flux disagrees with the MC prediction.
35
5.5. The atmospheric
neutrino
problem
The zenith angle dependence of the muon neutrino flux as well as R < 1 cannot be explained with SM massless neutrinos. This is called the atmospheric neutrino anomaly. In order to explain it disappearance of muon neutrinos is needed. While there are several exotic explanations [24], the most attractive explanation is active neutrino flavor oscillation. In particular, the best fit is achieved for v^ —> vT oscillation with Am 2 = 2.6 x 10" 3 eV 2 ,
sin2 29 = 0.97.
(60)
The hypothesis of neutrino oscillation can be further tested. First, terrestrial long baseline experiments probe the same region of parameter space as the atmospheric neutrino experiments. Indeed, as discussed in more detail below, the recent K2K long baseline experiment results agree with the atmospheric neutrino data. Second, search for tau appearance in the atmospheric neutrino data is also possible. The SuperKamiokande experiment found such indications [28], but they are not yet convincing. 6. Terrestrial neutrinos One disadvantage of solar and atmospheric neutrino measurements is that since the neutrinos are not produced on Earth, there is no control over the neutrino source. Neutrinos from accelerators and nuclear reactors have the advantage that their source is known. That is, we know to high accuracy the initial neutrino flux, the energy spectrum and the flavor composition of the neutrino source. Moreover, in some cases these parameters can be tuned in order to maximize the experimental sensitivity. The main advantage of solar and atmospheric neutrinos is that they are sensitive to very small Am 2 . That is, their sensitivity is much better compared to most terrestrial neutrino experiments. Therefore, it is not surprising that most terrestrial neutrino experiments did not find disappearance or appearance signals. Their data was used to set bounds on the neutrino parameters. There are, however, some exceptions. The sensitivity of the K2K [29] and KamLAND [30] experiments are similar to those of, respectively, the atmospheric and solar neutrino experiments. Recently, positive disappearance signals were found in these two terrestrial experiments. The situation is different with regard to the LSND [31] experiment. In that case there is a claim for an appearance signal. This signal can only be explained with neutrino masses that are much larger compared to the masses deduced from the solar and atmospheric neutrino data. Next we
36
describe these three experiments and their results. 6.1.
K2K
In order for accelerator neutrinos to be sensitive to the same range of parameter space as the atmospheric neutrino experiments, long baseline, of order 103 km, is needed. In such long baseline experiments it is possible to search for v^ disappearance or vT appearance. In these experiments a neutrino beam from an accelerator is aimed at a detector located far away. The first operating long baseline experiment is the K2K experiment. It uses an almost pure z/M beam that is generated at KEK and is detected at SuperKamiokande, which is about 250 km away. Recently, the K2K experiment announced their first result [32]. They observed 56 muon neutrino events with an expectation of about 80. The v^ disappearance can be explained by v^ —> vx flavor oscillation. The best fit is found for the following values Am 2 = 2.8 x 10" 3 eV 2 ,
sin2 26 = 1.0.
(61)
These values of the neutrino parameters are very close to those indicated by the atmospheric neutrino data, see eq. (60). The statistical significance of the K2K oscillation signal is smaller than the atmospheric neutrino ones. The importance of the K2K result lies in the fact that it provides an independent test of the atmospheric neutrino parameter space. 6.2.
KamLAND
Reactor neutrinos have much smaller energies compared to neutrinos produced at accelerator. Thus, long baseline reactor experiments are sensitive to smaller values of Am 2 . Another important difference is that reactors generate only electron anti-neutrinos, while accelerators produce mainly muon neutrinos. Thus, reactor experiments can be used to provide independent probes of the solar neutrino parameter space. The KamLAND experiment is placed in the Kamioka mine in Japan. This site is located at an average distance of about 180 km from several large Japanese nuclear power stations. The initial fluxes and spectra of the ve emitted in each reactor are known to a good accuracy, since they are related to the power that is generated in each reactor. Thus, measurement of the total flux and energy spectrum of the Ve at KamLAND can be used to search for disappearance. This is particularly interesting since the KamLAND setup is such that it is sensitive to the region of the parameter space indicated by the LMA solution to the solar neutrino anomaly.
37
Recently, the KamLAND experiment announced their first result [33]. They found - r ^ = 0.611 ± 0.085 ± 0.041
(62)
NMC
where N0bs is the number of observed events and NMc is the expected number. This result can be explained by Ve —> Vx neutrino flavor oscillation with the best fit at Am2 = 6.9 x 10" 5 eV 2 ,
sin2 29 = 1.0.
(63)
These values are very close to the ones indicated by the solar neutrino data, see eq. (56). The importance of this result is twofold. First, it provides a completely solar model independent test for the neutrino oscillation solutions of the solar neutrino problem. Moreover, it discriminates between the different possible solutions, pointing at the LMA as the favorable one. 6.3.
LSND
The set up of the LSND experiment [31] is as follows. Neutrinos are produced by sending protons on a fixed target that generates pions. Almost all the negatively charged pions are absorbed in the target before they decay. The neutrinos are produced via 7T+ - > / / + ^ ,
fj,+ - > e+veVIJi.
(64)
Then, the neutrinos travel for about 30 meter to the detector. Since the beam contains u^, ve and F M , with negligible fraction of Ve, it best to search for Vfj, —» Ve appearance. The LSND collaboration announced a 3.3<7 indication for Ve appearance P(Pn -» ve) = (2-64 ± 0.67 ± 0.45) x K T 3 .
(65)
This result can be explained with v^ —> ve oscillation where the best fit is achieved for Am 2 = 1.2 eV 2 , 2
sin2 2(9 = 0.003.
(66)
Note that the value of Am in (66) is much larger than the values indicated by the solar and atmospheric neutrino data. The LSND indications for neutrino masses are not as strong as the solar and atmospheric neutrino ones. First, the statistical significance of the LSND signal is rather low (traditionally, only a 5a effect is called a discovery). Second, the LSND signal still needs an independent confirmation.
38
The Karmen experiment [34], which has a setting similar to LSND, but with a shorter baseline, L (=s 17 meter, did not find an appearance signal. Soon, the MiniBooNE experiment [35] will be able to clarify the situation.
7. Theoretical implications As we have seen, there are solid experimental indications for neutrino masses. The most important implication of these results is also the most simple one: the SM is not the complete picture of Nature. Namely, there are experimental indications that the SM is only an effective low energy description of Nature. There are basically two ways to extend the SM. One way is to assume that there is new physics only at a very high scale, much above the weak scale. Then, neutrino masses can be accounted for by the seesaw mechanism. Alternatively, one can think about weak scale new physics. Neutrino masses that are generated by such new physics are, in general, too large and some mechanism is required in order to explain the smallness of the neutrino masses. In the following we concentrate on the former option.
7.1. Two neutrino
mixing
We start by considering only one type of results at a time. Namely, we draw conclusions from the solar and KamLAND data or from the atmospheric and K2K data. In that case there is one theoretical challenge imposed by the data: how to generate neutrino masses at the right scale? The atmospheric neutrinos and K2K results indicate z/M —> vT oscillation with Am^ N ~ few x 10" 3 eV 2 .
(67)
The solar neutrinos and KamLAND results indicate ve —> i/^tT oscillation with Am!N~l(r4eV2.
(68)
We use (9) in order to obtain the scale of the new physics that is responsible for neutrino masses
mu = -§M
=*
M=-^-. vav
69
39
In order to find the high energy scale, M, we need to know m„ and moThe most likely situation is that the neutrinos are not degenerate Am.2, 13 2 m - • 1.
(70)
Then, we use mv ~ v Am 2 . In general, the Dirac masses are of the order of the weak scale times dimensionless couplings. The problem is that we do not know the value of these couplings. In the following we make two plausible choices for their values. Motivated by S'O(IO) GUT theories, we assume that these couplings are of order of the up type quark Yukawa couplings. In particular, for the heaviest neutrino, the coupling is of the order of the top Yukawa coupling. Another plausible choice is to use the charged lepton Yukawa couplings as a guide to the neutrino ones. Considering atmospheric neutrino and K2K data we use mv ~ few x !CT 2 eV and get mD~mt mD~mT If instead we use mv ~ 10 LAND data, we obtain mD~mt mD ~ mT
M ~ 1016 GeV, M ~ 10 11 GeV.
=> => 2
(71)
eV as indicated by solar neutrinos and Kam-
=> =^
M ~ few x 1016 GeV, M ~ 1012 GeV.
, ^ '
We emphasize the following points: • Both eqs. (71) and (72) imply that there exists a new scale between the weak scale and Mpi ~ 10 19 GeV. • The values obtained assuming mo ~ mt in both eqs. (71) and (72) are very close to the GUT scale, M G U T ~ 3 X 10 16 GeV. Therefore, we can say that neutrino masses are indications in favor of unification. • In several new physics models, a new scale, usually called the intermediate scale, Mi n t ~ \fm~wMp\ ~ 10 11 GeV, is introduced. For example, supersymmetry breaking has to occur at this scale if it is mediated via Planck scale physics to the observable sector. The values obtained assuming mn ~ mT in both eqs. (71) and (72) are very close to Mint- This could be an indication that neutrino masses are also generated by such models.
40
7.2. Three neutrino
mixing
When we combine the solar, KamLAND, atmospheric and K2K data, we have to consider the full three flavor mixing. There are three mass differences that control the oscillations. They are subject to one constraint Am 2 2 + Aml3 + A m ^ = 0.
(73)
The data indicates that the mass-squared differences are hierarchical, |Am 2 2 | "C |Am 2 3 |. This implies that |Am| 3 | RS (ATO^I, and therefore, that there are only two different oscillation periods. There are three mixing angles. Solar neutrino oscillations depend mainly on 0i2 and atmospheric neutrino oscillations depend mainly on 0 23 . The third angle, #13, is known to be small, mainly from terrestrial experiments (see, for example, [4] for details). Thus, the oscillation phenomena are described by two mass-squared differences and three mixing angles Am? 2 - 1(T 4 eV 2 , 012 ~ 1,
Am^ 3 - few x 1(T 3 eV 2 ,
#23 ~ 1,
(74)
013 < 0.2.
As before, we assume that the neutrinos are not degenerate. Then, we learn from (74) that the neutrino masses are somewhat hierarchical with two large and one small mixing angles. The theoretical challenge imposed by considering three neutrino flavor mixing is to explain the flavor structure of (74). There are basically two approaches to do so. One is called neutrino anarchy [36]. It assumes that there are no parametrically small numbers. The two apparently small numbers in the neutrino sector m2 m3
rs^
1 5
#13~^r,
(75)
are assumed to be accidentally small. That is, numbers of order five are considered natural. Therefore, this mechanism predicts that 0i3 is close to its current upper bound. Consequently, it also predicts that CP violating observables, which are proportional to the product of all the mixing angles and the neutrino mass-squared differences, should also be close to their current upper bounds. The other option to explain (74) is to assume that there is an underlying broken flavor symmetry that controls the neutrino sector parameters. Such a symmetry is often assumed in order to explain the quark sector flavor parameters. Within this framework it is assumed that there is one small
41
parameter, e, such that all the small parameters of the theory are functions of it. In particular, — ~ en « 1, 913 ~ em « 1, (76) m3 where n and m are some positive integers. In that case we often refer to observables like 1112/1713 and #13 as parametrically small numbers. While flavor symmetries can explain the observed hierarchies in the quark sector, it is not straightforward to extend the treatment to the neutrinos. The main reason is that in general large mixing angles come without mass hierarchies and vice versa. Explicitly, we consider the two generation case. The neutrino mass matrix is
- - ( ; : ) .
P7)
and we distinguish the following three cases • a^>b,c ==>• sin 2 0 <^i 1, mi/m2 < 1. • a, 6 , c ~ l , det(m) ~ 1 = > s i n 2 # ~ l , mi/m2 ~ 1. • a, b, c ~ 1, det(m)
s i n 2 0 ~ l , mi/m2 < 1. We learn that in order to have large mixing with mass hierarchy the determinant of the mass matrix must be much smaller than the typical value of its entries (to the appropriate power). This is not a common situation in flavor models. In particular, this is not the case in the quark sector. 7.3. Four neutrino
mixing
While the LSND data cannot be considered as a solid indication for neutrino masses, one may like to try to accommodate it as well. In that case the situation become much more complicated. The reason is that three different mass-squared differences are needed A m | N ~ 10~ 4 eV 2 ,
Am2AN ~ few x 1CT3 eV 2 ,
Am£ S N D ~ 1 eV 2 . (78)
With three neutrinos, however, the. constraint (73) implies that there are at most two different scales. Therefore, at least four neutrinos are needed in order to accommodate all of these results. Experimentally we know that there are only three active neutrinos, and thus we need at least one additional light sterile neutrino field. There are two theoretical challenges: First, a sterile neutrino, by definition, is a fermion that is a singlet under the SM gauge group. As such,
42
it can acquire very large mass since there is no gauge symmetry that forbids it. Thus, a sterile neutrino naturally has very large mass. There are, however, several ideas that produce naturally light sterile states [37]. Even if there are light sterile states, complicated patterns of masses and mixing angles are required in order to accommodate all the data. In fact, even adding a sterile state does not help in obtaining a good fit to the data [38]. In the following we do not address the four neutrino mixing any further. 8. Models for neutrino masses As we saw there are three different kinds of theoretical challenges imposed by the neutrino data. Considering separately the solar and the atmospheric neutrino data, the challenge is to generate neutrino masses at the right scale. Combining them, we would like to find ways to generate the flavor structure, in particular, to explain mass hierarchy with large mixing. In order to accommodate also the LSND result, we should find a mechanism that generates light sterile neutrinos, and construct even more complicated flavor structure. The simplest way to generate neutrino masses is the seesaw mechanism. It is particularly attractive when it appears in models that are well motivated because they address various theoretical puzzles. Below we describe two models where neutrino masses are generated as one aspect of a model. (See [2,4] for more examples.) 8.1. Grand Unified Theories
(GUTs)
There are several supporting evidences for supersymmetric grand unification. First, in the minimal supersymmetric SM (MSSM) the three gauge couplings unify at one point at a scale of AGUT ~ 3 x 10 16 GeV.
(79)
Second, one generation of SM fermions fits into two 5*17(5) multiplets and into one multiplet for higher rank GUT groups. Neutrino physics provide further support for GUT, particularly, for GUTs that are based on 50(10) (or higher rank) gauge group. Next we describe two of the facts that make 50(10) GUTs attractive from the point of view of neutrino physics. The 15 degrees of freedom of one SM fermion generation fall into a 16 of 50(10). The one extra degree of freedom is a SM singlet. Thus, it can serve as a right handed neutrino field. Once 50(10) is broken, this singlet
43
becomes massive. Consequently, the SM active neutrinos acquire small masses via the seesaw mechanism. Namely, neutrino masses are predictions of 50(10) GUT theories. 50(10) also predicts the rough scale of the neutrino masses. To see it recall that 50(10) relates the up type quark masses to the Dirac masses of the neutrinos. In particular, the Dirac mass of the heaviest neutrino is of the order of the mass of the top quark. The Majorana mass of the singlet is expected to be of the order of the GUT breaking scale, Mjy = A AGUT where A is an unknown dimensionless number. Then, the seesaw generated mass of the heaviest active neutrino is of the order of rrii
10" 3 eV
,
,
For A ~ O(10 _ 1 ) it is the mass scale that was found by solar neutrinos. Somewhat smaller A is needed in order to get the scale that is relevant for atmospheric neutrino oscillation. Generally, it is fair to say that 5O(10) based theories predict neutrino masses in accordance with the experimental findings. 8.2. Flavor
physics
The most puzzling features of the SM fermion parameters are their smallness and the hierarchies between them. These features suggest that there is a more fundamental theory where the hierarchies are generated in a natural way. Generally, a broken horizontal symmetry is invoked. (By horizontal symmetry we refer to a symmetry that distinguishes between the three SM generations.) We demonstrate this broken flavor symmetry idea by working in the framework of a supersymmetric U(1)H horizontal symmetry [39]. We assume that the low energy spectrum consists of the fields of the MSSM. Each of the supermultiplets carries a charge under U{1)H- The horizontal symmetry is explicitly broken by a small parameter A to which we attribute a charge —1. Then, the following selection rules apply: Terms in the superpotential that carry a charge n > 0 under H are suppressed by 0(A n ), while those with n < 0 are forbidden due to the holomorphy of the superpotential. Explicitly, the lepton parameters arise from the Yukawa terms Z YijLiEjHd + ——LiLjHuHu. (81) Here Li (Ei) are the lepton doublet (singlet) superfields, Hu {Hj) is the up type (down type) Higgs superfields, Yy is a generic complex dimensionless
44
3 x 3 matrix that gives masses to the charged leptons, Zij is a symmetric complex dimensionless 3 x 3 matrix that gives Majorana masses to the neutrinos and M is a high energy scale. The selection rule implies H(Li) + H{Ej) + H(Hd) =n
=»
Ytj = 0(Xn)
H(Li) + H{Lj) + 2H(HU) =m
=>
Zi5 = 0(Xm)
(82)
where we assume that the horizontal charges of all the superfields are non-negative integers. We learn that the dimensionless couplings that are naively of order unity are suppressed by powers of a small parameter. Next we demonstrate a realization of the neutrino anarchy scenario using the above framework. Consider the following set of U(l)u charge assignments H{HU) = 0,
H{Hd) = 0,
(83)
H(Li) = 0,
H(L2) = 0,
H{L3) = 0,
H{E{) = 8,
H(E2) = 5,
H(E3) = 3.
Then, the lepton mass matrices have the following forms / A 8 A5 A 3 \ M ~ (Hd) A8 A5 A3 , \ A 8 A5 A 3 / l
/ l 1 1\ 1 1 1 . \1 1 1 / 2
M" ~ ^ M
(84)
We emphasize that the sign "~" implies that we only give the order of magnitude of the various entries; there is an unknown (complex) coefficient of 0(1) in each entry that we do not write explicitly. Equation (84) predicts large mixing angles sin #23 ~ 1,
sin #12 ~ 1,
sin #13 ~ 1,
(85)
non-hierarchical neutrino masses (Hu) (Hu)2 v 2 M M and hierarchical charged lepton masses v 1
me~(Hd)\8,
m^(Hd)X5,
v 3
(Hu) M
mr~(Hd)\3.
y
'
(87)
We see that this model predicts neutrino masses and mixing angles that are in accordance with the neutrino anarchy scenario. When we assume that both (Hd) and (Hu) are at the weak scale and that A ~ 0.2, this model also reproduces the order of magnitude of the observed charged lepton masses. Other, more complicated, flavor structure can also be generated using a similar framework. See [2,4] for more details.
45
9. Conclusions Neutrino physics is important since it is a tool for probing unknown physics at very short distances. Since we know that the SM has to be extended, it is of great interest to search for such new physics. In general, new physics at high scale predicts massive neutrinos. Therefore, there is a strong theoretical motivation to look for neutrino masses. In recent years various neutrino oscillation experiments found strong evidences for neutrino masses and mixing • Atmospheric neutrinos show deviation from the expected ratio between the fluxes of muon neutrinos and electron neutrinos. Moreover, the muon neutrino flux has strong zenith angle dependence. The simplest interpretation of these results is that there are v^ — vr oscillations. • The electron neutrino flux from the Sun is smaller than the theoretical expectation. Furthermore, the suppression is energy dependent. The total neutrino flux, as measured using neutral current reaction by the SNO experiment, agrees with the theoretical expectation. The simplest interpretation of these results is that there are ve — vx oscillations, where vx can be any combination of v^ and vT. • The K2K long baseline experiments found indications for muon neutrino disappearance. Moreover, the same neutrino parameters that account for the atmospheric neutrino disappearance also explain the K2K data. • The KamLAND experiment found evidences for F e disappearance. The data provide an independent test of the same parameter space that is probed by solar neutrinos. The theoretical expectation and the experimental data fit comfortably. Yet, there are many unsolved problems associated with neutrino physics. In the future we expect to have more data and thus we will be able to learn more about neutrinos, and eventually, about the short distance new physics. Acknowledgments I thank Yossi Nir, Subhendu Rakshit, Sourov Roy, Ze'ev Surujon and Jure Zupan for comments on the manuscript. Y.G. is supported by the United States-Israel Binational Science Foundation through Grant No. 2000133, by a Grant from the G.I.F., the German-Israeli Foundation for Scientific
46
Research and Development, and by the Israel Science Foundation under Grant No. 2 3 7 / 0 1 . References 1. A partial list of books includes: B. Kayser, "The physics of massive neutrinos", 1989, World Scientific; J. N. Bahcall, "Neutrino astrophysics", 1989, Cambridge Univ. Press; R. N. Mohapatra and P. B. Pal, "Massive neutrinos in physics and astrophysics", 1991, World Scientific; C. W. Kim and A. Pevsner, "Neutrinos in physics and astrophysics", 1993 Harwood Academic; G. G. Raffelt, "Stars as laboratories for fundamental physics", 1996, Chicago Univ. Press. 2. For some recent reviews see: S. Goswami, hep-ph/0303075; S. Pakvasa and J. W. Valle, hep-ph/0301061; R. N. Mohapatra, hep-ph/0211252; E. K. Akhmedov, hep-ph/0001264. 3. A. D. Dolgov, hep-ph/0208222; Phys. Rept. 370, 333 (2002) [hepph/0202122]. 4. M. C. Gonzalez-Garcia and Y. Nir, Rev. Mod. Phys. 75, 345 (2003) [hepph/0202058]. 5. The neutrino oscillation industry web page: "neutrinooscillation.org"; Juha Peltoniemi's ultimate neutrino page: "cupp.oulu.fi/neutrino". 6. Maury Goodman runs the long-baseline neutrino oscillation newsletters. These well written short monthly letters, are very nice summaries of neutrino news. It is highly recommended to sign up for the free monthly email service for anyone who likes to keep up with the developments in neutrino physics, "www.hep.anl.gov/ndk/longbnews/index.html" 7. See, e.g., M. E. Peskin and D. V. Schroeder, "Introduction to quantum field theory", 1995, Addison-Wesley. 8. K. Hagiwara et al. [Particle Data Group Collaboration], Phys. Rev. D 66, 010001 (2002). 9. H. V. Klapdor-Kleingrothaus et al, Eur. Phys. J. A 12 147 (2001). 10. Y. Grossman and H. J. Lipkin, Phys. Rev. D 55, 2760 (1997) [hepph/9607201]. 11. L. Wolfenstein, Phys. Rev. D 17, 2369 (1978). 12. See, e.g., H. Nunokawa, V. B. Semikoz, A. Y. Smirnov and J. W. Valle, Nucl. Phys. B 501, 17 (1997) [hep-ph/9701420]; S. Bergmann, Y. Grossman and E. Nardi, Phys. Rev. D 60, 093003 (1999) [hep-ph/9903517]. 13. S. P. Mikheev and A. Y. Smirnov, Sov. J. Nucl. Phys. 42, 913 (1985) [Yad. Fiz. 42, 1441 (1985)]. 14. S. J. Parke, Phys. Rev. Lett. 57, 1275 (1986). 15. J. Bahcall web site, "www.sns.ias.edu/~jnb"
47
16. See, for example, E. K. Akhmedov, Pramana 54, 47 (2000) [hep-ph/9907435]; S. T. Petcov, Phys. Lett. B 434, 321 (1998) [hep-ph/9805262]. 17. R. Jr. Davis, D. S. Harmer and K. C. Hoffman, Phys. Rev. Lett. 20, 1205 (1968). 18. B. T. Cleveland et al., Astrophys. J. 496, 505 (1998); K. Lande et al., Nucl. Phys. Proc. Suppl. 77, 13 (1999). 19. GALLEX home page: "kosmopc.mpi-hd.mpg.de/gallex/gallex.htm" 20. GNO home page: "www.lngs.infn.it/site/exppro/gno/Gno_home.htm" 21. SuperKamiokande home page: "www-sk.icrr.u-tokyo.ac.jp/doc/sk/index.html" 22. SNO home page: "www.sno.phy.queensu.ca" 23. Q. R. Ahmad et al. [SNO Collaboration], Phys. Rev. Lett. 89 011302 (2002) [nucl-ex/0204009]. 24. See, for example, S. Pakvasa, Pramana 54, 65 (2000) [hep-ph/9910246]. 25. P. C. de Holanda and A. Y. Smirnov, hep-ph/0212270; J. N. Bahcall, M. C. Gonzalez-Garcia and C. Pena-Garay, hep-ph/0212147; A. Bandyopadhyay, S. Choubey, R. Gandhi, S. Goswami and D. P. Roy, hepph/0212146; M. Maltoni, T. Schwetz and J. W. Valle, Phys. Rev. D 67, 093003 (2003) [arXiv:hep-ph/0212129]; V. Barger and D. Marfatia, Phys. Lett. B 555, 144 (2003) [hep-ph/0212126]. 26. A. Bandyopadhyay, S. Choubey, S. Goswami and K. Kar, Phys. Lett. B 519, 83 (2001) [hep-ph/0106264]. 27. T. Toshito [Super-Kamiokande Collaboration], hep-ex/0105023. 28. S. Fukuda et al. [Super-Kamiokande Collaboration], Phys. Rev. Lett. 85, 3999 (2000) [hep-ex/0009001]. 29. K2K home page: "neutrino.kek.jp" 30. KamLAND home page: "www.awa.tohoku.ac.jp/html/KamLAND/index.html" 31. LSND home page: "www.neutrino.lanl.gov/LSND" 32. M. H. Ahn et al. [K2K Collaboration], Phys. Rev. Lett. 90, 041801 (2003) [hep-ex/0212007]. 33. K. Eguchi et al. [KamLAND Collaboration], Phys. Rev. Lett. 90, 021802 (2003) [hep-ex/0212021]. 34. Karmen home page: "www-ik.fzk.de/~karmen/karmen_e.html" 35. MiniBooNE home page: "www-boone.fnal.gov/publicpages/index.html" 36. A. de Gouvea and H. Murayama, hep-ph/0301050; N. Haba and H. Murayama, Phys. Rev. D 63, 053010 (2001) [hepph/0009174]; L. J. Hall, H. Murayama and N. Weiner, Phys. Rev. Lett. 84, 2572 (2000) [hep-ph/9911341]. 37. For a review see, for example, R. N. Mohapatra, hep-ph/9910365. 38. For a recent paper see, for example, C. Giunti, hep-ph/0302173.
39. M. Leurer, Y. Nir and N. Seiberg, Nucl. Phys. B 398, 319 (1993) [hepph/9212278]; M. Leurer, Y. Nir and N. Seiberg, Nucl. Phys. B 420, 468 (1994) [hepph/9310320]; Y. Grossman and Y. Nir, Nucl. Phys. B 448, 30 (1995) [hep-ph/9502418].
It
KONSTANTIN MATCHEV
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PRECISION ELECTROWEAK PHYSICS
KONSTANTIN MATCHEV Department of Physics, University of Florida, Gainesville, FL 32611, USA and Institute for High Energy Phenomenology, Newman Laboratory for Elementary Particle Physics, Cornell University, Ithaca, NY 14853, USA E-mail: [email protected]
These notes are a written version of a set of lectures given at TASI-02 on the topic of precision electroweak physics.
Contents
1
Preliminary Remarks 1.1 What is it all about? 1.2 Useful references for further reading
53 53 54
2
The 2.1 2.2 2.3
55 55 57 58
3
Precision Measurements at the Z Pole 3.1 Z resonance parameters 3.2 Branching ratios and partial widths 3.3 Unpolarized forward-backward asymmetry 3.4 Left-right asymmetry 3.5 Left-right forward-backward asymmetry 3.6 Tau polarization
61 63 63 65 66 66 66
4
Precision Measurements at LEP-II
67
Tools of the Trade Theory Fundamental parameters, input parameters and observables . . . . Experimental facilities
51
52
5
Precision Measurements at the Tevatron 5.1 Top mass measurement 5.2 W mass measurement
71 71 73
6
Theoretical Interpretation of the Precision Electroweak D a t a 6.1 Testing the Standard Model 6.2 Fixed parameters 6.3 Floating parameters 6.4 Comparing the values for the electroweak observables 6.5 The Higgs mass prediction 6.6 Impact of the My/ measurement on m^ 6.7 Impact of the asymmetry measurements on m^ 6.8 A Higgs puzzle?
74 76 77 78 79 80 80 86 89
7
Testing for N e w Physics 7.1 S, T, U parameters 7.2 Constraining new physics scenarios
90 92 92
8
Concluding Remarks
94
53
1. Preliminary Remarks Precision experiments have been crucial in both validating the Standard Model (SM) of particle physics and in providing directions in searching for new physics. The precision program, which started with the discovery of the weak neutral currents in 1973, has been extremely successful': it confirmed the gauge principle in the Standard Model, established the gauge groups and representations and tested the one-loop structure of the SM, validating the basic principles of renormalization, which in turn allowed for a prediction of the top quark and Higgs boson masses. In relation to new physics models, precision measurements have severely constrained new physics at the TeV scale and provided a hint of a possible (supersymmetric) gauge coupling unification at high energies. 1.1. What is it all about? On the one hand, the term Precision Electroweak Physics (PEW) refers to quantities which are very well measured. How well? Let us say at the % level or better. However, not every well-measured quantity is of interest here. For example, the amount of cold dark matter in the Universe £ICDM, Newton's constant GN and the strong coupling constant as are all very well known, yet they do not belong to the electroweak sector of the Standard Model: SU(S) x SU(2)W QCD
x U(1)Y
Electroweak
The observables which are typically included in the precision electroweak data set are conveniently summarized in Tables 1 and 2. Our goal in these lectures will be the following: (1) we shall learn the meaning of some of the quantities in Tables 1 and 2; (2) we shall find out how these quantities are measured by experiment; (3) we shall discuss the prospects for improving these measurements in the near future; (4) we shall find out their implications about the validity of the Standard Model; (5) we shall learn how they can be used to predict or constrain as yet undiscovered physics - for example, the mass of the Higgs boson, or the existence/lack of new physics beyond the Standard Model near the TeV scale.
54 Table 1. Principal Z-pole observables, their experimental values, theoretical predictions using the SM parameters from the global best fit as of 1/03, and pull (difference from the prediction divided by the theoretical uncertainty). T(had), T(inv), and r{(.+(.~) are not independent, but are included for completeness. From Ref. 1. Quantity Mz [GeV] Tz [GeV] T(had) [GeV] T(inv) [MeV] F(e+£-) [MeV] Thad [nb] Re Rfj. RT
AFB(e) AFB(P) AFB(T)
Rb Re Rs,d/R(d+u+s) AFB(b) AFB(C)
AFB(s) Ab Ac As ALR (hadrons) ALR (leptons) A» Ar Ae{QLR) AT(VT) Ae{Vr) QFB
Group(s)
Value
Standard Model
pull
LEP LEP LEP LEP LEP LEP LEP LEP LEP
91.1876 ±0.0021 2.4952 ± 0.0023 1.7444 ± 0.0020 499.0 ± 1 . 5 83.984 ± 0.086 41.541 ± 0 . 0 3 7 20.804 ±0.050 20.785 ± 0 . 0 3 3 20.764 ± 0.045
91.1874 ±0.0021 2.4972 ± 0.0011 1.7436 ±0.0011 501.74 ± 0 . 1 5 84.019 ± 0.027 41.470 ± 0.010 20.753 ± 0.012 20.753 ± 0.012 20.799 ± 0.012
0.1 -0.9
LEP LEP LEP LEP + S L D LEP + S L D OPAL LEP LEP DELPHI.OPAL SLD SLD SLD
0.0145 ± 0.0025 0.0169 ±0.0013 0.0188 ± 0.0017 0.21664 ± 0.00065 0.1718 ±0.0031 0.371 ± 0.023 0.0995 ± 0.0017 0.0713 ± 0.0036 0.0976 ±0.0114 0.922 ± 0.020 0.670 ± 0.026 0.895 ± 0.091
0.01639 ± 0.00026
-0.8 0.4 1.4 1.1 -0.2 0.5 -2.4 -0.8 -0.5 -0.6 0.1 -0.4
SLD SLD SLD SLD SLD LEP LEP LEP
1.2. Useful references
0.15138 0.1544 0.142 0.136 0.162 0.1439 0.1498 0.0403
for further
±0.00216 ±0.0060 ±0.015 ±0.015 ±0.043 ±0.0043 ±0.0048 ± 0.0026
0.21572 0.17231 0.35918 0.1036 0.0741 0.1037 0.93476 0.6681 0.93571
± 0.00015 ± 0.00006 ± 0.00004 ±0.0008 ± 0.0007 ±0.0008 ± 0.00012 ± 0.0005 ± 0.00010
0.1478 ±0.0012
0.0424 ± 0.0003
1.9 1.0 1.0 -0.8
1.7 1.1 -0.4 -0.8 0.3 -0.9 0.4 -0.8
reading
For the current status of the precision electroweak observables, one should consult the "Electroweak model and constraints on New Physics" review [2] in the Particle Data Book [3]. There are nice lecture summaries from previous schools [4-11], as well as book collections of relevant review articles [12-14]. The long-term prospects for improvement in the precision measurements have been discussed in several workshop reports, e.g. the Tevatron Run II Workshop [15] and Snowmass 2001 [16-18]. The website of the LEP Electroweak Working Group is another excellent source of information, with its continuously updated electroweak summary notes [20].
55 Table 2.
The recent status of non-Z-pole observables, as of 1/03. From Ref. 1.
Quantity mt Mw Mw
[GeV] [GeV] [GeV]
Group (s)
Value
Standard Model
pull
Tevatron LEP Tevatron,UA2
174.3 ± 5 . 1 80.447 ± 0.042 80.454 ± 0.059
174.4 ± 4.4 80.391 ± 0 . 0 1 8
0.0 1.3 1.1
NuTeV NuTeV CCFR CDHS CHARM CDHS CHARM CDHS 1979
9l Rv R" Rv R? RD R"
CHARM II all CHARM II all
97 97 97 9A
Qw (Cs) Qw(Tl) 1 0 3 r(fc-.. 7 )
-0.035 -0.041 -0.503 -0.507
± 0.017 ± 0.015 ±0.017 ± 0.014
-2.9 0.6 -0.3 0.1 -1.7 -0.1 1.0 -1.0
-0.0398 ± 0.0003 -0.1 -0.5065 ± 0.0001 0.0
Boulder Oxford/Seattle
-72.69 ± 0.44 -116.6 ± 3 . 7
-73.10 ± 0 . 0 4 -116.7 ± 0 . 1
0.8 0.0
BaBar/Belle/CLEO
3.48i°;«|
3.20 ± 0 . 0 9
0.5
direct/Be /B^ e + e ~ / r decays BNL/CERN
290.96 ± 0.59 ± 5.66 56.53 ± 0.83 ± 0.64 4510.64 ± 0.79 ± 0.51
291.90 ± 1 . 8 1 57.52 ± 1 . 3 1 4508.30 ± 0.33
TT [fs] 10 9 (a„ -
0.30396 ± 0.00023 0.30005 ± 0.00137 0.03005 ± 0.00004 0.03076 ± 0.00110 0.5820 ± 0.0027 ± 0.0031 0.5833 ± 0.0004 0.3096 ± 0.0033 ± 0.0028 0.3092 ± 0.0002 0.3021 ± 0.0031 ± 0.0026 0.3862 ± 0.0002 0.384 ± 0 . 0 1 6 ± 0 . 0 0 7 0.403 ± 0.014 ± 0.007 0.3816 ± 0.0002 0.365 ± 0.015 ± 0.007
£)
-0.4 -0.9 2.5
2. The Tools of the Trade As usual, we need to have a theory and experiments to test it. Before we go on to discuss the observables from Tables 1 and 2 in the next few sections, we first need to introduce the necessary theoretical ingredients and describe the relevant type of experiments. In Section 2.1 we review the electroweak Lagrangian and in Section 2.2 we discuss its input parameters. Sections 2.3 provides a general review of some of the types of experiments which have contributed to the results in Tables 1 and 2.
2.1.
Theory
Achieving the remarkable precision displayed in Tables 1 and 2 is only possible because the Standard Model provides a well-defined theoretical framework for computing the different electroweak observables in terms of a few input parameters and thus predicting different relations among them.
56
The SM electroweak Lagrangian for fermions is given by
CF = J2i;i[iP-mi-^T7-)^i 2MW e^Q^a^iAp
(!)
(QED)
(2)
i
-^^^7M(i_75)(r+w+
2cos#i^
• E ^ " ( 0 V - 0 A 7
5
+
T-W-)^
) ^
(CC)
(^C)
(3) (4)
i
where the vector and axial vector couplings are g^=Tl-2Qism20w,
(5)
9A = It, (6) Q is the electric charge of fermion i, TJ is its weak isospin, 9w is the Weinberg angle ( t a n # ^ = g'/g), g (g') is the SU(2)w (U(l)y) gauge coupling constant, and e = gsiu.9w is the electromagnetic coupling constant. The photon (A/j) and Z-boson (Z^) fields are given in terms of the hypercharge gauge boson B^ and neutral W-boson W? as A^ = B^ cos 9W + Wl sin 6W, (7) l
Z„ = -Bp sin 9W + W* cos 6W.
(8)
Often the neutral current interactions are equivalently written as - ^ ^ E ^ ( f f i ( l - 7
5
) + s k ( l + 7
5
) ) ^
(JVC)
(9)
i
with the redefinition 9v = 9L+
9R
9A=gL~gR
(10) (11)
Exercise: Use (5), (6), (7) and (8) to derive (1-4) from the SU(2)w x U(l)y gauge theory lagrangian supplemented with the Yukawa terms for the fermions. Hint: you will need to know the Biggs vacuum expectation value v, which is computed by minimizing the tree-level Higgs potential VH = -
1
- ^
2
+ ^
(12)
57
2.2. Fundamental observables
parameters,
input parameters
and
Fundamental parameters are the parameters appearing in the original gauge theory Lagrangian. In the case of the electroweak Lagrangian, the fundamental parameters are the gauge couplings g' and g. The Higgs sector of the theory (12) adds two more fundamental parameters: the Higgs mass parameter /i and self-coupling XH • The remaining fundamental parameters are the Yukawa couplings in the Yukawa sector (1). Putting all together, the set of fundamental parameters is {
(14)
Often, as was the case above in Eq. (1-4), it is convenient to rewrite the Lagrangian in terms of derived quantities, i.e. input parameters. Through a simple redefinition we can trade one fundamental parameter for a combination of a new (input) parameter and the remaining fundamental parameters, e.g. g' = gtan0w
(15)
eliminates g' from the Lagrangian and replaces it with 9w- Another example is 2MW v= , (16) 9
which eliminates the combination v = [ij \J\JI in favor of the V7-boson mass Mw- The latter is directly accessible experimentally, unlike either A# or fi. A similar trick allows to go from Yukawa couplings to fermion masses: gnu K
-
JiMw
(
}
which is very convenient, as most fermion masses are well-known experimentally while in contrast the Yukawa couplings are small (with the exception of the top Yukawa) and thus difficult to measure. Of course, one should make sure the total number of parameters stays the same for both the fundamental and input parameters. For the precision electroweak tests of the SM the following set of wellmeasured input parameters is typically used: {a,GF,Mz,mh,mi}, 2
(18)
where a = e /47r is the fine structure constant, GF is the Fermi constant, which is measured from the muon lifetime, and Mz (m,h) is the Z-boson (Higgs boson) mass.
58
Exercise Find the relation between the input parameter set (18) and the fundamental parameter set (14). Finally, observables are the quantities which are measured by experiment. An example is the set of electroweak observables listed in Tables 1 and 2. If the number of observables is larger than the number of input parameters, we can test the model. 2.3. Experimental
facilities
As can be seen from Tables 1 and 2, the electroweak observables have been measured at a variety of experimental facilities. We shall now briefly describe each type. Lepton colliders Lepton (e + e~) colliders have played a very important role in the precision program. Lepton colliders have numerous advantages, perhaps best summarized in the following aphorism: "At a lepton collider, every event is signal. At a hadron collider, every event is background." The main attractive features are: • Fixed center of mass energy in each event. This provides an additional kinematic constraint, useful for eliminating the backgrounds as well as extracting properties of new particles, allowing e.g. a missing mass measurement. • All of the available beam energy is used (minus beamstrahlung), i.e. we have an efficient use of particle acceleration. • Clean environment: easier identification of the underlying physics in the event, easier heavy flavor tagging, less backgrounds. • Polarizability of the colliding beams (SLC). The polarization of the colliding beams can be used to reweight the contribution of the different diagrams involving W's, Z's and 7's. Naturally, lepton colliders also have certain disadvantages in comparison to hadron colliders (see below), which makes both types of facilities equally valuable and to a large extent complementary. Figure 1 summarizes some of the history of e+e~ colliders. In reverse chronological order, they are: • The Next Linear Collider (NLC) with an initial CM energy of 500 GeV is now under active discussion as the next large international
59
10
-a
10
§ o
a u
10
10
10
Figure 1.
0
20 40 60 80 100 120 Center of Mass Energy [GeV]
Total e + e - cross section from CESR/DORIS to LBP energies.
particle physics facility. Its timescale is still uncertain, but the NLC was ranked as the top priority among mid-term science facilities in the Department of Energy's Office of Science 20-year science facility plan [21]. LEP-II at CERN (1996-2000). It had the highest CM energy so far among lepton colliders. LEP-II took data at several yfs before shutting down in November 2000 amidst much speculation and heated discussions. There were four experiments (detectors) at diametrically opposite sites: ALEPH, DELPHI, L3 and OPAL. LEP-I at CERN (1989-1995), which took data at several energies around s/s = MzSLC at SLAC (1989-1998). It reached energies up to 100 GeV and also took data around the Z-pole. Unlike LEP, it had an important advantage: the availability of beam polarization (80%). B-factories: PEP-II at SLAC (1999-present), 9 x 3.1 GeV; KERB at
60
KEK (Japan) (1999-present) 8 x 3.5 GeV; CESR at Cornell (1979present). Exercise: Why are the B-factories asymmetric? • Others (y/s < 10 GeV) (Novosibirsk, Bejing, Frascati).
Hadron colliders In turn, hadron colliders have their own advantages: • Protons are much heavier than electrons, which leads to a reduction in the radiation losses. As a result, for a given CM energy, the ringsize of a hadron collider is smaller. Equivalently, for a given ringsize (and fixed magnetic field), a hadron collider allows to reach higher CM energies. • Even though the CM energy of the colliding beams is large, the typical parton CM energy is much smaller, but usually still beyond the CM energy of LC competitors. The main hadron collider facilities are the following: • LHC (CERN) is currently under construction and is projected to begin operation in 2007-8 as a 7 x 7 TeV pp collider. There will be four experiments - ATLAS, CMS, LHCB and ALICE. The initial data taking rate will be 10 fb~ per year at low luminosity and will subsequently increase to 100 fb~ per year. • The Tevatron Run II at Fermilab is now operating as 0.98 x 0.98 pp TeV collider. There are two experiments: CDF and DO, and the hope nowadays is for 8 fb~ per experiment before the LHC. • The Tevatron Run I (1987-1996): discovered the top quark, operated in a 0.9 x 0.9 pp mode and delivered 110 pb~ of data per experiment. • SppS at CERN (1981-1990). 300 x 300 GeV pp collider, discovered W and Z. It is interesting that the last three SM particles (W, Z and t) were discovered at hadron colliders and chances are that the last one (the Higgs boson) will follow suit. Other facilities In addition to the collider experiments mentioned above, there are numer-
61
cms fixed target experiments, whose main advantage is the lower cost and large luminosity (because of the dense target). Their main disadvantage is the low center of mass energy, which scales with the beam energy Ef, only as ECM ~ \fE~b- Another class of experiments on muon dipole moments are done at muon storage rings at CERN, and more recently, at BNL. In conclusion of this section, we should comment on the experimental identification of heavy particles, which decay promptly and are detected only through their decay products. For example, a W-boson decays either to a pair of quarks, which later materialize into QCD jets, or to a lepton and the corresponding neutrino. The W branching fractions are B(W —> qq') ~ 2/3, and B(W ->• ivt) ~ 1/9 for £=e,n, r. The jets (or the lepton and its neutrino) may have had a different source, and the only indication that they may have come from a W decay is that their invariant mass is close to MwSimilarly, a Z-boson can decay to pairs of quarks, leptons or neutrinos, with branching fractions B(Z —> qq) ~ 0.7, B(Z —> vv) ~ 0.2 and B{Z —> £+£~) ~ 0.1 (summed over the three generations). When both decay products are visible, their invariant mass is again required to be near Mz. A word of caution: r is not always a lepton! A r-lepton can decay leptonically to e or \i with a branching ratio 0.18 for each, or hadronically, with a branching ratio 0.64. In the latter case it appears as a "tau jet", which is similara to an ordinary QCD jet. To an experimentalist, a "lepton" is either an e or a /i, which may or may not have come from a tau decay. By T'S, experimentalists often mean "tau jets".
3. Precision Measurements at the Z Pole At the Z pole the cross-section for e+e~ —> ff diagram. For / / e w e have
dafz _ 9 dn
sr e e r / 7 /M|
±{s-Ml)2
+ s2T2z/M2z
is dominated by the Z
(l + c o s ^ X l - i V y L
2cos9Af{-Pe
B
+ Ae)
Although not quite - a tau jet is more narrow and has lower track multiplicity.
(19)
62
where P e is the polarization of the electron beam (relevant at SLC), s is the square of the CM energy, Y z is the total Z width b , r e e and Y ff are the partial widths for Z —> ee and Z —> / / , correspondingly. (The partial widths are related to the Z branching fractions as B(Z —> / / ) = Tjj/Tz-) In (19) 9 is the angle between the incident electron and the outgoing fermion and Af is the left-right coupling constant asymmetry: 2gjgA
{g[f - {gfRf
(9fv)2 + (gfA)2
(gfL)2 + (9fR)2
A f
where in the second equation we have used (5) and (6). Notice that Af < 1. Since gv and gA only depend on the quantum numbers of the particles, it follows that Af is the same for all charged leptons, all up-type quarks and all down-type quarks. For example, for charged leptons gev = - i - 2 ( - l ) sin2 6W ~ -0.50 + 0.462 = -0.038 9A = ~ \ =
-0.50
(21) (22)
We see that because of an accidental cancellation g\?
_
2(-0.038)(-0.5) _, Q _ 15 (-0.038) 2 + ( - 0 . 5 ) 2
(23)
(compare to the values measured in Table 1). This small value of An makes it particularly sensitive to electroweak vacuum polarization corrections (which have an impact on sin2 6w)- In terms of sin2 9w> we have _ Ai
2(1 - 4 sin2 9w)
' l + (l-4sin2M2-
(
}
Therefore small changes in sin2 6w are amplified by a factor of 8 in the leptonic asymmetry Ag. Exercise: Confirm roughly the numerical values for A^ and Ac in Table 1. b
T h e result (19) already incorporates the s-dependent width
rz(s) = ^rz(s
= M2z),
which in turn accounts for the effect of the so called "non-photonic", or oblique, one-loop corrections, i.e. the corrections to the Z propagator. For details, see 22.
63
Let us now discuss the various measurements on the Z-pole. We shall need the following integrals for the forward and backward regions: I-K/2
F : B :
/ Jo J
,TT/2
4
(l + cos2(9)sm6W0 = - ; 3 (1 + cos 2 (9) sin 6W6> = - ;
JTT/2
3.1. Z resonance
3
/ J0
2cos0sin0d0 = 1; (25)
f
2 c o s 0 s i n 0 d 0 = - 1 . (26)
JTT/2
parameters
Scanning the Z peak and fitting to a'z yields measurements of Mz, Tz and the peak hadronic (/ = q) total cross-section (obtained for s = M\ by integrating (19) over the full solid angle):
'" = m! Sff
(27
»
Recall that the LEP beams are not polarized, so Pe = 0. Figure 2 shows the hadronic cross-section (Jhad measured by the four collaborations as a function of the center-of-mass energy (solid line). Also shown as a dashed line is the cross-section after unfolding all effects due to photon radiation. The radiative corrections are large but very well known. At the peak the QED deconvoluted cross-section is 36% larger and the peak position is shifted by ~ 100 MeV. 3.2. Branching
ratios and partial
widths
Looking at various exclusive final states one can determine the following ratios rhnd 0.7 21; (28) Ri — 0.0333 Te Rb = Rr =
1
had
(29)
)
•*- cc
(30)
J- had
with £ = {e,/x, r } . Basically this amounts to measuring the Z branching fractions. Having measured the total width from the peak scan and all visible partial widths, one can determine the invisible partial width (for Z —* vv): •L inv
=
-L Z
-L had
*• ee
-L /x/x
-L r r
l^-V
64 —
i
|
i
,
i
|
i
i
i
|
,
i
,
i
i
i
°° yy
40 ALEPH DELPHI L3 OPAL
. .
30
/ / / /
kj
20 - • measurements, error bars / increased by factor 10
10
- _ — c from fit
/
/
. . -
/
—
vti
-
/
•
\V -
/ / / /
... QED unfolded
N:
•
ass
i . . .
86
i . ,y¥z,
i . . .
88
90
, .
•
1
92 94 E [GeV] cm
Figure 2. The final hadronic cross-section averaged over the four experiments at LEP-I, as a function of the center-of-mass energy, as measured (solid line) and QED deconvoluted (dashed line).
and count the number of neutrino species Nv coupling to the Z. Assuming that Yi N T where Tu is the Z partial width to pairs of a single neutrino species, we have Nv
•L inv / *- £
{YU/Y()SM
(32)
The ratio of Tinv and the Z partial width into charged leptons is used in order to minimize the uncertainties due to the electroweak corrections which are common to both partial widths and cancel out in their ratio. The Z lineshape in the hadronic channel is shown in Fig. 3 along with the SM prediction for two, three or four neutrinos species. The result confirms that there are three light neutrino flavors with SM-like couplings to the Z.
65
ALEPH
C
o 1.05
b
w
1 0.95
88
89
-4-, 90
91
92
93
94
95
Energy (GeV) Figure 3. The Z lineshape in the hadronic channel along with the SM prediction for two, three or four neutrinos species [23]. The lower plot shows the ratio of the measured points to the best-fit values.
3.3. Unpolarized
forward-backward
asymmetry
The unpolarized forward-backward asymmetry is determined by a simple counting method in e + e _ —> / / scattering Af
—a =
-AeAi
(33)
A
FB
where aF (aB) is the total cross-section for forward (backward) scattering of / with respect to the incident e~ direction. Exercise: Use (25-26) in order to derive the second equality in (33) and then check the numerical values for ApB from Table 1 for f = £,b, c.
66
3.4. Left-right
asymmetry
The left-right asymmetry is defined as ,
_
iff/(-|Pe|)-g/(+|Pe|) Pe(Jf{-\Pe\) + af{+\Pe\) ~Ae>
ALR
~
^
where o-f(Pe) is the total (integrated over all angles) cross-section for producing / / pairs with an electron beam of polarisation Pe. Exercise: Derive the second equality. 3.5. Left-right
forward-backward
asymmetry
The left-right forward-backward asymmetry is defined as Af FB
_ 4(~\Pe\) ~ 4(~\Pe\) ~ 4(+\Pe\) + 4(+l^l) _ 3 p ^ 4(-|p e |) + 4(-|p e |) + 4(+|p e |) + 4(+|p e |) 4 e '•
It allows a direct determination of the A/ quantities which are presented in Table 1. Exercise: Derive the second equality. To summarize or discussion so far, there are three observable asymmetries, measuring either Ae, Af or their product. 3.6. Tau
polarization
The tau lepton is the only fundamental fermion whose polarization is experimentally accessible at LEP and SLC. The average polarization of r leptons is defined by 6
Ms) =
+
(s) - o * ° * ( s )
" ' , , \
(36)
where atot(s) is the r production cross-section via Z exchange, integrated over all angles, and the subscripts + and — refer to the r helicity states +1 and — 1. The dependence of PT at the Z pole on the polar angle 6 has the form [24] P(cozC) i^costfj -
Ml 1 +
+ ™s20) + 2Aecose • c o s 2 e + 2AAe cosg
W
The r polarization is measured using five exclusive decay modes which comprise about 80% of all r decays: ei/P, fii/i>, TV(K)I/, pv and a\v. The single 7r and K modes are not normally distinguished. The different channels do not all have the same sensitivity to the r polarization, TT(K)V being the
67
in
°. 1000 C 750 M
T^JTV
\
VI ' ' ' '
J
\ X2/dof= 60.4/ 39 !
/
1It
T-»pV
j
J
- -_
[t
:
500
'250
^L=r5^MO<X>8D?^KXXTCk7S*7c^
MCFit
•
Data
R551 tBkg
• • Pos. Helicity T
^ B Non-x Bkg
—
Neg. Helicity z
OPAL
0
Figure 4. Observed distributions of the polarization estimators for the different channels studied by OPAL [26], compared to the SM expectation for positive or negative r helicity.
best in that respect (see Fig. 4). The energy and/or angular distributions of the decay products can be used as r polarization analysers [25]. Figure 4 shows an example of the different distributions used and their sensitivity to the T polarization. Fitting the experimental data for P T (cos0) (an example from the ALEPH experiment [27] is shown in Figure 5) to Eq. (37) allows an extraction of AT and Ae which enter Table 1 as AT(VT) and Ae(VT), correspondingly. 4. Precision Measurements at LEP-II LEP-II was able to pair produce W's and perform measurements of Mw, Tw and the W branching fractions. There are three diagrams contributing to W-pair production: a i-channel v exchange, and an s-channel 7*
68
-0.1
-0.2
-0.3
No-Universality
i
-1
-0.8
-0.6
-0.4
-0.2
i
0
i
i
i
0.2
i
i
,
i
0.4
,
,
,
i
0.6
,
i
,
i
0.8
i
,
i
1
COS0
Figure 5. Tau polarization as a function of cos 9. The dashed (dotted) line is the result from a global fit to the data with (without) the universality assumption Ae = AT-
and Z exchange. The latter two diagrams contain triple gauge boson couplings, therefore LEP also tested the nonabelian nature of the SM gauge interactions. The W mass measurement involves reconstruction of the invariant mass of the W decay products. For WW, there are three possible final states: 4g, qqlvg and 2l2vi. The LEP measurements used only the first two channels. In the case of qqlvi, the energy and momentum of the missing neutrino are easily deduced from energy and momentum conservation. Then the four objects are paired up and the invariant mass of each pair is computed. The resulting invariant mass distribution in the qq^v^, channel is shown in Fig. 6. The mass peak is very pronounced, with virtually no background. The position of the peak allows an extraction of Mw, while its width is
69
cs 100 % 90 O 7^ 80
1 70
ALEPH Preliminary jxvqq selection •
Vs = 191.6,195.5, 199.5,201.6 GeV
Data (Luminosity = 237 pb"1)
—
MC ( m w = 80.60 GeV/c 2 )
H
Non-WW background
W 60
M w (GeV/c 2 )
Figure 6. Reconstructed W mass distribution from the ALEPH experiment for the qqiiv^ channel. The data are compared to the Monte Carlo prediction for M\y = 80.60 GeV.
indicative of Tw, where in addition one has to worry about detector resolution effects (smearing). All four experiments are consistent and so far the LEP Mw measurement is slightly better than the Mw measurement from the Tevatron (see below). The W mass measurement is of extreme importance — as we shall see later, Mw is one of the observables most sensitive to the Higgs mass TO/,. The W branching ratios were measured in WW production and the results displayed in Table 3 nicely demonstrate lepton universality. The measurement of the rise in the VF-pair cross-section near threshold provides an alternative method for determining Mw • The result for energies up to 207 GeV is shown in Figure 7. The method is statistically limited and yields inferior precision compared to direct Mw reconstruction. However,
70 Table 3. LEP measurements of the W branching fractions derived from WW production cross-section measurements [20]. Decay mode W -> eue W —> fiVit W -> r i / T
B r a n c h i n g f r a c t i o n (%)
10.59 10.55 11.20 67.77
W -^gg'
±0.17 ±0.16 ±0.22 ± 0.28
it clearly demonstrates the need for all triple gauge boson couplings in order to understand the data. LEP-II also measured the ZZ cross-section. However, the electron couplings to the Z are smaller, and in addition Mz > Mw- Thus the sample of ZZ pairs was much smaller and the measurement of the ZZ cross-section was not as good as for the WW case. Nevertheless, a comparison of the combined data to theory (Fig. 8) reveals good agreement within the large errors of the data.
.Q Q.
30 -1
11/07/2003
'
'LEP
/
',--'
PRELIMINARY
20/
/
_A—-*—-t-t—jr-"-* -
//y
10-
/// :!/ / ,/
lltlYFSWW/RacoonWW ... no ZWW vertex (Gentle) ... only ve exchange (Gentle)
r\
160
180
200
Vs (GeV) Figure 7. The measured WW cross-section at LEP as a function of the center-of-mass energy, compared to the predictions of the event generators RacoonWW and YFSWW. The magenta (upper dotted) line is the theoretical result ignoring both WW~y and WWZ couplings, while the red (lower dotted) line neglects WWZ only.
71 11/07/2003
.o
L E P PRELIMINARY ''.. ZZTOandYFSZZ
N N
1-
0.5
o
180
190
200
Vs (GeV) Figure 8. The measured e + e ~ —» ZZ cross-section at LEP as a function of the centerof-mass energy. The solid line is the SM prediction and the band indicates its ± 2 % uncertainty.
5. Precision Measurements at the Tevatron 5.1. Top mass
measurement
Since the top quark is so heavy, so far it can only be produced at the Tevatron. The dominant mechanism for pair-production is qq —> ti through a virtual s-channel gluon. Single top production is also possible. The top quark decays as t —> Wb almost 100% of the time. Top quark pair-production then gives W+W~bb events and there are three possible discovery channels, depending on the W decays: • Both W's decaying hadronically — 6 jet final state (2b4j). This is the largest signal cross-section, but with large QCD backgrounds. • One W decays hadronically, the other leptonically: £2b2jtfT (lepton plus jet sample). This was the best channel for Run I, because there was a sufficient number of events, yet the background was under control.
72
• Both W's decaying leptonically: 2£2bIfT (dilepton sample). This channel has the largest S/VB ratio, however it is limited by statistics. Both experiments (CDF and DO) have conducted measurements of mt in Run I and the results from the two experiments as well as from different channels are consistent with each other (see Fig. 9). The direct mt measurements from the Teavtron are in very good agreement with the best fit value for mt extracted from the electroweak precision fits (see Section 6.6). Right now mt is the best measured of all quark masses. In Run II one can anticipate significant improvements — the expected precision on the mt measurements is shown in Table 4. In fact, there are already some preliminary top results from Run II [29].
Tevatron Top Quark Mass Measurements 168.4 ± 12.8 GeV/c 2
I
I
w
I
Dilepton
173.3 ±
7.8 GeV/c2
Lepton+jets
172.1 ±
7.1 GeV/c2
Combined
f D0 "i'CDF 167.4 ± 11.4 GeV/c2 I I
'm* w
I I
176.1 ±
I
7.4 GeV/c 2
186.0 ± 1 1 . 5 GeV/c 2 176.1 ±
6.6 GeV/c 2
174.3 ±
5.1
Dilepton Lepton+jets All-Hadronic Combined
Tevatron combined
i
150
160
170
GeV/c 2
i i i i i
180
190
200
Mtop (GeV/c 2 ) Figure 9.
Run I Tevatron results for mt and the global average. (From Ref. 28.)
73 Table 4. Expected precision (in GeV) on the rat measurements in Run II for the lepton plus jet and for the dilepton channel.
fCdt bbijj$T
bb2e$T
5.2. W mass
(ft- 1 ) Statistical Systematic Total Statistical Systematic Total
2 1.7 2.1 2.7 2.4 1.4 2.8
15 0.63 1.2 1.3 0.87 1.0 1.3
30 0.44 1.1 1.2 0.62 1.0 1.2
measurement
W bosons can be singly produced at the Tevatron with a relatively large cross-section ~ 1 nb. The W is subsequently detected through its leptonic decay mode to a charged lepton and the corresponding neutrino. (The hadronic decays cannot be used because of the enormous dijet background from QCD.) The signature is l.I$T + X. Unfortunately, we cannot reconstruct the invariant mass of the W, because of the unknown longitudinal component of the neutrino momentum. Hence we must extract Mw from transverse quantities only. There are several possible methods [30]: looking at the transverse mass (M^Y), the transverse momentum of the charged lepton (pj,) o r the missing transverse energy I$T. Since different methods have different systematic errors, it is desirable to have as many independent measurements as possible. In Run I the transverse mass method was used. (The p^T method was limited by the number of leptonic Z events - see below.) The transverse mass c is defined as M^
= ^2plTpuT{l-cos<j>)
(38)
where (j> is the angle between p\ and p\ in the transverse plane. p\ is nothing but the missing transverse energy and is measured as
FT = - 0 + fT)
(39)
where U is total transverse momentum of the remaining hadronic activity in the event (typically a jet from initial state radiation). The transverse mass distribution exhibits a characteristic drop-off around Mw, as shown in Fig. 10. The Mw measurement is done by fitting Monte Carlo predictions c T h e name is appropriate since when the W has zero longitudinal momentum p™, the transverse mass M^f coincides with the invariant mass Mw- If, however, \p^\ > 0, then M™ < Mw.
74
^500 a
CDF(IB) Preliminary
> *400
X7df = 158/139 (50 < MT < 120) X2/df = 62/69 (65 < M T < 100) Mw = 80.430 +/- 0.100 (stat) GeV
300 200
90
100 110 120 Transverse Mass (GeV)
Figure 10. Transverse mass distribution of W —^ \iv events from the CDF Run IB data, with the best fit. The shaded distribution is the background.
for different values of My/ to the observed Mjf distribution. In addition, at large Mjf the shape of the distribution is sensitive to the intrinsic W width TV, as illustrated in Fig. 11, which allows for a direct measurement
of rV: Tw = 2.055 ± 0.125 GeV.
(40)
An alternative method for measuring Mw employs the lepton pr distribution, which cuts off around M^y/2. However, unlike M^f, the pj. distribution depends on the W boost, and therefore on the transverse momentum p^ with which the W was produced. It is difficult to compute p^ by theoretical means, especially in the low px region, and it is best to extract it from data. For this purpose the Tevatron experiments have used the observed pij, distribution, which has a similar shape, but fewer events. Hence, in Run I this method was statistically limited, but offers good prospects for Run II. The expected precision of the Mw measurements at the Tevatron by the two methods is shown in Table 5. 6. Theoretical Interpretation of the Precision Electroweak Data We can make use of the precision electroweak data in three different ways: • Test the Standard Model.
75
>
10'
O
• Tw=L6GeV • Tw = 2.1GeV *rw = 2.6GeV
in +->
in J 10
>
10
10
100
120
140
160
180 200
mp (GeV) Figure 11. Monte Carlo simulations of the transverse mass spectrum for different W-boson widths. The normalization is arbitrary. (From Ref. 31.)
Table 5. Expected precision (in MeV) on the Tevatron Myy measurements in Run II.
JCdt (fb" 1 )
Mf PT
Statistical Systematic Total Statistical Systematic
Total
2 19 19 27 44 10 44
15 7 16 17 16 4 16
30 5 15 16 11 3 12
• Predict the preferred mass range of yet undiscovered particles: top quark in the past, nowadays the Higgs boson. • Point towards specific new physics models (in case of some discrepancy) or constrain new physics models (if agreement is found).
76
We shall discuss each one in turn. 6.1. Testing the Standard
Model
Having made a variety of measurements for different observables, we can test the SM by comparing theory to experiment. For this purpose we need to compute the theoretical prediction (within the SM) for each observable. How do we do it? Recall that in the SM we start with the input parameters {p} = {a,GF,Mz,g3,mh,mt,mb,mc,ms,...}
,
(41)
where the first three are the parameters of the electroweak sector. Any other electroweak quantity, e.g. sin2 9w, Mw etc. can be expressed in terms of these and thus is predicted by the model. At tree level the expressions are simple and only involve the electroweak inputs a,Gp,Mz, e.g. the Weinberg angle can be found from
Similarly, the W mass is Mw = Mz cos 6W
(43)
with 6w given by (42). The radiative corrections, however, modify the tree level relations and introduce the remaining (non-electroweak) parameters into the game, so that each electroweak observable depends in principle on the full set of input parameters (41): Of e°ry({p})
= Oree(a,GF,Mz)
[1 + A,({p})].
(44)
By measuring enough electroweak observables (with sufficient precision so that we are sensitive to the radiative corrections) we can extract information about the values of the other parameters outside the electroweak sector. We can then test the model for consistency by comparing the values deduced indirectly with direct measurements of those parameters. So the strategy is as follows. First compute the corrections in a certain renormalization scheme, treating some parameters as fixed inputs (usually the best measured ones). Then perform a global fit to the electroweak data. This will result in "best fit" values for the remaining (floating) input parameters. Then
77
(1) Compare the "best fit" values of the floating parameters to their direct measurements (if available). For example, the fit will choose "best" values for Mw, rnt, a3, which have been measured by other means directly. On the other hand, the best fit value for rrih cannot be checked against experiment yet, but is perhaps the most valuable piece of information from the global fit. (2) For the best fit values found above, compute and quote the theoretical prediction of the SM for each observable. This is the number quoted under "Standard Model" in Tables 1 and 2. Then compare the experimental and theoretical values of the observables themselves. A large discrepancy in a certain place may signal new physics which affects that particular measurement but not the others... There are two popular programs on the market which would accomplish this program: • ZFITTER [32], which uses the on-shell scheme; • GAPP [33] (global analysis of particle properties), which utilizes the MS scheme and is used for the PDG review. 6.2. Fixed
parameters
The parameters which are held fixed in the global fits are the following. Fine structure constant a. It is measured at low energies, so in order to obtain g'(Mz) and g(Mz) we need to evolve it up to the Z scale: a Mz
^ ^
= i
f^(M
Y
( 45 )
1 — i\a{Mz) Aa has QED and (two-loop) QCD contributions. The latter are denoted as Aa;)J d (for five active quark flavors below Mz)- The global fit produces a "best fit" value for it [16] Aa
hld(Mz)
= 0-02778 ± 0.00020,
(46)
which can again be compared to theoretical estimates. The uncertainty arises from higher order perturbative and nonperturbative corrections, from the uncertainty in the light quark masses (most notably mc) and from insufficient e + e~ data below 1.8 GeV. Sometimes, however, the theoretical estimates are used as a constraint in the fit and predetermine the best fit value. Note that a " 1 = 137.03599976 ±0.00000050 is much better known than l a~ (Mz) = 127.922 ± 0.020, which explains why it is preferred as an input parameter.
78
Fermi constant GF = 1.16637(1) 10~ 5 GeV 2 . It is determined from the muon lifetime formula .
25
o - p ill,,,
n2\ airrifj)
1927T3
'»
a 2 (m M )' G2 " (47)
where F{x) = l-8x
+ 8x3-x4-12x2lnx
(48)
and Ci is a known number. The remaining uncertainty in GF is almost entirely from the experimental input. Fermion masses. For simplicity, the fermion masses are held fixed, with the exception of mt and mc. 6.3. Floating
parameters
The remaining parameters from the set (41), which were not mentioned in Sec. 6.2, are floating and their values are determined from the global fit. • Mz • It is measured very well, and since the experimental measurement is included in the fit, the "best fit" value for Mz is very close to the experimental one (see Table 1). • as. Its best fit value [1] as{Mz)
= 0.1210 ±0.0018
(49)
is in very good agreement with direct determinations from tau decays, charmonium and upsilon spectra, jet properties etc. • mt. The best fit value, including the Tevatron data, is [1] mt = 174.2 ± 4.4 GeV.
(50)
However, even if we take the Tevatron data out, the prediction of precision data alone [1] mt = 174.0l?;2 GeV.
(51)
is in impressive agreement with the direct Tevatron value 174.3 ±5.1 GeV. • Higgs boson mass m/,. The best fit value is [1] mh = 86+^ GeV.
(52)
The central value is well below the LEP exclusion limit from direct searches, but is consistent at la. The result for the Higgs mass is often shown as "the blue band plot" (Fig. 12). However, there are
79
two caveats which we shall discuss in more detail below. First, the blue band hides a potential discrepancy between individual contradictory measurements. Second, the dependence on rrih is only logarithmic, and new physics contributions at the same level may have an impact on the Higgs mass prediction.
6.4. Comparing
the values for the electroweak
observables
Once we have the best fit values for the floating parameters, we can predict the theoretical central values Oth for the remaining observables in Tables 1
CM
<
2H
0 ExcludedT ^ n ^ - r ^ 20 100
Preliminary
400
mH [GeV] Figure 12. Of particular interest is the constraint on the mass rrih of the Higgs boson, because this fundamental ingredient of the Standard Model has not been observed yet. The figure shows the Ax versus rrih curve derived from precision elecXji troweak measurements, assuming the Standard Model to be the correct theory of nature. The solid line is the result of the fit using all data while the band represents an estimate of the theoretical error due to missing higher order corrections. The vertical band shows the 95% CL exclusion limit on rrih from the direct Higgs search at LEP. (Prom Ref. 20.)
80
and 2. We can then compare theory (Oth) to experiment (Oexp). The results are usually presented as "pulls", which in Tables 1 and 2 are defined as /nth P"
11
=
/nexp atH
(53)
»
where ath is the error on Oth. All in all, the fit is successful, as evidenced from Fig. 13. One is tempted to conclude that the SM works pretty well, having been tested at the level of 1% and less. No major discrepancies are observed, and, with so many measurements, a few 2a — 3a deviations are simply inevitable. 6.5. The Higgs mass
prediction
Let us now understand better where the Higgs mass prediction comes from. Ideally, we should concentrate on those observables which exhibit the strongest dependence on the Higgs boson mass rrih- For this purpose, it is useful to plot the theoretical prediction for each observable as a function of rrih and contrast it with the experimental measurement (see Figs. 1417). We see that the observables most sensitive to m^ are the W mass Mw and the asymmetries. We will discuss these in more detail in the next two subsections. 6.6. Impact
of the Mw
measurement
on irih
An approximate formula for Mw (in the MS scheme), which exhibits the dependence on the relevant electroweak parameters, is [34] MW = 80.3827 - 0.0579!n ( j ^ )
- 0.008h,"
( ^ ^ )
1
-H^-fH(i^)'- ) 1 -H^- )-
(54)
The experimentally measured value for Mw seems to be a bit high (Table 2). As we can see from (54), a largish Mw can be explained by either a small rrih, a larger mt, a smaller as, or some combination of the above. The correlation between Mw, rnt and rrih from the precision data is shown in Fig. 18, where the solid contour delineates the 68% CL region resulting from a global fit omitting the Mw, Tw and mt measurements. Also shown is the
81
Summer 2003 Measurement
Fit
I^meas /-Jiti / meas
(}
1
2
J
r
m 7 [GeVl
r 7 (GeVJ °hari R:
^
91.1875 + 0.0021 2.4952 + 0.0023
91.1875 2.4960 •
41,540 ± 0,037
41.478 M M
20.767 ± 0.025
20.742
ii^^^^i§i§ii!i;l
s
0.01714 + 0.00095 0.01836
R
0.21638 ±0.00066 0.21579 _
b D rt c
0.1720 ±0.0030
™"
0.0997 ±0.0016
0.1723 ' 0.1036 —
0.0706 ± 0.0035
0.0740
Ab
0.925 ± 0.020
0.935
A
0.670 ± 0.026
0
A * M
f bc
A°' n
fb
A,(SLD)
0.1513 + 0.0021
0.668 0.1477
:T)-.,, \(.«'.:.y \
00.423 + 0.034
80.385
i .... iCoVj
2.139 ±0,089 174.3 ±5.1
c
mt [GeV]
-•••I
••WWl SHU I i:iilliiiii^
~
•
2.093 174.3 0.2223 — 1
0
1 2
3
Figure 13. Agreement between the measured values for some of the precision' electro-weak observables and their SM predictions, as of the Summer of 2003. (From the L E P EWWG[19].)
82
10
;\ v \ i \ \
10'
; •
\x\>
>
>
\v\ % K \
CD
o 10
•
1
2_
V.
i
i
i
i
CD
O
-
10 2H 41.6
0.02
0.02761 ± 0.00036 as= 0.118 ±0.002 m,= 174.3+ 5.1 GeV
1.992
2
2.008
o?ep [nb] Figure 14. of m/,. The SM band is of the band
Comparison of LEP-I measurements with the SM prediction as a function measurement with its error is shown as the vertical band. The width of the due to the uncertainties in Aah(^d(Mz), as(Mz) and mt- The total width is the linear sum of these effects. (From the LEP EWWG [20].)
SM prediction for this correlation, for mh = 114,300,1000 GeV. We first see that the indirect determination of Mw and mt from precision data alone is in very good agreement with the direct Mw and mt measurements shown
83
Preliminary
0.105
0.14
0.08
0.15
A,(P T ) 10' Measurement ^ AajfJd= 0.02761 + 0.00036
>
a = 0.118 + 0.002 jm,= 174.3 ±5.1 GeV 10' 0.14
0.15
A, (SLD) Figure 15. Comparison of LEP-I measurements with the SM prediction as a function of m^. The measurement with its error is shown as the vertical band. The width of the SM band is due to the uncertainties in A a ^ d ( M z ) , aa(Mz) and mt- The total width of the band is the linear sum of these effects. Also shown is the comparison of the SLD measurement of At, dominated by A°LR, with the SM. (From the LEP EWWG [20].)
84
Preliminary 10?
10'
>
> CD
CD
o
O
I
E
X
E 10*-
10' 0.216
0.218
Rg
m3
1
> CD
O
10 2-
: •
^
-
^
0.173
1
i
'
0.179
0.72
Measurement x(5)-
a s = 0.118 ±0.002 U m t = 174.3 ±5.1 GeV
0.234
Figure 16. Comparison of LEP-I and SLD heavy flavor measurements with the SM prediction as a function of mh. The measurement with its error is shown as the vertical band. The width of the SM band is due to the uncertainties in Aa^d(Mz), as(Mz) and m±. The total width of the band is the linear sum of these effects. Also shown is the comparison of the LEP-I measurement of the inclusive hadronic charge asymmetry Q1^ with the SM. (From the LEP EWWG [20].)
85
Preliminary
sirf9w(vN) Measurement W,\ A a f f ^ 0.02761 ± 0.00036 as= 0.118 ±0.002 | | | m t = 174.3 ±5.1 GeV Figure 17. Comparison of Mw and Yw measured at LEP-II and pp colliders, of sin 2 d\y measured by NuTeV and of atomic parity violation in caesium with the SM prediction as a function of m^. The measurement with its error is shown as the vertical band. The width of the SM band is due to the uncertainties in Aorh^d(Mz), cts{Mz) and mt. The total width of the band is the linear sum of these effects. (From the LEP EWWG [20].)
with the dashed contour. This fact may not look that impressive, now that the top quark and the W boson have been discovered, but nevertheless it should be considered as a triumphant success of the precision program. We also see that both the direct and indirect measurements of Mw and mt prefer a light Higgs boson.
86
6.7. Impact of the asymmetry
measurements
on m/,
2
The asymmetries (or equivalently, sin # e //) also exhibit significant sensitivity to nih- Examining Fig. 15, we notice a couple of things. First, At *b,c and ApB place contradictory demands on the Higgs mass: Ag prefers a \b,c very small mh while ApB prefer a heavier Higgs boson. There are a couple of At measurements — one from LEP and the other from SLD, and they seem to be in agreement. Second, since the best fit value for mh is low, this means that AFB will be off from its "SM prediction'"1. Finally, the
1
OU.D
1
1—[ —i
1
1
1
r 1 - i —1
i
'
'
'/
— LEP1, SLD Data -
80.5-
LEP2, ppLJSXB. 68% CL
> CD
—~^y /'
1
JL.
£ 80.4E
/
a
/ x&
80.3m H [GeV] 114/30QA on o OKJ.C. 150 1 C 30
000/ 170
/
Preliminary"
190
210
mt [GeV] Figure 18. A comparison of the indirect measurements of Myy and raj (LEP-I+SLD data) (solid contour) and the direct measurements (pp colliders and LEP-II data) (dashed contour). In both cases the 68% CL contours are plotted. Also shown is the SM relationship for the masses as a function of m ^ . The arrow labelled A Q shows the variation of this relation if a(Mz) is changed by \a. This variation leads to an additional uncertainty to the SM band shown in the figure. From Ref. 20.
d
Of course, if we compute the AjfB prediction with a large raj,, then AjfB will be OK, but a number of other well measured observables will deviate, most notably Ag and Mw< and the fit will become worse.
87
NuTeV measurement of sin2 9w (Figure 17) also seems to prefer a rather heavy Higgs boson. The results for the various asymmetries are often conveniently summarized as measurements of sin2 6^ (Fig. 19).
Final 0,1
0.23099 ± 0.00053 0,23098 ± 0.00026 0.23159 ±0.00041
A.(P_ had
-
-*-
Q fb
0.2324 ±0.0012 Preliminary
0,b fb i0,c A,fb
0.23212 ±0.00029 0.23223 ± 0.00081
Average 10
0.23150 ±0.00016
3
X2/d.o.f.: 10.5/5
>
CD E T ^ A a ^ 0.02761 ±0.00036
10 2i
m z = 91.1875 ±0.0021 GeV
ES23mt= 174.3 + 5.1 GeV
-i
0.23 •
0.232
p-
T
'
'
0.234
sin 2 e eff = (1 - g v /g A i) / 4 Figure 19. Comparison of several determinations of sin 2 # g? from asymmetries. Also shown is the prediction of the SM as a function of m/,. The width of the SM band is due to the uncertainties in Aa^d(Mz), Mz and roj. The total width of the band is the linear sum of these effects. (From Ref. 20.)
Summer 2003
r z [GeV] z > had [nb]
f|™
1
A|(P.) -,0
C
M^^p«^aa«m»««»»i«J
A,(SLD) sin 2 9^(Q fb ) m w [GeV] r w [GeV] sin29w(vN) Qw(Cs) •
f
y
y
^
.
^
^
,
^
j
B
i||i,»,||»,„||,„r,t„»|,
ggyBgMg^KHmg^iMMgMWgaWigg^p
2
10
10
10
M H [GeV] Figure 20. Preferred range for the SM Higgs mass m ^ from various electroweak observables. The shaded band denotes the overall constraint on the mass of the Higgs boson derived from the full data set. (Erom Ref. 20.)
89
6.8. A Higgs
puzzle?
Tables 1 and 2 contain a large number of observables, with different sensitivity to the Higgs mass rrih • Let us concentrate on a few selected observables with sufficient resolving power with respect to rrih and look at the range of rrih preferred by each one (Figure 20). We see that, as discussed previously, Mw and the leptonic asymmetries prefer a very light Higgs boson, while ApCB and NuTeV prefer a heavy Higgs boson. The current best fit value of rrih of just below the LEP limit arises from the combination of these somewhat contradictory measurements. Is this a problem? Chanowitz has made the argument that this discrepancy presents a "no-lose" case for new physics [35,36]: • One possible alternative is that ApCB is already indicative of new physics, and the discrepancy is real. However, any such new physics effect should not contribute too much to -R&, which seems to be in agreement with the SM. Proposed solutions to the puzzle include a heavy Z' boson with nonuniversal couplings to the third family [37-39] and mirror vector-like fermions mixing with the bji quark [40]. • Alternatively, the ApCB (and possibly the NuTeV) measurement could be wrong due to a statistical fluctuation or some unknown systematics. In that case, we should throw out the suspect measurements from the fit altogether. This significantly improves the goodness of fit, but at the expense of a much lower best fit value of rrih, whose upper bound from the fit becomes rrih < 120 GeV at 95% CL, leading to some tension with the direct LEP bound rrih > 114 GeV. In this scenario new physics is again needed in order to modify the radiative corrections and thus allow an acceptable rrih [41,42]. However, one should keep in mind that the tension between the indirect determination of rrih and the direct lower bound from LEP is statistically rather weak at the moment, and may be alleviated significantly, if, for example, the top quark turned out to be a little heavier, as suggested by the recent DO analysis of their Run I data [43] (which gives rrit = 180.1 ± 5.4 GeV). This is illustrated in Figure 21, which shows the effect of a l a upward fluctuation in mt on the indirect Higgs mass determination [44].
90
(5) Acc h a d -Aa
•0.0276110.00036 0.02768±0.00036
4-
MT = 179.4±5.1 GeV
CM
<
0
Excluded
Preliminary
mH Figure 21. Ref. 44.)
10 [GeV]
The effect of a la change in mt on the Higgs mass constraints.
(From
7. Testing for N e w Physics The precision data can be used to constrain or point towards new physics. To this end, there are different approaches. • Model-independent. If there is new physics, then upon integrating out the new heavy degrees of freedom we will end up with a set of higher dimensional operators in the low-energy lagrangian. The most model-independent approach, therefore, will be to write down all possible higher-dimensional operators consistent with the symmetries of the Standard Model. These operators will be suppressed by some scale A (presumably the scale where the new physics will appear) to the appropriate power. The size of these operators can
91
be parameterized by dimensionless coefficients of order unity. We can then use the precision electroweak data in order to constrain the dimensionless coefficients of the relevant operators [45-47]. The advantage of this approach is that it is completely model-independent. Unfortunately it is too complicated and time-consuming, and is rarely used for the case of more than a few operators. • Model-dependent. Alternatively, one can completely specify the new physics model of interest, e.g. minimal supergravity [48], minimal gauge mediation [48], minimal universal extra dimensions [49,50], little Higgs models [51-56], etc. One can then compute the contributions from new physics to the precision observables in terms of (41) supplemented by the new physics model parameters
&heory
=
&ree
^
G
^
M z )
[X
M
+
Af
({P})
+ A?P({p,
q})] ,
'(55) redo the fits and derive best fit values and constraints on {q} in a similar way as was done for {p}. In full generality (i.e. full leading order corrections to all precision observables), the method is again very time-consuming, so it is usually applied for a single (or a few) observables. • Oblique parameters. A third method, which sits somewhere in between, is the method of the so called S, T, U parameters (also called Peskin-Takeuchi parameters [57,58]). It amounts to making the approximation that the dominant new physics effects reside in the gauge boson propagators (self-energies). Notice that almost every electroweak observable involves some gauge boson propagator. So once we compute the new physics effects on the gauge boson propagators, we have essentially accounted for a whole class of corrections which appear in every observable, i.e. they are universal. In this method one neglects the process-specific corrections, i.e. the vertex and box corrections and the fermion (and Higgs) self-energies. Apriori we don't know whether this approximation is justified within a specific new physics model, however, there are many classes of models where it works. In scenarios with many new particles, there is a simple argument as to why the oblique corrections are most of the story - the gauge bosons couple to all particles charged under the corresponding gauge group, hence their self-energy corrections are enhanced by the multiplicity of the new particles. In contrast, the
92
flavor of the loop particles in the process-specific corrections is fixed by the flavor on the external legs. To summarize, the advantages of the method are: 1) simple calculations (there aren't too many relevant diagrams); 2) universality - i.e. the parameters are computed once and for all and affect all observables; 3) if S, T, U are added as free parameters to (41), their best fit values can be computed ahead of time by the fitting experts, and then supplied to model-builders, who in turn only need to learn to calculate S, T, U within a specific model and need not worry about the fitting procedure. 7.1. S, T, U
parameters
The S, T, U parameters are defined as [2] S
e
( M
_ ) nff%,(o) _ WzW T = ™ ^ - ^ ^ ,
z
Mir
(56)
M|
ae(Mz) _ U^(Ml) - H^*(0) 4?|c| * Ml 3e(Mz)fo ,m _ „w, (S + U)=
nwwy ^ ( Mw^ ) - nww ^(0) ' ~ ^>.
(57) •
(58)
where sfj ls defined in terms of the MS running couplings as s"| = 5 / 2 ( M z ) / ( 5 / 2 ( M z ) +
(59)
T = -0.15 ± 0.12(+0.09),
(60)
C/ = +0.32±0.12(+0.01)
(61)
for mh = 115.6 GeV (the parentheses show the change in going to m/, = 300 GeV). Best fit contours in the S — T plane (for U = 0) are shown in Fig. 22. We see that S = T = 0 is in the region of a light Higgs boson, i.e. in the absence of new physics, the fits prefer small mh, as we saw in the previous Section. However, a heavy Higgs is not out of the question, provided there are positive new physics contributions to T. 7.2. Constraining
new physics
scenarios
Previously we saw that precision electroweak data generally prefers a light SM Higgs boson (Figure 12), in which case any new physics effects
93 1.5
0.5
-0.5
—
r„ a „,, R., R Z*
had'
P
q
— asymmetries — —. - y scattering —
- Mw
—
Qw
all:M H =: 115 GeV all: M u = 1000 GeV -1.5 -1.5
0.5
1.5
Figure 22. la constraints on S and T from various inputs. The contours assume wih = 115 GeV except for the central and upper 90% CL regions (shaded, from all data) which are for m^ = 340 GeV and m^ = 1000 GeV, correspondingly. All fits assume U = 0. (Prom Ref. 2.)
appearing through 5, T and U should be small. However, this does not rule out a possible "conspiracy" [59,60], where the undesired contributions from a heavy Higgs boson in the SM are fortuitously cancelled by new physics effects. While a "conspiracy" is still a valid option in principle, the current thinking is that it would involve a large degree of fine-tuning and is therefore theoretically disfavored. As a result, we tend to like new physics models where no major deviations in the precision observables are expected. A simple and effective way to guarantee this is to have the new physics contribute only at the loop level, for example due to a conserved symmetry which distinguishes the SM particles from the rest. Some well
94
known examples are: supersymmetry with conserved i?-parity, Universal Extra Dimensions with conserved i^X-parity [49], and little Higgs models with conserved T-parity [61]. Conversely, models with tree-level contributions to electroweak observables (e.g. generic little Higgs theories) tend to have trouble with the data. 8. Concluding Remarks Overall, the SM is in good shape, and the agreement between theory and experiment has in fact improved since the time of TASI-2002. The results in Tables 1 and 2 are from 1/03 [1], while the electroweak data shown at the TASI school was from 7/01 [16]. Some notable changes and recent updates are the following: • Table 2 lists the Summer 2002 value for Mw (80.447± 0.042 GeV), but in the Winter of 2003 a revised ALEPH analysis lowered the LEP average to 80.412 ± 0.042 GeV, which is closer to the SM best fit prediction and will lead to a small increase in the best fit value for rah• New measurements of Mw and m-t are expected soon from Run II at the Tevatron. A new preliminary DO analysis [43] of Run I data gives rrit = 180.1 ± 5.4 GeV, which again will raise the best fit value for rrih• The NuTeV results on deep inelastic scattering in terms of sin2 0w (0.2277 ± 0.0016) are 3.0cr above the global fit value of 0.2228 ± 0.0004. Possible explanations of the NuTeV anomaly include an unexpectedly large violation of isospin in the quark sea [62,63], the effect of an asymmetric strange sea [62-64], nuclear shadowing [65,66] or NLO QCD corrections [67,68]. • The status of the g^ — 1 anomaly after the 2002 result from BNL was still somewhat uncertain, depending on the particular theoretical analysis used to extract the hadronic contribution to the Standard Model prediction for g^ — 2. In addition, last year the CMD-2 Collaboration at Novosibirsk discovered that part of the radiative treatment was incorrectly applied to their e + e~ data (a lepton vacuum polarization diagram was omitted) which prompted a reanalysis of the e+e~ data [69]. This subsequently brought the theory prediction in better agreement with experiment: the deviation was 1.9cr (0.7er) for the e + e~ (r-based) estimates [70,71]. The final result based on negative muons has just been announced [72].
95 It is consistent with the previous measurements and brings the world average to a^exp) = 11659208(6) x 1 0 " 1 0 (0.5 p p m ) . T h e difference between a^exp) and the SM theoretical prediction based on the e+e~ or r d a t a is now 2.7a and 1.4CT, respectively. New results from K L O E and B A B A R may help sort out the theoretical predictions. If the discrepancy stands, a n obvious candidate for a new physics explanation would be supersymmetry with relatively low superpartner masses and large t a n / 3 [73,74].
Acknowledgments It is a pleasure to t h a n k the organizers of TASI-2002 (Howard Haber and Ann Nelson) for creating a very stimulating atmosphere at the school, as well as the participants for their enthusiasm and insight. I would like to t h a n k Andreas Birkedal for helpful comments on the manuscript. This work is supported in part by the US DoE under grant DE-FG02-97ER41029.
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r\
IRA Z. ROTHSTEIN
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EFFECTIVE FIELD THEORIES
IRA Z. ROTHSTEIN* Carnegie Mellon University Dept. of Physics, Pittsburgh PA 15213, USA
These notes are a written version of a set of lectures given at TASI-02 on the topic of effective field theories. They are meant as an introduction to some of the latest techniques and applications in the field.
Contents 1
Lecture I: The Big Picture 1.1 A Toy Model 1.2 Matching On or Off the Mass Shell? 1.3 Non-Decoupling and Wess-Zumino Terms 1.4 Types of EFTs 1.5 EFT and Summing Logs 1.6 Heavy Quark Effective Theory 1.7 The Method of Regions 1.8 A Caveat About Scaleless Integrals 1.9 Power Counting 1.10 The End of Our Calculation 1.11 EFT and WS 1.12 The Force Law in Gauge Theories
105 105 115 116 120 121 125 128 130 131 131 132 133
2
Lecture II: Non-Relativistic Effective Theories 2.1 The Relevant Scales and the Free Action 2.2 Interactions 2.3 The Multipole Expansion and Ultra-Soft Modes 2.4 Soft Modes 2.5 Calculating Loops in NRQCD
135 135 138 139 142 145
*Work supported by DOE contracts DOE-ER-40682-143 and DEAC02-6CH03000. 101
102
2.6 2.7 2.8 2.9 2.10 2.11
Matching at Subleading Orders in D Matching at Subleading Orders in a Summing Logs: The Velocity Renormalization Group (VRG) The Lamb Shift in QED States in the Effective Theory Summary
Lecture III: Effective Theories and Extra Dimensions 3.1 UV Fixed Points: A Theory Can Be More Than Effective 3.2 Naive Dimensional Analysis 3.3 The Force Law in d Dimensional Yang Mills Theory 3.4 Grand Unification 3.5 Orbifolding 3.6 Classical Divergences as UV Ignorance 3.7 Effective Field Theory in Curved Space 3.8 Unification in AdS5
147 149 . . . 153 157 160 161 162 163 165 166 170 173 177 178 181
It is a little known fact t h a t the Pioneer spacecraft launched in the mid-seventies contains a d a t a repository with a s u m m a r y of h u m a n scientific knowledge. It is interesting to ask what would h a p p e n if an alien civilization found this satellite and studied its contents? Let us furthermore suppose t h a t while these aliens were/are greatly advanced technologically, they are not terribly well versed in basic research, mainly because of the alien government's short-sightedness 8 , and are thus very interested in the free knowledge t h a t we are offering them. One of the pieces of information in the d a t a repository is a copy of the particle d a t a group booklet (the hand sized one, not the tome). T h e booklet is handed over to the relevant scientists, and a massive project is launched to decipher the information. After a couple of years of going over the d a t a and a copy of Bjorken and Drell, which was also on board, the aliens are up to speed and ready to test our theories. They've come to the conclusion t h a t we claim to have a well defined theory of strong and electro-weak interactions u p to the TeV scale (let's not quibble over anachronisms), and t h a t while we don't know exactly what happens at the TeV scale there is something t h a t unitarizes the theory above t h a t scale. They also conclude t h a t our understanding of gravity is very limited. In particular, while we know how to calculate q u a n t u m corrections at energies which are small compared to the Planck scale, above the Planck scale we have no idea what is going on. a
We know that without basic research they probably would never have achieved space travel, but I'll invoke suspension of disbelief at this point.
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After some period of intense investigations, the aliens decide to test the theoretical predictions based on what they learned from us. A proposal is put forward by a prominent experimentalist for a high precision measurement of the level splittings in Hydrogen. The proposal is summarily rejected based on a review written by a top alien theorist. The rejection was based on the referee's argument that it is silly to try to calculate loop corrections in the human's field theory. The referee's justification for this statement is that given that gravitons run through the loops and the fact the integrals include momenta above the Planck scale, where we know the theory breaks down, there is no way that one can expect that the results will be correct. Moreover, even if gravitational couplings are suppressed by the Planck scale, there are power law divergences which can compensate for these suppressions, so it seems that any attempt at calculating loop corrections would be fruitless. Fortunately, for us, there were some wise aliens working at the funding agency, and they said, "let's for the moment ignore gravitational effects, and see if the data agrees with what we get using just electro-magnetic effects". So the experiment was done and the theory did indeed meet with great success. So where did the referee go wrong? Well, to be fair, the referee was not completely wrong. There is a kernel of truth to its claims. It is true that the theory breaks down and that we should indeed pause to think about the results of any calculation which involves loop momenta above the Planck scale (the scale where the theory is no longer reliable). Since we know we are not correctly portraying the physics at the high scale, whatever the result of the loop integration may be, it will have to be treated in some way so as to remove the dependence on the UV physics. Indeed, the loop integrals will, in general, be divergent and need to be regulated. But there is no preferred regulation procedure b and, therefore, the numerical value of the integral is ambiguous. However, the point the referee missed is that if we are not too ambitious, this is not a problem. The short distance physics, which is not properly accounted for in the loop integrals, looks local at low energies. Another way of saying this is, that if we think of the loop integral in coordinate space, the part of the integral from which we know we are getting nonsense can be contracted to a point and therefore looks like an insertion of a local operator into our Feynman diagram. This means that all of the effects of the unknown short distance physics can be
b
However, a prudent calculator would choose to use a regulator which preserves the symmetries of the action, though this may not always be possible.
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mimicked by adding some new operators into our Lagrangian (as we will see these operators may be of arbitrarily high dimensions). That is to say, all the effects of the short distance physics can be accounted for by either shifting the parameters in our renormalizable Lagrangian, or by adding higher dimensional operators which are suppressed by the UV scale, Muv • Since we needed to perform measurements to fix those parameters anyway we still have predictivity as long as we truncate the Lagrangian at some order in powers of 1/Muv- So we see now in what way the referee was right. It's not that the unknown effects of the short distance physics are parametrically small in any way, indeed these effects are very important as they contribute at order one to the parameters in the renormalizable part of the Lagrangian. It's that all the effects go into making up the electron mass and all the other low energy parameters of the theory.0 If you changed the short distance physics it could, and would, change the values of the low energy parameters. Thus, as long as we are not too ambitious, we don't need to worry about the fact that our integrals diverge! Who cares if they diverge, we didn't expect to get the answer right anyway. So we should really call our field theories "effective" field theories. This moniker warns the consumer that it should be used with caution, as at some scale the theory no longer makes any sense. What that scale is depends upon the interactions one is trying to describe. Let's for the moment consider a universe where the effects of short distance physics could not be absorbed into low energy parameters. We couldn't calculate anything without knowing the complete theory of quantum gravity. While, this line of reasoning seems almost obvious now, thinking in terms of effective field theories (EFT) only became generally accepted in the last few decades. Indeed, from an EFT point of view, renormalizability, which was a standard benchmark for acceptable quantum field theories not that long ago, d is no longer relevant, unless we're interested in theories which are supposed to correctly describe all the short distance physics. Does this mean that field theory as a mathematical construct is incapable of being a "complete" theory? Certainly not. There are many field theories which are valid to arbitrarily high scales. The classic examples being asymptotically free theories such as QCD. These theories possess trivial UV c As we will discuss at length in the text, the number of such low energy parameters is dictated by the desired accuracy. d A s far as I know the only text books prior to those by Peskin and Schroeder [2] and Weinberg[3], which espoused the point of view described above was Georgi's book on weak interactions [4].
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fixed points, which allow for the cut-off to be taken to infinity. In fact, the necessary criteria for completeness is the existence of a UV fixed point, and taking the cut-off to infinity is called taking the "continuum limit", following the lattice terminology. The reasoning behind this criteria is that at the fixed point, the correlation length £, (read inverse mass) diverges, and thus one can study physics at arbitrarily large distances compared to the cut-off (the inverse of the lattice spacing a), which is equivalent to saying that the cut-off can be taken to infinity since lattice artifacts will be suppressed by powers of £/
1. Lecture I: The Big Picture This lecture is meant to give a global overview of effective field theories. The first part of the lecture in an introduction to the basic idea and a tutorial on the matching procedure, all discussed within the context of a simple toy scalar model. I then categorize the various types of effective field theories into two main headings depending upon whether the theory admits a perturbative matching procedure. Most of the rest of the lecture is dedicated to some sample theories where we can match. I will assume throughout these lectures that the reader has a basic understanding of renormalization.
1.1. A Toy
Model
Let us now try to be more quantitative by starting with a simple toy model. What we would like to illustrate is that all the effects of ultra-violet (UV) physics may be absorbed into local counter-terms and furthermore, that all of the physical effects of the UV physics are suppressed by powers of the UV scale.6 To do so we will work with a model of two scalar fields, <&L and e
This idea is usually referred to as "decoupling" as first codified in [5].
106 &H, with mji S>TOL,in four dimensions. The interaction Lagrangian will be given by
~Lint = ±*l + | M | + ^l&H
+ |$ 4 H-
(1)
It is typical, in most pedagogical treatises, to impose a discrete symmetry to forbid tri-linear interactions, as such terms introduce dimensionful couplings, which complicate the systematics. However, such interactions will illuminate certain ideas which otherwise would not arise until higher orders in the loop expansion. The usual problems with tri-linear theories arise from issues of vacuum stability, but for our purposes these issues will not be an obstruction. I will assume that all the couplings, except for mi, take on their "natural values". Here I use the term "natural" in both the sense of Dirac and t'Hooft. That is, dimensionless couplings should be of order one (Dirac) and dimensionful couplings should be of order of the largest scale in the problem, unless a symmetry arises in the limit when the coupling vanishes (t'Hooft). Thus, we take A2 ~ 1 and Ai ~ m # . Furthermore, since there is no symmetry protecting the light scalar mass, choosing m^
= Full Theory Result - Effective Theory Result.
(2)
In a sense what the left hand side of this equation is telling you is exactly what the effective theory is missing. Since the effective theory is designed to reproduce the IR physics of the full theory, SLEFT will have a well defined
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Taylor expansion around p — 0. Thus the holes in the effective theory can be plugged by adding local operators. If this seems a little obscure at this point don't worry as we will discuss many concrete examples of this matching procedure below. Notice that matching each n-point function will generate distinct local operators in the effective theory, with higher n-point functions obviously generating higher dimensional operators. Thus the number of n-point function one needs to calculate depends upon the desired accuracy. The more accuracy we want the more operators we must keep, as each operator of successively higher dimension will be suppressed by additional powers of the UV scale. Let's see how this works in our toy theory. Consider the amputated four point function for the light fields. At tree level, there are four diagrams which contribute to this process. One is the usual contribution involving only the light fields gives — i\o, and the other three have intermediate heavy scalars as shown in figure 1. These latter contributions are given by
Figl =
-i\f
+s
{t,u).
(3)
°H
Figure 1. Tree level contributions to the full theory four point functions involving the heavy scalar (thick lines).
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After expanding (3) we can see t h a t the four point function can be reproduced, u p to corrections of order p2 /m2H by using SLM
= C { ^ .
(4)
Here I have established notation for the coefficients (called "Wilson coefficients" ), the subscript counts the number of fields while the superscripts denote the order in the expansion in coupling and derivatives (more generally powers of 1/mjj), respectively. We can now see clearly what was emphasized in the introduction. As far as a low energy observer is concerned this looks like a simple <&4 theory with a coupling t h a t we had to fix via experiment anyway. T h e low energy observer would be completely oblivious to the UV physics unless the experiment were sensitive enough to see the deviations from $ 4 theory due to p/mn corrections. If we wanted to, we could add additional operators to ensure t h a t we correctly reproduce the higher n point functions as well. Suppose the low energy observer upgrades the machine, either by raising the energy or luminosity, to the point where the d a t a can no longer be fit by pure 0 4 theory. We know exactly what to do to correctly reproduce the data, simply add the higher derivative interaction SLint
=
J 77 • (5) mzH 4! Other operators with two derivatives and four fields can all be p u t into the form above via integration by p a r t s and use of the equation of motion. T h a t this latter manipulation is allowed will be discussed later. As the energy of the experiment is further raised one needs to include higher and higher derivative interactions to reproduce the data. Notice t h a t until we get to high enough energies to actually produce the heavy scalar we will be in the dark as to the true underlying theory. This is because in principle there will be an equivalence class (called a "universality class") of theories which look the same in the infra-red. As we fit more and more of the couplings of the higher dimension operators we would be able to eliminate theories, but removing all ambiguities would be difficult without including some other selection criteria. So far, we've shown t h a t to tree level we can reproduce the full theory by adjusting coefficients in an effective Lagrangian, this is called "matching" . This matching procedure can be carried out to arbitrary order in the couplings and p/ran- Indeed, suppose we wish to match at higher orders in the couplings. T h e matching procedure now takes on a new twist, but the
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idea is the same. We Taylor expand the full theory result and compare it to the result in the effective theory. We then modify the effective theory so as to get the answer right. I should mention at this point that in most theories the loop and coupling expansions coincide. But in our toy model, because of the Ai coupling this is no longer true. Nonetheless we can overcome this issue by assuming that (Ai/m# = Ai) is of order A/A~O- In this way all tree level contributions are of order Ao — A2 — A3. This is why I labelled the (i 2) Wilson coefficient above C\ . Returning to our calculation, let us see if we can reproduce the full theory at the next order in the couplings, beginning with the two point function. There are two leading order (dimension four) operators which get renormalized when calculating the two point function. The mass m\<&\, and the kinetic energy $ i D $ i . The one loop contributions to the two point function are shown in figure 2. Diagram 2a is the usual contribution found in $ 4 theory and is given by ^. „
Pig2a
i\0m2r
(\
, ,m? .
- = i^U-
ln(
#
A
)+1
,„.
(6)
)
where I will use the notation i = i - 7 + ln(47r). e e
O 2a.
_Q 2b.
(7)
Q 2c.
_2_ 2d.
Figure 2.
One loop contributions to the full theory two point function.
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The diagram in 2c. gives the same result as 2a., but with the replacement (mi, Ao) —> (mu, A2). Both of these contributions are momentum independent and thus only renormalize the mass. Figure 2b, on the other hand, will have non-trivial momentum dependence. We expect to have a cut beginning at (ran + rriL)2, and thus this contribution will be analytic in the neighborhood p2 ~ m2L. Doing the one-loop integration gives
^•=M\-^t)+i)
(8)
where I have expanded in I/ma, and kept only the leading term. Finally, there is a contribution from the tadpole diagram 2d which is given by
Fig2d. = - J ^ 4 ( ' - m ( 4 ) + 0-
0)
Now let's calculate in the effective theory. We must note that to be consistent we must use the 0(A) effective Lagrangian which we generated at tree level when we calculate on the effective theory side (the aforementioned twist). This is in contrast to the tree level matching where there is nothing to calculate on the effective theory side. At leading order in l / m # , the only relevant operator is the <&\ term and so we find that the effective theory gives the same result as Fig 2a. except now the coupling is not Ai but Cf , 0 ) . To match we take the renormalized (here in MS) full theory result expanded around small {rh2L,p2 = p2 /m2H), and subtract the renormalized effective theory result, finding C (i,o)
1 32TT 2
A2mk(l-ln(^))
2
2 2
ta +i ,2
^( >"<^)- - 5< ^ » (10) To insure that perturbation theory is well behaved, we should choose [i to minimize the large logs. This scale is called the "matching scale". Intuitively we would think that this scale should be the high scale m # . Given that the effective theory is supposed to reproduce the infrared physics, we would expect that all the dependence on the low energy parameter m^ should be identical on both sides of the matching equation, thus leaving all logs in the matching independent of mi- However a glance at our results shows that choosing \i = m # does not get rid of all the large logs. Clearly we have done something wrong. The problem stems from the fact that we
Ill
have kept some terms which we had no business keeping. In particular, diagram 2d is power suppressed relative (remember that Ai scales as run) to the other diagrams. Thus, to be consistent we need to expand our full theory result for diagram 2b. to one more order. So we should make the replacement
A? m\ ,_ m\
F i g 2 b . ^ ( 8 ) + i - ^2 (( - f2 l n - f ) . '16TT rn H m\{
(11) 1,0)
The log term will exactly kill off all of the large logs in Cm'
«0, = ii« 1 + 5 ) + A^-
, leaving
(12)
Notice that this correction wants to pull up the light scalar mass to the scale mn since Ai c± mjj. This is just what we called "unnatural" in the sense of t'Hooft, since as m^ is taken to zero we do not enhance the symmetry. Keeping mi,
=
^MS
2
+ 0(\/(32n
))(m2H),
(13)
and we will need to delicately choose m2LMS to cancel the large 0{m2H) piece. The momentum dependent terms will simply shift the field normalization in the effective theory with Cp = ,» J* . If we wish to go to higher O^TT Tit TT
orders in the l / m # expansion we will generate the operator
02=cf2)$L^UL.
(14)
"H
(1 2)
Exercise 1.1: Calculate C\ ' Now let us consider the four point function. A partial list of the relevant full theory diagrams are shown in figure 3. The diagrams with external leg corrections will automatically be reproduced by the effective theory, since we already matched the two point function. In the effective theory, there will be only one diagram, and it is identical to diagram 3c except that at each vertex we use the coupling C\ . By considering the combination of
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3a.
3b.
3d
3c.
-
3e.
Figure 3. functions.
A partial list of the one loop contributions to the full theory four point
couplings coming from (C\ ' ')2, we can begin to see that we will indeed reproduce diagrams 3a, 3b and 3c. However, we can also see that this diagram will not be able to reproduce the log of m^ from diagram 3e. To match this log we will have to first reproduce the six point function at tree level. Then diagram 3e will be reproduced by tying together two external legs. You can see how this will happen by noting that heavy field propagators can be contracted to a point in the full theory graph. Diagram 3d will have no partner in the effective theory and will contribute ( in the
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s channel), after renormalization and expansion in s = -%-,
We see that since we are matching at the scale /x = m # , this diagram will not contribute to C\ ' , at a least in the MS1 scheme. The other channels give similar contributions.
Exercise 1.2: First draw all the remaining diagrams which contribute to the four point function in the full theory. Then show that all the dependence on ln(mi) cancels in the matching for the four point function. Hint: By working at zero external momentum the calculation greatly simplifies. What happens if we go to first order in p2/m%, staying at one loop? At this order when we calculate in the effective theory, we will need to include the graph depicted in figure 4, where the circle represents an insertion of the operator (5). The integral for this Feynman diagram will have a contribution which is quadratically divergent d v / (2TT)4
D
(p2 - ml + ie)((p! + p2 + p)2 ~ m% + ie)'
(16)
Figure 4. One loop contributions to the effective theory four point functions, with the dark circle representing an insertion of the derivative operator (5).
Now, suppose we regulate this divergence with a cut-off, then this contribution will scale as A2 /m2H. What order is this in our momentum expansion? If it's order one, it would seem that we have lost all hope of calculating in a systematic fashion, since no matter how many powers of ran may suppress a given operator, it may give an order one contribution
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if we stick it into a multi-loop diagram. This would be a disaster. However, this reasoning is misleading. The fact that something can't be right can be immediately seen by working with a different regulator. Suppose we work in dimensional regularization. In this scheme, there are no power divergence as they are automatically set to zero [6]. It had better not be that our answer depends on our choice of regulator, otherwise we're clearly lost. So what's going on? Which regulator is giving us the correct result? The answer is, both. The difference between the two is "pure counter-term", which means the discrepancy in the two results can be accounted for by a shift in the couplings of local operators. Since the couplings are measured, and not predicted, there is no physical distinction between the two results. The only difference is in the bookkeeping, that is, the counter-terms in the two schemes differ but that's it. The nice thing about dimensional regularization is that we don't have to worry about mixing orders in l/rnn- We can see that if we use a cut-off and take it to be of order m # , then operators which are formally suppressed will "mix up" with operators of lower dimension.f But if we stick to dimensional regularization, then powers of IjmH are explicit, as they only appear in coefficients of operators (modulo the caveat about Ai in our toy theory), so we can read off the order of a loop diagram immediately. Thus, in general it is wiser to always work in "dim. reg." when doing effective field theory calculations. This is not to say that the cut-off doesn't have its time and place. In particular the cut-off makes quadratic divergences explicit, whereas in dim. reg. they show up as poles in lower dimensions. Another interesting point which arises from the calculation of the diagrams in figure 4 is the fact that there are divergences which can only be subtracted using counter-terms for operators with dimensions greater than four. The effective theory is non-renormalizable, an anathema in ancient times. That is, there are new UV divergences in the effective theory that arise in each order of perturbation theory. These divergences were not there in the full theory. This should not surprise us, since the effective theory is only built to mock up the IR of the full theory, not the UV. Thus the UV divergences will not cancel in the matching. This isn't a problem, we simply choose a scheme, and subtract. In the effective field theory approach we should be careful with out choice of scheme. If we're not careful we could mess up our power counting by introducing a scale. So we should choose a nice simple mass independent scheme like MS. I will have more I am using term "mix" in the technical sense of operator mixing.
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to say about this later on in these lectures. Notice that the matching coefficient will clearly be prescription dependent, but any such prescription dependence will drop out in low energy predictions. Exercise 1.3: Calculate the anomalous dimensions of the dimension six operator $ | D $ | to one loop. The additional UV divergences which arise in the effective theory correspond to IR divergences in the full theory. This is easily understood from the fact that we have taken m # to infinity in the effective theory. So the "IR" divergence ln(m_ff) now looks UV. Thus effective field theory allows us to resum IR logs in a simple way using standard renormalization group techniques. We will see an example of this in the coming sections.
1.2. Matching
On or Off the Mass
Shell?
Before moving on I'd like to discuss one more technical aspect of matching. In particular, it's important to understand that it doesn't matter whether or not one chooses to match with external states on-shell or off-shell. The reason is that putting external lines off-shell only changes the IR behavior of the theory, and since the effective theory is designed to reproduce the non-analytic structure of the full theory, all the information regarding the virtuality of the external lines cancels in the matching. This can greatly reduce the amount of work one has to do to match. For instance, if one is not interested in matching operators with derivatives, then one could choose the external momenta to vanish identically, greatly simplifying the necessary integrals. Also if one matches off-shell one can induce operators in the effective theory which vanish by the leading order equations of motion. For example, in our toy model we could induce $ ^ 0 $ ^ . However, it is quite simple to show that we can eliminate this operator via a field redefinition, which are known to leave S-matrix elements unchanged due to the equivalence theorem [7]. The proof [9] is inductive. Suppose we have set all operators of order {p/rriH)n which vanish by the leading order equations of motion (by leading order I'm referring to the momentum expansion) to zero. If there exists an operator at order (p 2 / TO ir)™ +1 of the form F ( $ L , <J?#)n£, then we make the field redefinition 5$L = -F($L,QH)
(17)
so that variation of the leading order Lagrangian 8LLO = {5§L)U§L
(18)
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exactly cancels the offending term. Notice that since F is order n +1 in the derivative expansion the lower order operators remain unchanged but we do induce a change in the subleading set of operators, i.e. at order n + 2. However, since in an effective theory we generate all possible operators consistent with the symmetries, all this redefinition will do is to shift the value of some matching coefficients. 1.3. Non-Decoupling
and Wess-Zumino
Terms
Let us consider the case of a chiral gauge theory. These are theories for which a fermion mass term is disallowed by gauge invariance. We know that if the theory is to be unitary and gauge invariant, the particle content must be such that all the gauge anomalies arising from triangle diagrams sum to zero. The classic example of such a theory is the standard model itself, where the fermions get their masses from the Higgs Yukawa couplings. Taking the fermion masses large while holding the symmetry breaking scale fixed corresponds to the limit of a large Yukawa coupling A. Since the coupling is proportional to the mass, we might expect that low energy observables (below the scale of symmetry breaking) could be enhanced by a large mass. The classic example of this "non-decoupling" arises in K — K mixing. After integrating out the top quark one generates a term in the AS = 2 Hamiltonian Ht=m2t^r,204,
(19)
where O4 is a four quark operator. A similar effect shows up in the p parameter, which is a measure of the ratio of charged to neutral currents. These effects would disappear in the limit where the bottom and top quark become degenerate, the former vanishes as a consequence of the GIM mechanism. Both of these effects arise due to SU{2) breaking. If we took the limit where both the bottom and the top quark masses become equal and large, then we would expect that the effects of this heavy pair could all be absorbed into SU{2) symmetric operators. The thoughtful reader might be confused by this last statement if they are familiar with anomalies. Suppose that one of the SU(2)L doublets is much heavier than the weak scale, and we wish to integrate it out. The low energy theory would then have anomalous matter content and would apparently no longer be gauge invariant. Actually things can look even worse. If we start with an even number of SU(2)L doublets and integrate out one such pair, then naively, the partition function would vanish due to
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the non-perturbative anomaly [12].g Now it should not come as no surprise that at "low energies" we do not have manifest gauge invariance. Nor should we be worried about this fact, since the lack of gauge invariance will only manifest itself in a pernicious way (a breakdown of unitarity) at energies near the masses of the gauge bosons. Nonetheless we should be able to add a tower of higher dimensional operators which will allow us to calculate sensibly at energies above the masses of the gauge bosons. The question is, what scale suppresses the effects of the fermions in these higher dimensional operators? The answer is the symmetry breaking scale v, and not the mass of the fermion. Integrating out a heavy fermion will generate a set of higher dimensional operators such as those arising from the graph in figure 5. This graph will generate a whole string of operators once
Figure 5. by 1/v2.
A diagram which contributes to a dimension six operator which is suppressed
we expand it in momentum. Consider the contribution to the dimension 6 operators, which are suppressed by l/MyV. Just from counting vertices and dimensional analysis we see that it must scale as 1/My-V oc A 2 /m 2 = 1/v2. Thus we can't write down a local effective theory which is valid in the region v < E < Xv, at least with these variables. The trouble all starts from the fact that fermions are getting their relatively large masses from dimensionless couplings. Even so, we should be able to calculate in a reliable fashion up to the scale v even with our low energy theory which appears anomalous. Imagine we're taking the limit g
This anomaly arises as a consequence of the fact that TT4(SU(2)) = Z2 and a topologically non-trivial gauge transformation multiply the partition function by a factor of -1 for each SU(2)L doublet.
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I will only briefly summarize. The serious student may wish to spend some time looking at the original work [13]. If we allow for A S> 1, then the heavy fermions will be strongly coupled, but this should not stop us from matching onto an effective theory. We know that we will not be able to match perturbatively. However, the large Xv limit lends itself naturally to a derivative expansion which will keep our matching under control. We will simplify the problem by considering a toy Yukawa theory with SU(N)i x SU(N)R symmetry. The field content is a left (right) handed fermion in the fundamental and a Higgs in the bi-fundamental, which transform as fa -> 9L^L,
i)R - • 9RfpR,
$ - • gL$gR,
(20)
LW>L + ^RIWR
-
ML^R
-
WI&HL,
(21)
where the Higgs doublet, $ is written as $ = vU, with U being a unitary matrix. That is we have frozen the modulus of the Higgs field to simplify the problem. Allowing the modulus to fluctuate wont change our conclusions. Now we would like to match on to an effective theory without the heavy fermions. The matching involves only calculations in the full theory since the are no graphs in the effective theory. If we were working in the standard model and wanted only to integrate out the top quark, this would no longer be true. Thus matching corresponds to formally do the path integral over the fermions then expanding the effective action W(U) = -iIn
f d[ipL}d[i)R]exp fi f d4xL)
(22)
in powers of l/(Xv), keeping only the terms which do not vanish in the A —> oo limit. Here I will only sketch how this calculation is done. The trick is to perform a change of variables to absorb the Higgs field into a gauge field. This is accomplished by the redefinition NL = U^L,
NR = Vfl,
(23)
which results in L = NL{i$ - 4L)NL + NRi$NR
- Xv(NLNR
+ NRNL),
(24)
where AL = U'd^U. The advantage of using this form is that a derivative expansion corresponds to an expansion in the number of external fields.
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We may write the new effective action denned in terms of an integral over the N fields, as W with W = W - U(U)
(25)
[ # L ] = J(U)[dNL].
(26)
since
Where J{U) is the Jacobian for the change of variables (23). W can be calculated using the usual Feynman diagram techniques, and, in general, contains many interesting pieces, such as the famous "Goldstone-Wilzcek" current [14] which can endow solitons with fermion number. However, in this model, all of these terms will be invariant under the anomalous symmetry, so we will not concern ourselves with them at present. We will be mostly concerned with the Wess-Zumino term which resides in the Jacobian [10]. This term can not be written as the four dimensional integral of an 5(7(2) x SU(2) invariant density but can be written as an invariant five dimensional integral [11] wz =
240TT 2
d4x
dx5eabcdeTr
&dauifidbUu*dc&uddUutdeu . (27)
U is an arbitrary (differentiate) extension of U into the x 5 direction which need only to satisfy U(x5 = 0, a;") = 1 ; U(x5 = 1, x?) = U{x»).
(28)
The reason that the interpolation is arbitrary is that the integrand is invariant under general coordinate transformations. Moreover, under a small variation of U, the integral is a total derivative whose value is fixed in terms of U(x><). Exercise 1.4: Prove that under small but arbitrary variations in U, the Wess-Zumino term changes by a total derivative, and thus depends only on the vale of U in four dimensional space-time. If we were to include the gauge fields, then a variation of the WZ term would exactly reproduce the usual ^vfXTF^Fpa contribution to the anomaly. It would also account for the non-perturbative anomaly [12] in the case where the gauge group is SU(2). The result for the gauged theory can be found in [13], and is slightly more complicated then what I have
120
described above. The non-local WZ term can be expanded in powers of v and the resulting theory will give well denned predictions in this standard model-like theory up to the scale v. 1.4. Types of EFTs I will separate EFTs into two types. Those for which the underlying UV physics is known and the matching can be done perturbatively, which I call "Type I", and those for which it is not possible to match, either because the UV physics is unknown, or because matching is non-perturbative, which I will call "Type II". Some examples of type II theories for which the underlying theory is unknown are • The Standard Model • Einstein Gravity • Any higher dimensional gauge theory11 whereas for the following effective theories • Chiral perturbation theory: Describes low energy pion interactions. • Nucleon Effective theory: Describes the low energy interaction of nucleons [15]. the underlying theory is just QCD, but the matching coefficients are not calculable, at least perturbatively. Examples of Type I theories are • Four Fermi Theory: For low energy weak interactions. In this case the underlying theory is the standard model, which in itself is a type II theory. • Heavy Quark Effective Theory: Describes mesons with one heavy quark [18]. • Non-Relativistic Effective Theories (NRQED, NRQCD): For nonrelativistic bound states such as onia or atomic physics [20,21]. • Soft-Collinear Effective Theory (SCET): For large light-like interactions such as those in heavy to light quark transitions [24], high energy jet processes [25] or quarkonia decays [26]. h
There are some higher dimensional SUSY field theories which are conjectured to have UV fixed points [16]. In these cases the theory is not effective, in that it is valid up to arbitrarily large momenta.
121
For type I theories, one may ask the question: "Why bother with an effective theory when we know the complete theory?". The answer is that in general the full theory can be quite complicated and going to an effective theory simplifies matters greatly. In particular, going to an effective theory can manifest approximate symmetries that are hidden in the full theory and, as we know, increased symmetry means increased predictive power. Furthermore, when the full theory contains several disparate scales (mi,mh) perturbation theory can be poorly behaved as typically one generates terms of the form a\n(mi/mh). When these logs become large, they need to be resummed in a systematic fashion in order to keep perturbation theory under control. Working within an effective theory simplifies the summation of these logs. The classic example of such a resummation arises in B meson decays, which I will now explore. 1.5. EFT and Summing
Logs
Let us consider the decay of a free b quark. This calculation is discussed in the text by Peskin and Schroeder [2] using slightly different language, but I will sketch it here for the sake of completeness . I will also simplify it some by ignoring operator mixing. At first sight there is no reason to believe that free quark decay has anything to do with the actual meson decay, but we'll come back to that point later. The tree level decay rate is proportional to the usual muon decay calculation, while at one loop we have diagrams such as those in figure 6a. This correction is finite and contains a large log of the form Diagram 6a = Ai—
m(mL/?7i|),
(29)
47T
The diagrams which do not traverse the W propagator do not contribute large logs and will not be of interest to us. If we are to save perturbation theory we must resum these logs. Indeed, the perturbative series for a generic Greens function of a theory which contains a large disparity of scales whose ratio is <5, will contain the series oo
n
n
C ln m
Giog = J2^ T, ^ ^ n—1
(30)
m—0
If we take (f* ln(<5)) to be order one, then we must resum all terms of the form (§£ ln(6))n. This is called the "leading log (LL)". approximation. This resummation as well as the "next to leading log (NLL) ( ( | £ \n(S))n~1)" and so forth are easily performed once we work within the framework of EFT.
122
6b. Figure 6.
One loop correction to heavy quark decay.
The way EFT makes the resummation of the logs simple is by converting the problem to one with which we are more familiar. We know how to sum logs arising from UV divergences using the renormalization group (RG), so all we need to do is to figure out how to relate the logs in (30) to RG logs. The logs in (30) can be thought of as IR logs in the rrib goes to zero limit, or UV logs if we take Mw to infinity. But this second limit is exactly the limit we take in constructing the EFT. In the four-Fermi theory Mw is taken formally to infinity, usually keeping only the first non-trivial term in the 1/Mw expansion. Let us see how the four Fermi theory reproduces the log in (29). The loop correction in the effective theory corresponding to those in diagram 6a is shown in diagram 6b, and is given by Diagram 6b = \
1
^ - ( -
47T
\e
Hml/v2)
(31)
Note that without doing any calculation we could have guessed the coefficient of the log in the effective theory from the full theory result. Since, as we discussed in the context of our toy model, the effective theory must reproduce the IR physics of the full theory, so the logs of m,b must be identical in both theories. Using this result we may match onto the effective theory. The Wilson coefficient of the four-Fermi operator is CiF(jj) = GF[l
+ X i ^ - (\n(Mw/m2b) A-K
+ l n K / M 2 ) + p)
(32)
Notice that we work in a mass independent prescription (usually MS) where
123
we subtract only the pole and possibly some dimensionless constants. Thus, the constant in (32) is scheme dependent. However, this scheme dependence is order 0(a), which is sub-leading in the LL approximation. To cancel this 0(a) scheme dependence we would need to go to the NLL, where the shift in the two loop anomalous dimension, generated by shifting schemes, will account for the change in the matching coefficient. Details of higher order calculations can be found in [17].
Exercise 1.5: Prove that the anomalous dimension is scheme independent only up to one loop. Also show that the beta function is scheme independent up to two loops. Assume that the coupling in the new scheme is analytic in the coupling in the old scheme, that is gl(g) = a±g + a2g2 + . . . Now in order to match we must make sure that series for C^F is converging. Thus we should choose fi to be order Mw, m which case dp — Gj?(l + A i " ^ ' p ) . In making this choice we have removed the large log from the matching, but if we are not careful the logs can still reappear. To see this we must consider the observable of interest. Using the optical theorem we may write the B meson decay rate as [18] T = Im [d4x— J
| ClF{n) I2 (B | T(O{F(x)OiF{0)
\ B),
(33)
TOB
Any fi dependence in the Wilson coefficient must be cancelled by the \i dependence of the matrix element. If we choose [i to be order Mw, then the matrix element will also be renormalized at this high scale. This would clearly be a blunder, since the physics responsible for the decay is all taking place at the scale irn,. This blunder will reprise the log, since it will now show up in the matrix element as ln(mt//x). To truly vanquish the log we want to renormalize the four Fermi operator at the lower scale m.(,. But we know how to do this. We use the fact that the bare operator, OB = ZOR, where Z includes both the operator and wave function renormalization, is fi independent ^ then since Z =
+ Pi9)§-g + 7 o 4 F ) OfF = 0,
(34)
Z^/ZoiF
loiF = ^g- i-S0iF
+ 4 x -S2J
(35)
124
where So4F = — |^if^ is the counter-term necessary to renormalize the four Fermi operator, and 82 is the fermionic wave function counter-term •
**
=
—
-
•
W
3 47T e
The solution to the RG equation for O^f is 0
»
4
= O 4 > 0 ) e x p [f™
^
^
2
) 1
#
),
07)
leaving r,
,
04F(mw)
^
r,
,
= 04F(mb)
J l n ( m ^ / A
ln(m2/A2)
(Al/(26<,))
(38)
where bo is the leading order beta function coefficient 11 — 2/3n/, and 7o 4 F = f^O^i = 2Ai — ^ ) - We are now free to evaluate the time order product, renormalized at the low scale mj without fear of generating large logs. Note that we can not claim to be accurate at order as. We can achieve this level of accuracy only if we run at two loops. Since at order as we must keep all terms of the form jj™ ln(<J)™_1. Thus keeping the constant in the matching really bought us nothing. The systematics follow the rule that to achieve accuracy at order an, you must match at order an and run at order n + 1. We have successfully removed that large and irrelevant scale Mw from our calculation. Doing so enabled us to easily sum the large logs. But we are not quite done removing scales. In the decay process itself there are two relevant scales, namely rriQ and AQCD, the scale of hadronic physics. So we would expect that in our matrix elements we could induce logs of the ratio of these scales. But this is not a large log, so should we bother integrating out the scale TUQI The answer is, of course, yes. Because if you recall in the introduction to this section we said that effective field theories do much more than just sum logs. They make approximate symmetries manifest. Just as the scale Mw was irrelevant for the decay process, so is TTIQ. Since rriQ 3> AQCD, the quark effectively acts as a static color source. As far as the light degrees of freedom are concerned, the heavy quark is a brick wall. If you increased its mass, the effects on the bound state would be essentially nil. Thus in the limit where the quark mass is taken to infinity, a new symmetry, "flavor symmetry", emerges. If we have more than one quark with TUQ 3> A, then to leading order in A/mg, the quark decay is flavor independent. Furthermore, the spin of the heavy quark should
125
decouple since the color magnetic dipole moment scales inversely with the mass, so we would expect the emergence of a spin symmetry as well in the large mass limit. 1.6. Heavy Quark Effective
Theory
In this section I will briefly introduce HQET. I wont go into detail as this is now textbook material [18]. But some of the concepts inherent in this theory will be necessary for the second lecture, so I will briefly recap the highlights and refer the reader to [18] for more details. What we would like to do is eliminate the scale ITIQ from the theory. However, we obviously need to keep the heavy quark field in the theory, so how do we keep the field and lose the mass? We begin by studying the dynamics of a heavy meson decay. As I said above the heavy quark should be static up to corrections of order A/TTIQ . We may write the heavy quark momentum as PQ
= "»««" +fcM,
(39)
where v^ is the quarks' 4-velocity ((1, 0) in the rest frame) and k^ is what's known as the "residual momentum" which is of order A. By studying the propagation of a heavy quark in a hadronic environment, It is easy to illustrate that the heavy quarks only role is to be a static color source Consider a heavy quark sitting inside a hadron being constantly bombarded by "soft" gluons(i.e. gluons with momentum of order A), as shown in figure (7). In the large mass limit we may simplify the intermediate propagators as follows Umrf + t + mchu w 7„(l + fl7. (rriQV + k)1 — m^ 2v • k
+
Q{k/m
}
(4Q)
so that the dependence on the heavy quark mass has disappeared. We expect the mass only to appear at sub-leading order in the mass expansion, and in phase space integrals. Furthermore, we also have
Mb) = «(*>) + 0{k/mQ)
(41)
so we may make the replacement 7^ —> v^ and the spin structure becomes trivial. In the real world where we have effectively two heavy quarks with mQ 3> A, charm and bottom, 1 we can see that in the large mass limit we 'The top quark decays too quickly to be relevant to this discussion.
126
mv
Figure 7. Scattering of a heavy quark off of gluons with momenta of order A.
should generate an SU(2)fi avor x SJJ{^)Spin symmetry, and this symmetry should be manifest in our effective theory. We can determine the action for the effective field theory by matching. We consider the full theory propagator, again splitting the momentum into a large and small pieces, and expand in the residual momentum
G™(p) = v
*
«
i{l+i,)
.
(42)
' mQi> + ft-mQ+ie 2v • k + ie Now we need to find an action which has the proper quadratic piece to reproduce this two point function. We do this in two steps: First we notice that the numerator of the effective propagator is a projector onto the large components of the heavy quark spinor. So we should split up the heavy quark spinor into "large" and "small" components. Then, to remove the heavy quark mass yet retain the field itself, we simply extract the mass dependence via a field redefinition [19], q(x) = e~imv-x (hv(x) + Hv(x)) ,
(43)
where
hv(x)=ei™-xl(l
+
Hv{x)=eimv-X\{l~i)q{x).
(44)
Here is where HQET differs from the usual effective theories where we remove entire fields from the action. The fields have labels v. A field with a label v, destroys a particle-* whose four momentum is fixed to be within J
If we were interested in anti-particles we would chose an opposite sign in the re-phasing.
127
A of mvp. That is, the field itself only has Fourier components less than A. This is in accordance with our physical expectations. Once we integrated out the hard modes, the only modes left have momenta of order A whose effect is only to jiggle the heavy quark. So we write the momentum of the heavy quark as p = TTIQV + k, where k is called the "residual momentum" and is of order A. Thus, derivatives acting on this rescaled field are of order A. We may now substitute this parameterization of the full QCD field into the Lagrangian to arrive at LQCD = q(il? ~ mQ)q
= V hv(iv • D)hv „
+ 0[
).
(45)
\mQJ
Notice the sum over velocities. It is needed because we should allow for arbitrary four velocities of the heavy quark, and remember that once we fix a label the momentum is fixed up to some small an amount of 0(A). Technically, this means that there is a super-selection rule, which is defined by the statement that states with four-velocities which differ by more than A exist in different Hilbert spaces. So to allow for differing velocities we have to "integrate in" the different sectors. If we are only interested in processes with no weak decays this point becomes moot since we can go to a frame where the heavy quark is static. However, if we want to look at for instance a b to c transition then a large "external" momentum is injected which drastically changes the velocity of the heavy quark. In the effective theory this is described by the flavor changing current, whose labels reflect the connection between different Hilbert spaces
J(x)=J2 ~KYhbv,{x).
(46)
v,v'
These type of label changing operators will play an important role in the next lecture. We performed a Taylor expansion of our tree level propagator, should we expect this replacement to work even in loop diagrams? The answer is of course, yes, and the reason is (not to flog a dead horse) that the part of the loop integral for which the Taylor expansion is ill suited comes from UV physics, which is local and can always be absorbed into counter terms. Technically, this can be see in a simple fashion. Take any full theory integral, / and write it as I = (I — IEFT) + I
EFT
(47)
128
If / is finite, then Taylor expanding and integrating commute and there is no work to be done, the EFT will reproduce the full theory to any desired order in the inverse mass. If the integral diverges, then all we need to do is differentiate it enough times with respect to the external momentum to make it finite and then perform the Taylor expansion. The value of the full integral will differ from the expanded integral by some polynomial in the external momentum, i.e. a counter-term. Using this type of argument inductively an all orders proof has been provided both for four Fermi theory [27] and HQET [28]. HQET is particularly simple because there is only one relevant IR region, k ~ AQCD • So we expect that Taylor expanding around small loop momenta will correctly reproduce the IR physics. Other theories may have more relevant IR momentum regions as will be discussed later on in these lectures. 1.7. The Method
of
Regions
This line of reasoning can greatly simplify matching calculations. Suppose we wish to match onto to HQET at one loop. The standard recipe is, take the full theory result, expanded in powers of k/rriQ, and subtract the value of the corresponding effective theory result. k But there is actually a way to make life a lot less complicated by utilizing some of the magic of dimensional regularization and asymptotic expansions [29]. Instead of first calculating in the full theory and then expanding, one can Taylor expand at the level of the integrand, assuming that the loop integrals are dominated by momenta much larger than the external momenta. By working in this way we will miss out on non-analytic pieces, but that's OK since we know that they cancel in the matching. How do we know that this method correctly reproduces the matching coefficient? One way to think about it is in terms of the "method of regions" [30]. The idea is that in dimensional regularization the integral will receive contributions from momenta of order the heavy quark mass ("hard modes") as well as momenta of order the residual momenta ("soft modes"). This is true even for divergent integrals since the contribution from modes much larger than the quark mass lead to scaleless integrals which vanish. Let us see how this works in a simple one loop example. Consider the two point function integral dnq /
1
(2TT)" (q2 + ie) ((pQ
1 + k - q)* - m2Q + ie)'
(48)
In addition one needs to worry about how the states are normalized in the two theories.
129
k is the residual momentum of order AQCD- TO extract the hard contribution we assume q is of order TUQ and Taylor expand the integrand up to order I/TUQ dnq x 2 g (i, n 2 ' 2 2 fr- >-*n. (27r) (q + ie) (q - 2p • q + ie) \ q - 2p • q + ie /
(49)
Whereas the soft part of the integral is found by assuming the loop momentum is order k. Keeping only the leading order piece in this region we find f dnq 1 1 Js = J (27r)ra (q2 + ie) 2p-(k~ q) + Ye' ^ which is exactly the HQET contribution. So if we can show that the sum of these two contributions is equal to I, then we've shown that the hard piece is exactly what we would call the matching. The full integral can be performed using text-book methods /
=I^(l-ln^ + 2+(^-1)ln(1^2))'
where p2 = 1 +
PQ 2
m
(51)
. The hard piece is given by
Q
The soft integral has denominators of differing dimensions, so it's helpful to use the identity
i -2^4
anbm
ry ^
I — F ,™-, JI T[n]T[m] 0 1
H /„ + i 26A)" O L \ \ n.-\-rr> (a
_
. ^
'
Then following standard techniques we arrive at k 2
-P«(i_+2_2HZ^)].
16-7T TTIQ \e
(54)
M
Expanding the full theory result (51) and comparing it to the sum of (54) and (52), we find agreement between the two. Notice that both 1^ and Is have spurious divergences which cancel each other. For more complicated theories, such as NRQCD, there are more regions to worry about, and one must be sure that they are all accounted for in order to correctly reproduce the full theory result. Furthermore, one must be careful when matching at two loops using the above trick. Doing so would necessitate keeping e dependence in the effective theory action [31,41].
130
Exercise 1.6: In the last example I have expanded in e before expanding in l/mq. Show that the equality holds true even if you don't expand in e by calculating the full result exactly and then expanding in I/TTOQ. The method of "Asymptotic Expansions" has been proven to work to all orders for both the large momentum and large mass limits for Euclidean momenta. However, there are no known counter-examples to two loops in Minkowski space.
1.8. A Caveat About Scaleless
Integrals
Suppose we chose to match exactly at zero external momentum. We are free to do this if we are not interested in corrections which are sub-leading p. It is often the case that at this point the integrals in the effective theory will be scaleless and thus vanish (this is true in HQET as well as NRQCD). However, this does not mean that we can conclude that the effective theory operator under consideration has no anomalous dimension. To calculate the running in the effective theory we need to account for the fact that the scaleless integral IEFT is giving zero because of a cancellation IEFT
= C (—
- )
.
(55)
Given any logarithmically divergent scaleless integral we can always perform this split algebraically [ ddk J J2^¥
1 _ f ddk ( 1 ~ J (2^)2 \(k2){k2-m2) 167T2 \euv
m2 \ ~ (fc )(fc 2 -m 2 )J 4
eIRJ
However, if and only if, we know that the full theory has no IR poles, can we correctly conclude that the anomalous dimension in the effective theory vanishes when we have scaleless integrals. When the full theory has no IR poles, the effective theory has no IR poles and since in the effective theory (for scaleless integrals) there is also a one to one correspondence of UV and IR poles we may conclude that the effective theory has no UV poles. This is an important point: We can't ignore dimensionless integrals in dimensional regularization in the effective theory unless we are sure that we have correctly accounted for all of the IR poles in the full theory.
131
1.9. Power
Counting
In most effective field theories the power counting is as simple as keeping tr.ack of powers of the mass. However, this will no longer be true in more complicated EFTs. Power counting is trivial in HQET, but I want to go through it pedantically, since such an analysis will bear fruit later. Once we've removed the heavy quark mass, there is only one relevant scale, A. All of the fields have support only over regions of order 1/A so we should expect that dimensional analysis will yield the correct power counting. Let's see if this is true. The leading order action, SHQET
Id4x
= v
'
J2hv(iv
• D)hv,
(57)
v
should scale as 0{1). Each field scales as A 3 ' 2 while the derivative scales as A. Finally, since the fields only have support over region of size A, d4x oc A~ 4 , the leading order action does indeed scale properly. In general, there may be more than one scale involved and the scaling of the fields is instead fixed according to the leading order action. All other operators in the action should scale as positive powers of A, since if there were operators with negative powers, power counting will be jeopardized.
1.10. The End of Our
Calculation
We now have all the ingredients to finish our inclusive B decay calculation. We started with a theory with three scales, Mw, vn-b and A. We eliminated Mw, ran down to nib and are now have the power, via HQET, to remove 77i{,. Removing this scale is crucial, even if we are not interested in the enhanced symmetry. To see this, we have to figure out how to evaluate the time ordered product (33). I will only sketch this final part of the calculation since it's is explained in details in [18]. The main point is that the non-local operator product can be simplified using an operator product expansion (OPE) much as in deep inelastic scattering (DIS) [8] (though the use of the OPE in the present context is not rigorously justified as it is in DIS). The TOP can be expanded into a set of local operators, schematically
/
d^xe-^xO{x)0{Q)
* &(
+ ... (58)
132
where q here is the large scale in the process (for us q ~ m^). The utility of the expansion is predicated on our ability to truncate it.1 As long as when we take the matrix elements of the RHS, we don't introduce a scale of order q upstairs, we can safely ignore terms of higher order in 1/q2. Now we can see that in the case of B decays we will run into exactly this problem since we will be taking matrix elements between B states. So that while each term in the expansion may be down by powers of q, i.e.TOj,,taking the matrix element will generate expectation values which scale with positive powers of 7715. The solution to this problem is simply to go to the effective theory, where the scale m,b is gone and all matrix elements scale only as A. How the OPE is performed in HQET is discussed in [18].
1.11. EFT and MS I want to emphasize the fact that using a mass independent prescription is crucial for this, as well as most other, EFT calculations. In fact, EFT and mass independent prescriptions go usually go hand in hand (lattice effective theories are exceptions to this rule). To see the need for this marriage, suppose we're trying to calculate the running of a gauge coupling. Our prescription can not affect any physical prediction, but a poor choice of prescription can lead to an ill behaved perturbative series. Let's compare and contrast the contributions of a massive fermion to the QED vacuum polarization in the MS and momentum subtraction (ms) schemes.
n
r/=^(^-p2v)(rf"(i-^)in[m2
n ™ = -^{V»VV
- P29^)
/ dx x(l - x) In
p2x{1 x), M2
•p2x(l
M2x(l~x)
(59) (60)
where M is the scale at which we have chosen to define the coupling, so that in the ms scheme IL(—M2) = 0. For p2
133
difference is that the ms scheme is a "decoupling scheme" while MS is not. That is, in the MS massive particles do not decouple in the low energy limit. That is exactly why we should work in an EFT, where we remove the heavy particles by hand. This is the small price we have to pay in order the easily sum the logs. Indeed, in the ms scheme we will need to calculate as{—M2), given its value at some other momenta at which it was measured MQ, and solving the RG equations in this scheme is more difficult. I should also say that even if you work in a mass independent scheme you don't have to use an EFT, it's just that without EFT calculations become more cumbersome since the low energy observables will receive contributions from all the heavy fields which re-introduce large logs. But in the end if you are careful you should always get the same result. Using an EFT will just make your life a lot easier. 1.12. The Force Law in Gauge
Theories
When working in an effective theory, or any theory for that matter, it is easy to get confused if one does not work with physical quantities. Coupling are in general scheme dependent and have no physical meaning, thus it is always best to have in mind some measurable observable when doing a calculation. Often in these lectures instead of talking about measuring a coupling, I will instead talk about measuring the force between static sources. In this way we can be assured that our result is in no way ambiguous. This force or potential is intimately related to what we normally call the coupling, but is a measurable quantity and thus scheme independent. We begin by showing that the static potential can be extracted by calculating the expectation value Z[J] = (0 | T{e-%e$ •^•"(xM/.c*) | o)
(61)
for a stationary charge distribution which turns off adiabatically at ±oo. then using standard arguments (see [2]) lim Z[J] oc eiE^T,
(62)
T—>oo
where E(J) is the vacuum energy in the presence of the source J. It is left to the reader to show that Z[J] is nothing but the expectation value of the gauge invariant time-like Wilson loop W(r, T) = (0 | Tr (p{e$ A»d^)^j | 0) where P stands for path ordering.
(63)
134
For a free Abelian theory, i.e. no matter, things simplify tremendously since the action is quadratic in the gauge field, and hence the functional integral can be done exactly, resulting in f dixd4yJ^(x)A^(x,y)J,'(y)
Z\J] OC e~^
/g^
where AM1/ is the photon propagator. We will be interested in the energy of two static sources, of opposite charge, separated by a spatial distance r. So that Mx)
= e5^5\x
- r) - e^0<53(x)
(65)
then it is straightforward to show that ^
(66)
= 4^-
Diagrammatically, one can arrive at result by using Wicks' theorem on (61). At leading order in the coupling there is one diagram corresponding to one photon exchange, resulting in the integral h
1 f ~ ^ J
(il
dtxdti _i2)2_r2-
. , (67)
At next order there are two Wick contractions, but these sum to give the square of Ii with a combinatoric factor of 1/2. One can then see that subsequent Wick contractions will just fill out the power series expansion of the exponential [32]. Now when we include matter, the only new diagrams which arise correspond to corrections to the photon propagator. Thus all the logarithmic corrections to Coulomb's law will arise from loops in the photonic two point function. Resumming the vacuum polarization graphs results in the corrected form of the potential o2
E{r) =
e fy
/
'
(68)
47IT
where e2(r)e// =
n+ ^>w/tt-
(69)
i 1 + "6^ ln ( r /W) When we go to QCD, things get slightly more complicated but, at least up to two loops, the same reasoning goes through [32,33]. The potential is given by the logarithmically corrected Coulomb potential with the appropriate beta function. However, at three loops one encounters an infrared divergence [34] arising from the fact that there can be intermediate
135
color octet states that live for long times. These colored states can interact with long wavelength gluons since they have non-vanishing dipole moments. However, this should not bother us. The vacuum matrix element contains long distance physics, and thus it need not be infra-red finite in perturbation theory.
2. Lecture II: Non-Relativistic Effective Theories 2.1. The Relevant
Scales and the Free
Action
In the previous lecture we displayed the utility of encapsulating the UV physics in an effective field theory. In our toy model we completely removed a heavy field, while in HQET we kept the field but removed the mass. In both of these cases the non-analytic momentum dependence came from rather simple kinematic configurations. In the toy model all the cuts were generated from intermediate light particles going on shell. In HQET the cuts come from either gluons or heavy quark lines going on shell with kfj, c± A. But there are certain kinematic situations where the momenta configurations which determine the cuts of a Greens function can be more complicated. Consider for instance a non-relativistic bound state such as Hydrogen or quarkonia. m In this case the on-shell external lines in our Greens functions will obey a non-relativistic dispersion relation, leading to the propagator
G^(E,p)=
I
,,
(70) -2
and the cuts will come from a momenta region where E ~ ^ . Notice that, as opposed to the case of our toy model and HQET where there was only one relevant low energy scale, TUL and A, respectively, a non-relativistic theory contains two scales namely E ~ mv2 and p ~ mv. That is, the typical size of the bound state is the Bohr radius TB — l/(mv) and the typical time scale is the Rydberg R ~ 1/mv2. The existence of these two scales complicates matters. As in our previous cases we'd like to eliminate all the large scales from the theory, leaving a nice simple theory with only one low energy scale. Unfortunately, as we shall see this is not possible due m For the quarkonia case we will assume that the bound state is Coulombic such that mv2 > > A. This hierarchy most probably does not hold for the case of the J/tp and may not for the T either. Attempts at alternate power countings which arise when this criteria are not met can be found in [35].
136
to the fact that the two scales mv and mv2 are correlated, and must exist simultaneously. Suppose we try to treat the scale mv in the same way that we treated the mass in HQET, i.e. by rescaling the quark fields. This would indeed remove it from the theory, and all derivatives would scale homogeneously as mv2. However, since spatial fluctuations on the scale mv are crucial to the dynamics of the bound state, we know that this can't be the end of the story as far as this scale is concerned. In fact, as we shall see, the Coulomb potential is built from the exchange of gluons with three momentum of order mv. So if we eliminate the modes with momenta of order mv by re-phasing the field we had better figure out how to put (integrate) them back in. At first, this seems like a silly thing to do, but we will see that this formal manipulation will simplify the construction of the effective theory since all spatial derivatives will scale homogeneously. If we did not do this re-phasing we would not have manifest power counting, as we shall see. But we are jumping the gun a little. Let us treat one scale at a time from top down. We begin by first eliminating the mass from theory as we did in HQET by a simple rescaling of the field Q = e - i m « t ( ^ + x),
(71)
,& = e ' m « t i ( l + 7 o ) Q ,
(72)
X= eim«^(l-7o)Q,
(73)
where
are the large and small two spinors of the full Dirac quark spinor, respectively. As opposed to HQET, there is now a preferred frame, namely the center of mass frame. So we will assume that we are working in this frame from now on. Thus when we rescale the field we won't sum over all four velocities, as we did in HQET. Also I won't bother labelling the fields by their four velocity, as there is only one relevant label velocity (1,0). If we wanted to allow for weak decays we would have to include a sum over four-momenta labels as we did in HQET. Now we can separate the next largest scale, mv, from the other scales in the theory by writing
V = Ee_i?'^W
(74)
p
where the label on the field fixes the three momentum to be order mv. Spatial derivatives acting on ipp(x) are of order the residual three momenta
137
raw2. This way of splitting up momenta into a large label piece and a residual momentum is depicted in figure 8.
<
mv
*•
P
>-
** 2
mv Figure 8. This diagram depicts how the momenta are divided by the labelling process. The largest label Vy. has been frozen to be (1, 0, 0, 0). The three momenta are split into a large label p oi order mv, and a small residual momenta of order mv2 depicted by the smallest boxes.
The free piece of the action can then be read off by substituting the expansions (71) and (74) into the full QCD action, solving for the small component x m terms of the large component ip, and keeping only the leading order terms in an expansion in the small parameter v, L
° = E <4 (ido - ~)
^5 + (^ - 0-
(75)
p
Notice that we have dropped all spatial derivative terms, such as p • <9, as they are higher order in v. Remember that the p which appears in the action is a C number and not an operator. £ is the large component of the
138
anti-quark field, which arises when we re-phase with the opposite sign in (60), and its action on the Fock space is to destroy an anti-quark.
(E.p + k)
•
(E.-p-k)
_ ^
,
"V
fr
^
(E.p'+k')
(E.-p'-k')
Figure 9. Full theory matching which generates the Coulomb interaction. The momenta are split up into a large piece (p) which is order mv and a residual piece k which is of order mv2. The large piece corresponds to the label of the field.
2.2.
Interactions
The interactions can be fixed by a matching calculations, though some interactions are more simply read off by gauge invariance, as we shall see. Let's start by considering the t-channel exchange of a gluon between a quark and anti-quark in the full theory as shown in figure 9. Since the initial and final state quarks are on shell, the gluon momentum must scale as (mn 2 , mv). We split the external three momenta into a piece of order mv, corresponding to the label of the field, and a residual momenta fc, which if of order mv2. We then expand the full theory result in v and write the full theory spinors in terms of the two spinors (72). The leading order piece generates the operator
Lc = 5 2 E 4 ' T ^ P ( ^ 7 ) l C l p , n - p .
(76)
This operator is nothing but the Coulomb potential. It is called a "potential" because the interaction is non-local in space but instantaneous in time. Notice that Lc includes a sum over all possible labels. This is in stark contrast to HQET where an external current is needed to change labels. Here in NRQCD, there are terms in the action which themselves change labels. What we have done seems a little perverse at first. We have integrated out
139
modes (via a field rescaling) only to integrate them back in by summing over labels. However, we will see that there is a method to the madness. Now let us power count to see if the operator (76) we matched onto should appear in the leading order Lagrangian. To do so we first need to determine how the fields scale. The easiest way to do this is to use the fact that the kinetic piece should be order one in our power counting. So, following the line of reasoning developed in section 1.9, we first power count the measure in the action. The quark field has support over 1/v2 in the time direction11 and order 1/v in the spatial directions so we conclude that d4x ~ v~5 and ip ~ v3/2. In this way the kinetic piece (75) scales as v°. Now let us consider the operator (76). The measure again scales the same way as in the kinetic piece since the operator contains only heavy quarks, so that Oc~v-5{v3/2)\r2~v-\
(77)
Thus Oc certainly should be treated as a leading order piece in the Lagrangian. Indeed, the fact that it actually is enhanced relative to kinetic piece is worrisome as it could lead us to the conclusion that there is no sensible v —> 0 limit, which is the limit in which the leading order action is exact. We will come back to this issue later. For now we will conclude that the Coulomb potential will have to be treated non-perturbatively in our theory. The operator can be interpreted in terms of the exchange of a non-propagating Coulomb gluon. If we wish we can re-write this operator in terms of an auxiliary field (i.e. a field without a kinetic term) [37]
Lc = E -9(4^P> + 4'V) A V' + ^ o , ) 2 •
(78)
Writing things in this way will make our relevant degrees of freedom more explicit when we discuss NRQCD in terms of the method of regions as discussed in (1.7). But this is just a formal manipulation. In general, "off shell" modes will not appear in the Lagrangian. 2.3. The Multipole
Expansion
and Ultra-Soft
Modes
What interactions are there in the one quark sector? We should allow for all interactions which leave the fields on-shell to within order mv2, which is the scale of their residual momentum, as these modes will certainly generate non-analyticity in our amplitudes. One obvious interaction we can have is n
All units are scaled to m.
140
Figure 10. Self energy diagram is NRQCD. The labelling technique automatically implements the dipole expansion.
of the usual form ip^Aip, if A is a gluon which carries momentum scaling as (mv2,mv2). In fact, gauge invariance seems to lead us to this type of interaction, since we would expect that the do in (75) should be elevated to a covariant derivative. Let's see if this new interaction does indeed to correspond to (mv2,mv2) gluons. The self energy diagram in figure 10 is given by
i£ = g2CF I ,f9,. . - 1 . .
^
.
(79)
Now we will show that the gluon running through the loop does have momentum which scales as (mv2,mv2). The important point to note here is that the fermion propagator does not carry any of the three momentum of the gluon. The p 2 piece is just the label of the external line. This is an implementation of a dipole expansion [38,39] Aa0(t, x) = Aa0(t, 0) + x • VAa0(t, 6) + • • • .
(80)
Exercise 2.1: Working with a coordinate space dipole expanded Lagrangian reproduce equation (79) for the self-energy. It is interesting that the act of separating the scales via the re-labelling technique has automatically generated a dipole expansion. This demonstrates a persistent fact in effective field theories. If you have properly separated the scales, then each diagram in the effective theory should scale homogeneously in the expansion parameter. In order for this to be true any given integral should be dominated by just one region of momenta. Let us see how this happens in our self-energy diagram. Doing the integral by contours we see that the go ~ | g | ~ mv2, there are no other relevant regions. Note that if we had not dipole expanded then the integral would have received a contribution where g was of order m, and the integral would not scale homogeneously in v. This would spoil the power counting in the
141
theory [39]. Gluons which have momenta of order (mv2,mv2) are called "Ultra-Soft (US)", and are on shell-modes which have the usual F2 kinetic term in the Lagrangian. These modes are sometimes also referred to as "radiation" or "dynamical" gluons which can be thought of as contributing to higher Fock states in the onia. Note, however, that the A0 US mode can be eliminated from the leading order action by redefining the quark field in the following fashion
tf(x) = Pe19^
Ao x+ dx
^ ^ i;(x),
(81)
where P stands for path ordering and n^ is the time like unit vector (1,0). The AQ US gluons will then reappear in the action at higher order in v. Thus without doing any calculations we can conclude that the interactions due to AQ US modes must vanish at leading order. There are additional US spatially polarized gluons (A) which play an important role at sub-leading order in v. We can determine the coupling of these gluons in a very simple way using what is known as reparameterization invariance [42] (RPI). The idea behind RPI is quite simple, yet its ramifications are powerful. The point is that when we pulled out the three momenta from the heavy quark field, there was an ambiguity.0 We could just as easily rescaled by e(imv + k) where k is order mv2. Thus our action should be independent under small shifts in the label. This just means that every time we see a p it should be accompanied by a d. Then by gauge invariance we know that we should make the replacement p - • p + it)
(82)
everywhere we see p. Notice that this form has to hold to all order in perturbation theory. This is quite a useful piece of information, as it tells us that many operators in our action will have vanishing anomalous dimension. The net effect of this substitution is the generation of subleading terms in the dipole expansion (80). For instance, the kinetic term p 2 /(2m) will generate the subleading dipole interaction V ^ ( P ' A/m)^>. Note that D = d — igA, scales as v2, since when the derivative acts on the heavy quark field it brings down a factor of the residual momentum which is of order v2, and, as you can prove for yourself, the ultra-soft gluon field scales as v2 as well.
°A similar ambiguity exists in the H Q E T rescaling. See [18] for a discussion.
142
f
\
Figure 11. A graph which one might have hoped would induce a running potential, which does not exist in the effective theory.
2.4. Soft
Modes
So now we have shown that there are two types of gluonic modes which contribute to the non-analytic behavior of the low energy theory: potential gluons (which are not dynamical), whose momenta scale as (mv2,mv), and "ultra-soft" (US) gluons whose momenta scale as (mv2, mv2). But this can not be the end of the story. In particular, we know that the Coulomb potential should run even once we are below the scale m, due to the non-Abelian nature of the theory. We might expect that a diagram such as figure 11 could account for the running of the coupling/potential. However, this diagram does not exist in the effective theory, since the Coulomb gluon is not a propagating degree of freedom (it is simple to show that this diagram vanishes). The Coulomb gluon sometimes called an "off-shell" mode because it can never satisfy the gluon dispersion relation g\ = g 2 , and thus never appears as an external state. To generate a running Coulomb potential we must include a new set of propagating degrees of freedom [30,40], namely, "soft gluons", whose momenta scale as (mv,mv). How could such degrees of freedom interact with the heavy quarks? We notice that because their energy scales as mv, any such interaction would throw the heavy quark off-shell by an amount of order mv. So we know that we need at least two interactions. The simplest such interactions are shown in figure 12. Now since these modes have a "large" component, in both their energy and momentum, we need to rescale the field so that all derivatives acting on the field scale as the lowest scale in the theory, mv2. So as in the case of the heavy quark field we write
Wx) = T,e~i9'XAMx)9
(83)
143
Figure 12. Leading order matching graphs between heavy quarks and "soft" gluonic modes with momenta of order (mv, mo). Not shown are the similar diagrams with ghost fields,
Notice the field is now labelled by a four momentum. Now we match the Compton graphs in figure 12 onto the effective theory. To do so we split the external momenta of the gluon fields into a label piece and a small residual momentum QP,
= q„ + k„ ,
(84)
and expand the diagram in powers of v. Keeping only the leading order result we find r int soft
L
4
™* £
\^4>[<>,<]^
q,q'v,p' At
gv0(q'-P+P'T-
SM0 ( « - P +P'r + ^iq-q')^ (p' - p )
" (p' - 0 p)2 ^ ^ '
Cq
^
rA,^A^^
2
f
+
^ ^
X
'
T
^
f
i
(85)
where I have also included the contribution from the ghosts cq, which are not shown in the figure. As we will see, this interaction will be responsible for the running of the Coulomb potential. Note that it vanishes for the
144
Abelian case, which is consistent with the fact that there is no change in the force law below the scale of the electron mass in QED. This cancellation would not arise if we did not drop the ie in the denominator. Keeping the ie would mean including a contribution where the gluons are potential and not soft. To see this write the intermediate fermion propagator as
- 4 - 7 - = P ( - ) - in5{q0) •
(86)
The delta function forces the gluon to have vanishing energy, i.e. it becomes a potential gluon [30]. In the effective theory this potential contribution is accounted for by operators which describe the scattering of soft modes off of a four fermion potential (see [45] for details). Furthermore, to include the effects of the "massless" quarks we will need to include a soft fermion field for each such flavor. So now we have two propagating gluonic degrees of freedom, the soft (mv,mv) and ultra-soft (US) (mv2,mv2). The kinetic terms for these fields are LKB
= -\F»VF^
+ J2
I P^P
~P"A$
I2 •
(87)
p
Here the fields strengths are composed of purely US gluons igF^ = [D^jD"]. While the second term in (87) generates the propagator for the soft gluons. We may treat the US field as a background gauge field. Given the relative time scales of these two modes, the US modes are frozen, as far as the soft gluons are concerned Thus in a background field formalism (see [2] for a review), we may gauge fix the US fields and the soft fields independently. The US gauge invariance is manifest while the soft gauge invariance is only seen when combined with re-parameterization invariance. Note that there will also be soft (labelled) ghost fields as well. The full soft Lagrangian, including ghosts, can be found in [41]. We are now prepared to see if the soft interaction Lagrangian is leading order in v. Each soft gluon field scales as v, as can be read off from (87), while each fermion field gives a factor of v 3 / 2 . How does the measure scale? The fermion has (temporal,spatial) support on the scale (l/(i> 2 ), l/(v)), whereas the gauge field has support only on scales of order 1/v. Thus, the measure scales as l/(i>) 4 , since outside this volume the integrand vanishes as depicted in figure 13. Thus the soft interaction Lagrangian I j " ' , t scales as v° and is indeed leading order. However, since this interaction is also order as we need not treat it non-perturbatively.
145
Figure 13. The soft gluons and heavy quarks have differing spatio-temporal extent, The overlap regions, which determines the scaling of the measure in the action is of order (y, v)
2.5. Calculating
Loops in
NRQCD
Now let us take stock in what we have built. So far we have a leading order action which contains the kinetic pieces for the fermions and gluons (75) and (87), as well as two leading order interactions (78) and (85). If we want to match beyond leading order in a, we need to know how to calculate using our effective theory. Calculating loop graphs seems rather complicated given that we will have loops with sums over labels. However, it will turn out in the end to be quite simple. As a first example suppose we're interested in a loop consisting of two insertions of Lc, as shown in figure 14. The external fermions have energy E and momenta ± p , r. In the effective theory, the external fermions have soft momentum (labels) ± p , ±r, and the ultra-soft momentum (E, 0). The intermediate fermions in the effective theory have soft momentum ±q, and ultra-soft momentum (E, 0) ± k. The graph in the effective theory is proportional to the integral
l
Coul
rf
1 ( p - q ) 2 ( ? - q ) 2 k° + E-q2/2m
?/ V
-k° + E-(i2/2m
+ ie'
+ ie (88)
146
(E.O)+k+(0.q) (E.O)+(0.p)
(E.O)+(0.r)
(E.O)+(0.-r)
(E.O)+(0.-p) (E.O)-k+(0,-q)
Figure 14. A one loop graph in the effective theory with two insertions of the coulomb interaction (solid dot).
There is an integral over the ultra-soft energy k°, the ultra-soft momentum k, and a sum over the soft momentum q. Summing over q and integrating over k is equivalent to integrating over the entire momentum space. Thus one can replace Eq. (88) by 1 1 1 Icoui = Id q 2 2 ( p - q ) ( r - q ) q° + E - q 2 /2m + ie /• X
1 -q° + E-q2/2m
(89)
+ ie'
where
dk° ^dq°,
Yl [ dk~+ f dq.
(90)
Soft loops are treated in a similar fashion. Consider the diagram with the insertion of two soft interactions as shown in figure 15. It gives a
(E.0)+(0.p)
(E.0)+(0.r)
(E.0)+(0.-p)
(E.0M0.-T)
Figure 15. A one loop graph in the effective theory with two insertions of the soft interaction (solid dot).
147
contribution of the form
/wt-E/d4*^^
(j5_f)2 (g0)2_(|2 ( g 0 ) 2 _ W +
^_ f ) 2 ' ( 9 1 )
where I have used the last interaction in Eq. (85) for the soft vertices. The sum on q is over a four-vector. As in the case of the potential loops, we make the replacement
J dAk-> f d4q,
(92)
Q
where the replacement must be done for all four components of q, since soft gluons carry a four-vector label q. There is a much simpler way to arrive at the effective theory integrand. Just take the full theory graphs, assume the proper scaling for the gluon loop momentum, i.e. soft, potential or US, and just keep the leading order term in each propagator. 2.6. Matching
at Subleading
Orders in v
Let us now consider to matching at higher orders in v, but still at tree level. We can guess the first correction to the kinetic energy from the dispersion relation
Formally this piece can be derived simply by keeping the sub-leading terms in the expansion of the QCD action in terms of the non-relativistic fields (74). The more relevant correction comes from higher order terms in the multipole expansion,
The ultra-soft gluon field inside D scales as v2, as does the derivative since it brings down a factor of the residual momentum of the heavy quark. Now let's consider the corrections to the quark/anti-quark potential. These are derived by expanding the full theory graph in figure 9 to higher orders in v.
Exercise 2.2: Show that, at order asv the subleading potentials generated by T-channel gluon exchange are given by
148
V{1)
= {TA ® TA)
^ C + ^2 S 2 TO2k2
m
+
^A(p',p) 2 v m
V,
'
T(k)
(95)
where k = p — p ' and • S-(p'xp)
A(p',p)
T(k)
=3l-S2
1
=;
k2
3 k • CTik • $2
and
V h f •• Vt
4:Tras 3 iras
(96)
(E.P1)
(E.P2)
(E.-P1)
(E-- P2)
Figure 16.
Tree level full theory diagram contributing to the potential.
T h e annihilation diagram in figure 16 generates terms of order asv in the potential. Using Fierz identities and charge conjugation, the contribution of this diagram can be transformed into the basis in Eq. (95) and give additional contributions to t h e matching. Only Vhf receives a non-zero contributions at tree level from annihilation: Vhf,a = —iras,
(97)
149
where the subscript a stands for annihilation. In addition this diagram generates the a hyper-fine interaction with a different color structure v(i)
jy« = (lxl)^fs2 with V^f = OL-K ^ 2.7. Matching
(98)
. The color factor arises from the Fierz rearrangement.
at Subleading
Orders in a
Let's now consider matching beyond tree level. But before we plunge into a calculation we must address an intricacy of NRQCD which I touched upon previously. In particular, we noticed that the Coulomb potential scales as 1/v. On the face of it, this presents a serious problem to the theory for the following reason. Suppose we wish to calculate the matrix element of some operator in the effective theory, (ip \ O \ tp). at order in vk. In this case we have (iP | O | V) = / f i i • • • (Txn x(iP\T(OH1(x2)...Hn(xn)exp(i
I dAxLG{x))
| if,) (99)
where Hi are terms in the sub-leading Hamiltonian such that their net order in v is vk. Given that the contribution from the Coulomb potential is of order 1/v and must be treated non-perturbatively, it appears that the time ordered product will not have a definite scaling in v. However, we are saved by the fact that while the Coulomb potential is of order 1/v it is also of order a. Furthermore, in a Coulombic state the Virial theorem tells us that n aAl/r mv2 ~ -^-^
~ mv) r
/„„„^ (100)
so that as(mv) — v.p So when we calculate matrix elements we should treat the Coulomb potential to be order one. However, when we match, we must account for the fact that the Coulomb potential can enhance a sub-leading operator, at the cost of a power of as. For instance, if we want to match at order a2s, then we have to include contributions of the form (L^LD^al p
(101)
If we are not in a Coulombic state, for instance we could be looking at threshold production, then we must have a < v.
150
where l/ 2 ) is an order v2a°s piece of the Lagrangian. As a practical example let us consider the matching of a production current at order v°. This matching calculation is relevant to top-quark threshold production [36]. At tree level we match by expanding the full QCD current in terms of the NRQCD fields, 97*9 = E ^liaiX-P
(102)
+
This operator produces a quark/anti-quark pair in a relative 3S\ state. Exercise 2.3: Show that at order v2 one generates the operators
(103)
O^E^P-^W-P-
(104)
°2 = E 4 ( P V X V
a.
b.
Figure 17. One loop matching diagrams in the full and effective theory. In the effective theory there are additional diagrams involving temporal US gluons which vanish.
At one loop we must calculate the graphs shown in figure 17. Since the full theory current is conserved, it has vanishing anomalous dimension once the wave function renormalization is included. We can simplify the matching calculation by working at threshold. In this way the effective
151
theory diagram vanishes, since it is scaleless. Calculating the full theory diagram gives Figa = f i r ' x
—
" 1 euv
2
31n(m 2 /V)-9
CIR
(105)
where I have explicitly separated the UV from the IR divergences. Adding the contribution from the wave function renormalization, all the UV and IR poles cancel. We then find the matching coefficient to be C=(1-2CF-).
(106)
IT
You should be able to show for yourself that this short cut to calculating the matching coefficient is identical to calculating the hard piece of the graph using the method of regions. Notice that at one loop the current has no anomalous dimension. We can conclude this from the fact that all of the IR poles in the full theory cancelled (see Section 1.8.2) In general we would not expect this since in the effective theory the current does not generate a symmetry. In fact we will see that at two loops we will generate an anomalous dimension. At two loops one must account for the possible Coulomb enhancement discussed previously in this section. So if we concerned with contributions of order a^v°, we must include all possible operator insertion such that the net power is v°. This means we must also include any order v°a2s potentials which might be generated. At this order the full theory generates the v° potential V<°> = Vk^m |k |
(107)
where Vk=o?{^--CACF),
(108)
and I have projected the potential onto the color singlet channel since that is all we need when calculating the production current. We must include one insertion of this potential in addition to insertions of the order v potentials already discussed in the previous section. The relevant diagrams are shown in figure 18. In addition there are corrections which contain Coulomb singularities 1/v, i.e. those contributions whose net scaling has inverse powers of v. However, these won't contribute to the matching since they will cancel in the full and effective theory. The purpose of this example was just
152
Figure 18. Diagrams contributing to the matching of the current at order a2v°. The large hatched circle is the current insertion. The small filled circle is a Coulomb potential while the square represents insertions of order v1 potential. The cross corresponds to the insertion of the p 4 / ( 8 m 3 ) operator. The triangle is the order v° potential.
to illustrate that when you match you must remember that sub-leading operators can be enhanced due to the 1/v nature of the Coulomb potential. To my knowledge, as of the time of this writing, this subtlety is unique to NRQCD. It is interesting to note that had we done the two loop calculation using HQET propagators we would get a vanishing anomalous dimension. This is most easily seen in AQ = 0 gauge.
Exercise 2.4: Prove that in the MS scheme the anomalous dimensions of the production current is gauge invariant. To prove this write down two equations for the current. One imposing its independence from fi and the other its independence of the gauge fixing parameter a. Then commute the two equations to derive a set of relations which allows you to complete the proof. If you get stuck consult the original reference [43].
The reason HQET gets it wrong lies in the theory's inability to properly describe fermion-fermion propagation. What happens is that HQET misses
153
out on the Coulomb singularity. This fact emphasizes an important point about NRQCD. You can't treat the scales mv and mv2 independently. Usually we would argue that between the scales m and mv, mv2 should be treatable as a perturbation, much as we would treat a light quark mass when working at a scale E 3> mq. But the fact that we need to keep the kinetic piece of the action at leading order is telling us that both mv and mv2 must exist simultaneously in the theory. After a brief diversion we will see the consequences of this correlation of scales.
2.8. Summing (VRG)
Logs: The Velocity Renormalization
Group
You have probably noticed that I have been careful not to specify the scale at which to evaluate the running couplings as(u). Such a specification implies we have control of the logarithmic corrections, which I will only now discuss. As shown in the previous lecture the recipe for summing large logs is typically straightforward. Suppose we have a theory with two "large" (relative to the scale at which experiments are to be performed E) scales mi > mi. In a general diagram we would expect to generate logs of the form Inimi/E), ln(mi/E), ln(mi/m2). We would sum these large logs in steps: 1) Match at the scale mi onto a theory where (p± has been integrated out. 2) Use the RG to run down to the scale m^. 3) Match onto a theory where $2 has been integrated out. 4) Run down to the scale of the experiment. We will call this procedure "two stage running". In the case of NRQCD, where we have two large scales m and mv and a low energy scale of mv2, we will get logs of the form \a(m/mv2) and \s\{m/mv). So we might be tempted to follow the standard machinery and turn the crank. Match at m, run to mv integrated modes of momenta mv and then run down to the scale mv2. However, NRQCD is a lot more interesting than that. The basic point is that the two scales mv and mv2 are correlated. They shouldn't be treated independently, since they coexist in the fermionic propagator. If we were to follow the two stage running in NRQCD, we would first match at m onto an HQET like theory by rescaling the fields only by their masses. In this case we should allow for quark virtualities q2 < m2Q and one might wish to treat ^ as a perturbation leaving the HQET form of the propagators. But as we discussed above, we know that this will give the wrong anomalous dimension for the current [21]. Instead, in NRQCD what we must do is match directly, at the scale m, onto the theory in which E ~ %—. So we can see immediately that the two stage
154
running will not be appropriate. Instead we do what is known as "one stage running". Given that we have to match directly onto a theory in which two scales exist simultaneously, how can we sum the large logs? In general, potential and soft loops will generate ln(VmE/n), while US loops will introduce logs of the form \n(E/fi). So in one-stage running we introduce two different H parameters, /is and nus- However, we are not free to choose /xs and pus independently, since /x| = m/ius- Instead we should write us = mv and nus = mis2, and think of a subtraction velocity v instead of a subtraction scale /x. This is the basis of what is known as the "velocity renormalization group" (VRG) [21]. The renormalization group equation is then derived using the fact that bare quantities are independent of the subtraction velocity, i.e. V-^-GB
= 0.
(109)
dv Then by solving these equations and running from v = 1 to v = v, we can sum all the large logs. How does the VRG differ from the standard RG [44]? In VRG we have d .dus d dfius d d d 1/ T~=l/^^r~ 1— + ^ — 1 '=^s^—+2/2US-, . (110) dv dv d/j,s dv d^us dfis d^us Note, the crucial factor of 2. In the two-stage method one would first integrate from m to mv using, 7s + "/us, then continue down to the scale mv2, using only jus • Whereas in one-stage one integrates 75 + 2"/us from v — 1 to v = v. If we assume that the anomalous dimensions are constant and don't run, then in the two stage running we would find for the running of some generic Wilson coefficient C(mv2) — C(m) = (75 + jus) \r\.{mv/m) + jus m(mi; 2 /'(mv)), (111) while in one-stage we have C(mv2) - C(m) = (7s + 2luS)
ln(v).
(112)
So we have agreement. However, in general the anomalous dimensions depend upon some couplings which are scale dependent. This scale dependence will lead to a discrepancy between the two methods at higher orders. Suppose we are interested in the anomalous dimensions ji of some set of Wilson coefficients [/j.q These anomalous dimensions will, in general, q
Operators and Wilson coefficients have opposite anomalous dimension since their product is RG invariant.
155
depend on t/j themselves. 7* = li{Ui(lA)-
(113)
7i will be some function of t = \n(/j,) = ln(w), which we can expand1"
*<'>-*<°>+<(KL+= 7.(0) + ' ( ^ )
M
•
+ -
("4)
Suppose the [/$ have both soft and ultra-soft anomalous dimension (this typically will occur at two loops). Then in one-stage running, integrating the anomalous dimensions will give S , n„,US\
Jo
J2 (ihfs
, &y?
us
d(21rs)..s , dW)
us
(115) whereas two-stage running yields ln(u)
jls'us\t')dt'+
0
H7
/-ln(w 2 )
^s\t')dt'
=
Jln(u)
*
+
7i
J +
2 ^(7 f c 7 f c + 5C/ f e l 7 f e
J +
5?7 fc
lk
+
3Uk
{lk
V (116)
and we see that the cross terms differ by a factor of two. Let us now return to the question of the running of the Coulomb potential. This running is particularly simple because the renormalization only involves soft loops. The matching coefficient is simply 4iras(m), and the running is generated by figure (15), with each vertex giving a factor of Vc
«, 1 5 _g ( W ) (H C j (i + h (g)) + ^ ) . r
(117)
This expansion is only for the purposes of illustrating the difference between one-stage and two-stage running.
156
There are also corrections stemming from US temporal gluon loops, but these vanish for reasons which were previously discussed. Calling the coefficient of the Coulomb operator in the action Vc, and using the fact that its bare value (VCB) is independent of v, we have
and we are left with v±Vc
= -2(30a2s{v),
(119)
where I've used /?o = ^ C U to make it clear that Vc obeys the same equation as the gauge coupling. Using our matching result for an initial condition, along with the explicit form for a3(u), we find Vc{mv) =A-Kas(mv).
(120)
At this order the running of the potential is identical to the running of the beta function. The large logs can be avoided by choosing a(\ k |) in the potential. This agrees with our intuitive picture of the force law (Wilson loop) getting quantum corrections due to the running of the coupling. However, this correspondence does not persist at higher orders as the running of the Coulomb potential includes effects from higher order potentials which vanish in the static limit [48,47].
Exercise 2.5: Show that the ^-CA gets replaced by the the full beta function (3Q = ^-CA — | n / i once nj soft light quark fields, q, are introduced. To begin this calculation first show that the soft Lagrangian gets an additional piece given by L's = -92
£
(7g73Bp(4'rA^)(^'7oT%))+(^^X)-
(121)
There is a simple way to determine when, where and how the fis a n d Hus show up. We just make sure that in 4 — 2e dimensions, the operators have the same v scalings as they would in 4 dimensions. For instance, in the Coulomb potential the matching comes with a factor of g2, which arises from an exchange of a Coulomb gluon. In 4 —2e dimensions, should we write this as g2^2g or g2fJ>2jS'? The coupling should still have units (mass)e but the v scaling appears ambiguous until we impose the fact that the operator should scale as v~x. The fermion fields now scale as v3^2~e and we have a factor of 1/v2 from the Wilson (label) coefficient. But now the measure
157
scales as v~2v~3+2c. So to insure that the operator scales as v'1 we should write g2 —> g2^\f. If we consider an operator where a US gluon attaches to the potential [41], the scaling of the measure won't change (see section 2.4) but since the US gluon scales as v2~2e then we should include a factor
2.9. The Lamb Shift in QED I would now like to utilize this formalism in a simple setting, namely, the Lamb shift in QED. This is a beautiful example because, not only does the formalism simplify the calculation, but it also provides one with immediate information about sub-leading orders, as we shall see. Let me remind you that the Lamb shift is the splitting between the 2SJ=1/2 and 2PJ=1/2 levels of Hydrogen. Recall that these levels are "accidentally" degenerate up to a2 corrections. Their splitting comes in at relative order a 3 , corresponding to a over all energy shift of order a 5 . This splitting is dominated by the so-called "Bethe log", a5 In v. Prom an effective field theory point of view, all logs are RG logs. If they look like "long distance" logs, its only because the scales have not been properly separated, i.e. you haven't written down the proper effective field theory. Thus, we should be able to also resum these logs if there are more of them. In the case of the "Bethe log" one can ask the question, is this log just a first term in a series? i.e. AE « a5 In v(l + a In v • a2 In2 v + ..
•
)
•
(122)
Figure 19. The QED diagram responsible for the Lamb shift. The photon over the top is non-instantaneous, i.e. ultra-soft, while the ladder gluons are Coulombic and must be resummed.
158
Typically the Lamb shift comes about from a low energy matrix element involving diagrams as shown in figure 19. But in NRQCD we know that there should be no large logs in any low energy matrix element. The log should arise from running. Let's see how this comes about. Recall that the leading order energy levels scale as v2, or using the Coulombic counting, a oc v, a2. So one would expect at order v4 there would be a splitting due to the subleading contribution to the Hamiltonian (123) and (107). However, we must account for the fact that in Hydrogen the masses of the constituents are not equal. In fact, we may take the mass of the proton to infinity.
Exercises 2.6: Show that for hydrogen we have 'V„r
yd)
•A(p',p) + ^ 2
(123)
where V2 is a contact delta function potential which vanishes in the equal mass case (considered in (95)). Matching at v = 1 we find ira I 1 1 \ not V2(y = !) = —-[ w —r. (124) K ' 2 \me mpj 2ml In addition, at this order, we should include corrections from the operator
4-i = £ 4 ( | ^ P + W-x)-
(125)
p
Thus the mass splitting is given by AE = ( 2 5 1 / 2 I L%>n + V2 12Sl/2) - ( 2 P 1/2 I L™n + ^ f A(p', p) 12P1/2).(126) Calculating these matrix elements is nothing more than solving the Coulomb problem to first order in perturbation theory, which we can solve quantum mechanically [46]. That is why it is simple to see that V2 does not contribute to the energy shift in the P wave since its wave function vanishes at the origin. The degeneracy is only split once we consider the (2) running of V2 and V\. L\(n is not renormalized due to reparameterization invariance but V2 gets an anomalous dimensions from the diagram shown in figure 20. A short calculation yields the result 2a ^ = 3 W
, N (12?)
159
Figure 20. The one loop diagram which generates an anomalous dimension for V2. The heavy lines are protons while the photon line with a line through it is ultra-soft. In the limit where the proton mass goes to infinity there is no soft anomalous dimension for V2 • The filled circle is a Coulomb vertex, while the US gluon couples to the electron via the order v dipole operator (94).
where Vc — —Aita. Because we are looking at a one loop diagram involving only US loops the VRG reduces to the usual RG (except of course for the factor of two arising from the fact that nus = mi/2) and we have v ^V2 = 7 2 (128) dv since in QED the coupling doesn't run below the scale me, the solution to the RG equation is trivial V2(v = v)=j2Vc\n{v) + V2{l). (129) We thus see that the series does indeed terminate. QED has a hard time generating logs because the coupling freezes below the scale me. At leading order the spin orbit potential has a vanishing anomalous dimension, so the energy splitting can be determined by taking the matrix element (126), but this time renormalized at the scale v = v, using =72Khu»|V(0)|2
(130)
and
| 2 = (i^2!
(13i)
AE = ~^-\n{v).
(132)
i m
we find the famous result
Using effective field theory we can go a step further and ask questions about the next series of logs, a5(aln(v)+a2ln2{v)
+ ...).
(133) s
It turns out [49], that this series terminates as well at order a In (v).
160
2.10. States
in the Effective
Theory
Before closing out this lecture I would like to emphasize an important point about the states in any effective theory. If we wish to keep our ducks in a row we should be sure to treat the states of the effective theory as eigenstates of the lowest order piece of the action. Power corrections to the states come from taking time ordered products in the low energy matrix elements. This is exactly what we do in quantum mechanics. We perturb around the lowest order wave functions. In this way we can keep track of various power law corrections. It is a simple exercise to see exactly how this works generically. Suppose we are considering a correction to the expectation value for some transition operator T = =
J A(*i(Pi) I T(O(0)ifi(y)) | *2(p2)>
(134)
where $i are eigenstates of HQ. The two time orderings correspond to corrections to $ i and $2- Let's consider the contribution from yo < 0. Using the representation of the theta function 0(-yo) = ~ / dr—— in J-oo T + ie and inserting a complete set of states yields
T = Yl I d4y— f
(135)
dTeil")(pa-1'»-T)(*i(pi) | O(0) | n'0') ;>(°>|ffi(0)|S2(p2))_
E2 — En (136) Then recalling from basic time independent perturbation theory that the first correction (ip^) to the zeroth order eigenstate (ip°) of the full Hamiltonian H = Ho + Hi is given by
,„,.») = E|»..»)'-7'f'
,137)
we explicitly see that the time ordered product accounts for corrections to the wave function. Let me emphasize that there is nothing wrong with treating the physical states as eigenstates of the exact Hamiltonian. This is often done in the literature. One often reads about "higher Fock states" of a bound state. For instance in the case of NRQCD one often hears the term "dynamical
161
gluons" as being part of a higher Fock state of the onium. Again, there is nothing wrong with this, it just goes against the spirit of effective field theory as espoused in these lectures. But there are advantages to treating the states as lowest order eigenstates. For instance, in the case of HQET the leading power corrections to the meson masses coming from the operators Oi=hv-
D2 2mQ
hv,
- acr 02 = h v ^
G^
4TOQ
hv
(138)
are cleanly separated from higher order power corrections. This would not be true if we took the meson to be an eigenstate of the full Hamiltonian since the expectation values of 0\ and O2 would contain contributions of order 1/m 2 . Of course this would only be an issue if we were interested in 1/m2 corrections.
2.11.
Summary
In this second lecture I have introduced some new concepts into the game. In particular, the idea that one can integrate out massless particles by breaking up the field into various components. In the case of NRQCD we split up the gluon field into four types of modes: hard (m,m), though I didn't use this term previously. These are just the gluons which we integrate out when we match at the scale m. Potential (TOW2, mv), soft (mv, mv) and ultra-soft (mv2,mv2). These last 3 modes are believed to be sufficient to correctly reproduce all the non-analytic structure of non-relativistic Greens function. To date this has only been proven up to two loops. Once we determined the modes, we fixed their couplings by matching, although gauge invariance and RPI fix many such couplings without doing any work. The way we arrived at the proper mode decomposition was far from systematic. Indeed, finding the proper modes is more of an art form at this point. The potential and US modes fell out directly from matching, while we deduced the need for the soft modes from physical argument. Of course had we carried out the matching in the two quark sector we would have immediately seen the need for a soft mode. In particular, if you calculate the full QCD box diagram you will see that just including the potential and the US modes is not sufficient to reproduce the full result [30]. There is no general rule for determining what modes are necessary. However, there are some guiding principles. First we must determine the scales involved in the external states of the theory. In the case of NRQCD, those would be mv and mv2. Then there are some obvious modes that one can pick out that are necessary. Those are modes which leave the
162
external lines on shell.s So for a fermion with momentum (mv,mv2) we know that US and potential gluons should contribute to the non-analytic behavior of the Greens functions. After that, finding more modes starts to get harder and you have to do some matching calculations to be sure. But its pretty clear that the other modes should also be formed from these scales. So the soft (mv,mv) modes are an obvious choice. Since these modes throw quarks off-shell, its clear that you will need more than one interaction vertex to generate an operator in the effective theory. Then what about (mv,mv2) modes? Physically, these modes would mediate interactions which are non-local in time but local in space, which is clearly unphysical, and in fact a simple calculation shows that such modes can't contribute. What about modes with higher powers of v such as mv3? It is simple to see that these modes would give zero since this scale can be dropped in all propagator, and so loop integrals would be independent of the loop variable itself. The interested reader should study SCET [24,25] where similar mode deconstructions are performed.
3. Lecture I I I : Effective Theories and Extra Dimensions I would now like to discuss the application of effective field theories to the speculative topic of extra dimensions. The basic idea of using extra dimensions to unify forces goes back to the early part of the twentieth century with the work of Kaluza and Klein. But while extra dimensions have always played a role in string theory, from a phenomenological stand point the topic was dormant until a few years back. The reason for this dormancy was that the size of the extra dimensions was always assumed to be of order the Planck scale, and therefore, the measurable effects of the extra dimensions are so suppressed that they are irrelevant for all intents and purposes. However, with the advent of D-brane ideas, it was realized that the extra dimensions can be of arbitrarily large size (up to the experimental limits for gravity at short distances [52]) since string theory provides a mechanism for field localization. That is, the usual problems involving observable effects of large extra-dimensions are eliminated because the standard model fields live on a fixed three dimensional space-like hyper-surface (the D-brane) and are thus impervious to the extra dimensions. This led to a flurry of model building, including new solutions to the hierarchy problem. Many of these models were discussed by other lecturers at this school [50,51]. My purpose s
By "on-shell" I mean its virtuality is on the same scale at the residual momentum.
163
here will be to treat these models from the point of view of effective field theories. Please note that I will be abusing the term D — brane throughout the rest of these lectures. You many substitute the term D — brane with a "domain wall of Planck scale thickness" for the purposes of these lectures. Let us consider a generic theory on a manifold M which can be written as a direct product M = M4xMd
(139)
where Md is some d dimensional compact manifold. We will assume for simplicity that there is only one relevant scale which characterizes Md, namely JR. So we would expect that for energies less than l/R, the effects of the compact extra dimensions should be suppressed by ER
(140)
Furthermore, naturalness tells us that g ~ Rd/2. Since for d > 0 the coupling is dimensionful, we would call this theory "non-renormalizable" in the old fashioned language. In our parlance we would simply say that its only an effective field theory, with limited predictive power. We would expect that this description of the theory would only correctly describe the physics up to a scale of order A = R-1. Typically the theory should contain a whole tower of higher dimensional operators which are suppressed by powers of A, which all become equally important when we have external momenta on this scale. 3.1. UV Fixed Points: Effective
A Theory Can Be More
Than
Now before going on I should say that the above conclusions need not necessarily hold. In particular, there could be a strongly coupled fixed point, where our power counting would break down. Indeed, as I previously mentioned, it is believed that there are five and six dimension supersymmetric Yang-Mills theories with 16 supercharges which posses non-trivial fixed points, although it is not known how to give Lagrangian descriptions of these theories. Our argument in the previous paragraph seems to exclude this possibility. So where is the loophole? The answer, is that our power counting arguments were all based upon reasoning which only necessarily holds at weak coupling. At strong coupling the power counting can change.
164
While its not clear how this happens in the higher dimensional SUSY theories, there is a nice example in three dimensions [53] which illustrates a mechanism by which naive power counting breaks down. Consider the theory of N fermions in three dimensions interacting via a four Fermi interaction L = $iifil>i + 2-$i1>i)2.
(141)
In three dimensions the coupling g2/N has mass dimensions — 1. So naively this theory is only an effective theory and breaks down at the scale N/g2. However, it has been shown that in the large N limit this is not the case. The conclusion that the theory is non-renormalizable was deduced from a weak coupling analysis. We know that quantum corrections can lead to scaling modifications. Typically, we study weakly coupled theory, where the quantum corrections only change the power counting by logs. But in strong coupling it can happen that this naive power counting breaks down, and a theory which we thought was non-renormalizable can indeed be renormalizable. This is exactly what happens if you study (141) in the large N limit. The way this comes about is seen by re-writing (141) in terms of an auxiliary scalar field L = Tpiipi/ji + a4>iipi-—a2. 2g-s
(142)
Now naively, the a propagator is constant. However, in the large N limit we are forced to resum sets of diagrams, and in doing so the a propagator scales as 1/p, if the bare coupling satisfies a certain bound. Given this scaling it is not hard, using textbook arguments, to show that all the divergences can be absorbed using only the operators shown in (142). This means that we can naturally set all the higher dimension operators in the action to zero without worrying that we will regenerate them via quantum corrections. The point of this little diversion was just to let the reader know that sometimes field theories can actually be more than just effective, they may be complete. The trouble is that the strong coupling, which is necessary to generate large anomalous dimensions, makes it difficult to perform controlled approximations. So finding strongly coupled fixed points is a difficult task. I should also point out, that even without such a fixed point, there could well be a field theoretic description of the theory above the cut-off in terms of a different set of variables. For instance, the underlying theory could consist of a quiver gauge theory [54].
165
3.2. Naive Dimensional
Analysis
Returning now to our non-renormalizable effective gauge theory, it actually turns out that there are simple arguments which lead to the conclusion that the scale at which the effective theory breaks down is numerically enhanced relative to the compactification scale by a geometrical factor. These arguments are based on what is called "naive dimensional analysis" which was first discussed in the context of chiral perturbation theory [55]. A discussion of NDA within the context of super-symmetric theories can be found in [56]. The basic argument goes as follows. Suppose we wish to calculate some n-point function (G\ ) in a non-renormalizable field theory. Generically there will be a tree level contribution from some unknown counterterm (Cj), as well as some calculable (in terms of lower order coefficients) non-analytic terms which arise from loop corrections. Both contributions will be suppressed by powers of the UV scale A. Furthermore, dimensional analysis tells us that there is a correlation between the order in the loop expansion at which the observable is generated and the dimensionality of the operator which contributes to the observable at tree level. Schematically the amplitude M can be written as M = (p2n/A2n)(CM
+ {C{\+
ln(p 2 /^ 2 )) + «)) •
(143)
Here the piece proportional to Ci is the contribution from some local operator, while the log piece comes from a loop integral involving lower order interactions whose couplings are related to C by a geometric factor generated by the loop integration (p). That is, if Co is the lower order coupling then C ~ CQP. The divergence will get absorbed into C,. The momentum p represents some generic set of external momenta. Varying the renormalization scale on a interval of order one generates an effective change in result of order C(p 2 n /A 2 n ). This is roughly equivalent to the statement that even if we set d to zero at tree level, it will be generated at the loop level. Thus, we would expect that the size of the counter-term is thus naturally of order Cp2n JA2n. Now given that the log arises from a loop, p is just a geometric factor which comes about from the angular integration and is of the form l/(77r r ) where 7 and r are a function of the number of dimensions. Thus NDA tells us that the cut-off is more like p _ 1 A instead of A. While this does not seem like a huge distinction, the chiral Lagrangian description of low energy QCD would be far less powerful if it were no for this factor of p — 47T. It is important to realize that NDA is a naturalness argument and assumes that couplings are all of order one. As such, it should only be taken as a guiding principle.
166
Let us now see how this works for a 4 + d dimensional Yang-Mills theory. The leading interaction is suppressed by k~d/2, so that any loop contribution to the effective action will be suppressed by Ad. Following the argument in the previous paragraph, let us compare the tree level contribution to the photon two point function arising from the higher dimensional operator O z = ^FABDFAB
(144)
to the contribution arising from loop corrections. The one loop correction to the two point function arising from interaction with a massless field is just the usual vacuum polarization graph, which in 4+d dimensions is given by -(«2) <* ^f^i-^d/2n-d/2}.
(145)
where, as usual -K^u(q2) = (q2g^v — q^qu)ir{q2). First we notice that the result only diverges in even dimensions. We could have guessed that since in odd dimensions there are no allowed (i.e. analytic) counter-terms which could absorb the divergence. So in odd dimensions we cannot use this particular calculation to learn anything from NDA. In even dimensions, e.g. d — 2, we would conclude that the true cut-off is not g^1 but (g(47r)3'/2)~1. Notice that I didn't worry about the overall constants in (144). The full result could include group theory factors or gamma functions which might arise from doing Feynman parameter integrals. However, as I said before, NDA is only supposed to give you a rough guess of the true cut-off. I would not wager much on its exactness, but it does make qualitative sense that the cut-off will in general be enhanced by a geometric factor, which grows with the dimensionality.
Exercise 3.1: Using a physical argument determine whether one would expect the NDA guess for the strong coupling scale to change due to compactification.
3.3. The Force Law in d Dimensional
Yang Mills
Theory
It is interesting to try to understand the force law in a compactified version of a d dimensional Yang-Mills theory. Let us calculate V(r) a la Exercise 1.6 for a five dimensional gauge theory compactified on an S 1 with radius R. If we KK reduce the theory (I am assuming here that the reader is familiar
167
with KK reduction, otherwise see [51]), then the force will get contributions from zero mode as well as KK mode exchange between our heavy sources. For distances much larger than R we would expect that only the zero mode force is non-negligible since the KK modes, whose masses scale with n/R, mediate a Yukawa force which dies off exponentially fast e~rn/R. Thus, in this regime, we reproduce the four dimensional result, with a rescaled coupling g/VR. The rescaling arises from the the norm of the zero mode. On the other hand, for r R: r<^R:
V(r) oc | - , Rr a2 y V(r) oc — .
(146)
How will this result be modified by quantum corrections? At large r 3> R we know that we should be dominated by external zero modes as well as zero modes in the loop. Whereas at small r < i J w e will get contributions not only from corrections to zero mode exchange but also from corrections to the exchange of external KK modes as well. At large r the dominant correction will arise from the pure zero mode vacuum polarization diagram which generates a correction of the form ^(r)K^ln(r),
(147)
whereas dimensional analysis tells us that the correction to the full five dimensional potential, at small r, will be of the form 8V(r) oc ^-,
(148)
so that at small r the quantum corrections dominate the classical result as per our previous discussion. Now let's do an honest calculation to see what the exact result looks like. Instead of calculating the full five dimensional potential, I will concentrate on the potential generated by external zero modes and its correction. The reason being that this will be the relevant object to calculate when we discuss grand unification. I will do the calculation in two ways. First by working exactly in five dimensions, and then by matching and running, as discussed in the first lecture.
168 I will specialize to the case of scalar QED for simplicity. In this case there are two contributions to the vacuum polarization. The result for a massless scalar can be written as [69] TI>(P2)
R
v f
(2fc + p)M(2/c + p)„ 1 ) ( / c - m 2 ) _| (p+k)2 4
25,flis
2
= (P>,PV-P29^MP2)2
nip )
•L R
256
If1 —-r2 /
(149) /— xdxyl x 2 ln(l
8TT JO
')
(150)
Exercise 3.2: Prove equation (150). A few points are worth noticing here. First off the result is finite. This is because dimensional regularization sets the power divergence to zero. This finiteness may seem strange given that each mode gives a divergent contribution. What happens is that the sum of the poles cancels. Also, we would expect that at long distances we should exactly reproduce the four dimensional non-analytic behavior. So if we expand for small values of the momentum we should get a log and nothing else non-analytic. Defining 6 = R^ and expanding the log we find ln(l
-+ln{S)+a152
• a254
(151)
where the rest of the series contains only even powers of S. The first term in the expansion cancels the power dependence, the second term is the 4-D log and the higher order terms are analytic. At pR 3> 1, the result is dominated by the power dependence and taking the Fourier transform of the first term in the Dyson series we get back the radius independent result we would expect from dimensional analysis (148). We can also see from dimensional analysis that in the regime pR 3> 1 the theory becomes strongly coupled as all loops will be of the same order. We should be able to reproduce the force law p dependence by the running and matching procedure. For long distances p « i ? _ 1 , all of the effects of the KK modes can be absorbed into local counter-terms and we simply reproduce the four dimensional result from the zero mode contribution. The higher KK modes just go into making up the low energy (measured) coupling. It is interesting though to see how we can reproduce
169
the short distance behavior as well. If we are interested in the two point function at the scale p, then we should sum over all the KK modes with masses less than that scale. Each mode will contribute a log(p/fj,) along with some scheme dependent constant. It seems clear how the linear dependence on p will arise from summing over the constant pieces, but how does the log(p) dependence becomes power-like? Suppose we have measured the coupling at some scale below the first KK mass Mi. To predict the coupling at P ~ M\, we run the coupling to the scale M\ using the four dimensional beta function, then we match, at the scale M\, onto a theory which contains the zero mode as well as the first KK mode. In fact, let's generalize this procedure. Suppose we know the value of the coupling at the scale Mn and we wish to predict its value at some higher scale. We just match in steps as we discussed in the first lecture. Since we are only interested in the log piece let us ignore additive constants in the matching (even though in this particular case the constants are of the same order as the logs, so this calculation does not follow the standard systematics). Thus, at the order we are working we may use the relation a(/x) = . R a ^ \ , , y (152) 1 - /30raa(/Jo) ln(/V/z0) where /3o is the beta function for one massless particle, and n counts the number of massless particles in the theory.* So the value of the coupling in a theory with n KK modes, an, at the scale Mn+\ is given by n+1 a„(M„ + 1 ) = an(Mn) + n(30a2n(Mn) H~^~) (153) where I have used the fact that Mn oc n and dropped higher order terms in the couplings since ln( IL ^ L ) is not a large log. Now we match onto a theory with n + 1 KK modes with a coupling we label an+\. To determine an+\, we equate the potentials of the two theories Vn(Mn+1)
= Vn+1(Mn+1)
(154)
Note that, in general, this relation does not mean we equate the couplings. Now we may use this result to derive the value of the coupling at the scale Mn in terms of a(l/R). a{n/R) = ail/R)
+ ^
a 2 (l/i?)n/? 0 m ( ^ ^ ) ,
(155)
'Remember once we are above threshold for a given particle it is treated as massless in the running. The mass effects are power suppressed.
170
taking A ^ > l w e have a{n/R) « a{l/R)
+ a2(l/R)f30N
(156)
Then using N ~ pR
(157)
we see that we have reproduced the power behavior we found when doing the full five dimensional calculation (150). But we also notice that the result is linearly sensitive to the matching scale. That is, an order one variation in the matching scale (157) changes the result by an amount of order one. Usually such a variation would lead to a subleading correction, but in this case the power law behavior leads to linear sensitivity to the UV physics. 3.4. Grand
Unification
One of the first uses of effective field theories was in the context of grand unification [57,58]. In the standard model the couplings approach each other near the scale 1 0 1 5 - 1 6 GeV, and in the minimal super-symmetric standard model (MSSM) the coupling are within errors of unifying. What unification of the couplings really means is that in the context of a GUT we get the correct post-diction for the weak mixing angle sin 2 9w • This postdiction is a compelling reason to believe that there is a desert of physics between the weak scale and the GUT scale." This desert would seem to exclude the existence of any extra dimensions whose scale is less than the usual (10 16 GeV) GUT scale. It is thus interesting see whether or not we can retain the grand unified prediction for sin2 9w within the confines of a theory of "large" (R_1 ~ TeV) extra dimensions [59]. Let us see what a compactified five dimensional GUT would predict for sin 2 9\y • I will discuss this calculation in the context of a simplified Abelian model. Suppose we have a unified 5-D theory with gauge group [/(l)ixC7(l) 2 x f7(l) 3 , which is compactified on a circle of radius R. We will furthermore impose a permutation symmetry to insure that the bare couplings for the three gauge groups are equal. We will assume that each gauge group has a matter content composed of two scalars. We will choose the first gauge group to have all its scalars massless. The second group will have one "Another hint that this might be the correct scenario is the long life time of the proton. Since higher dimensional baryon number violating operators need to be sufficiently suppressed. This is an additional challenge to extra dimensional model builders.
171
massless and one of mass M2, while the third group will have all its matter content of mass M3. All the masses for the scalars will be of the same order which we will call MQUT- These masses will mimic the effect of GUT symmetry breaking. For instance, they could be generated by choosing a Higgs potential in an appropriate way. Well above the scale of the masses, the values of the two point functions will be identical. Let us first review how the prediction for sin2 9w comes about in four dimensions in this model. Suppose that we've measured the low energy (say at the scale Mz) values of two of the couplings, gi(Mz) and g2{Mz)- For each gauge group we may write down a relation between the MS coupling, <ji, and the physical coupling, g(q)i, (the object sitting in front of the F2 piece of the effective action) -27T = ^27-T + "T MQ/H) » 9i(l) SI (M) ^ 1 1 +Ain(g//l)+*in( gl{q) glbx) ' 2 ^ " ^ ' ^ ' 2-K* ^7T = ^ n
aim
(158) M
+ ~2 l n ( M 3 / ^ ) -
aiw
^),
(159) (160)
^
In writing these results I have ignored constants of order one, as they are subleading in the leading log approximation. Furthermore, I have dropped the q dependence in the contributions from massive scalars, assuming that q < MGUT- When q is order MGUT this log will be small and sub-leading, so the q dependence will be irrelevant. We have at our disposal two measurements as well as the relation gi(q) = 92(9) = 53(9) for q > MGUTNaively, this is not enough information to make any predictions given the number of unknowns. However, it is the magic of the log that will save the day. First we solve for g~\ and §2 in terms of the measured couplings 1 1 , 0o H9/Mz) (161)
m-#wz)'**
1 1 gl{q) g\(Mz) Using this result we may solve for 2
(^ MGUT = Mz
exp
—-
'
, A) \n{q/Mz) 2n*
•
(162)
"
(163)
MGUT
\
l
1 2
—-2-
\ fa L51 ( M | ) 5i(M|)_ Now 33 is frozen at the GUT scale, so we can determine its value by solving for gi{MGuT) HMGUT/MZ). (164) g\1 g\(Mz) ^ 1 . A)
172
Notice that in making this prediction it is crucial that M3 ~ MQUT, otherwise #3 would run between those two scales. Perhaps more importantly the prediction relies upon the fact that the sensitivity to the threshold mass M3 is only logarithmic, so that even if M 3 differs from MQUT by a number of order one, the running between these two scales is negligible. If we only consider flat manifolds), then this property is unique to four dimensions. Exercise 3.3: This calculation was done in the context of a non-decoupling scheme. Redo the calculation in terms of an effective theory calculation by matching and running to each threshold. Let us now see how this prediction runs into trouble in five dimensions. Notice that the couplings do not "run" in the five dimensional theory since there are no log divergences. That is, there is no dimensional transmutation, the theory was not conformal classically. Thus it is always true that g\ = 92 = 5 3 , but is not true that the force between two charges will be the same for all the gauge groups at all distances. Thus instead of a running coupling I will define an "effective coupling", ge^ which will represent the strength of the force. Calculationally g^* is sum of the bare coupling gi plus radiative corrections. We will assume that we have compactified on a circle of radius R, and that the symmetry breaking masses are such that MiR > 1. For smaller masses we would expect that we would just reproduce the four dimensional log running which we know gets the answer for the weak angle wrong when the GUT scale is near a TeV. Consider the prediction for sin2 9w in this model. Assume we have measured the force between two charges at some low scale r _ 1 = Mz for g\ and g2- For the group with only massless fields in the loops the strength of the force at the scale E = Mz is
^ =ff|+ S l n ( 2 7 r M ^ ) -
(165)
This result is what we would expect. At low energies the corrections to the potential are governed by loops of zero modes of massless fields and we get back the four dimensional result. For the second gauge group we get 1 e// 2
G?2 )
1
< P0.H2irMzR) 9l W
+C
2
^ .
(166)
The term proportional to /3o is the usual massless result. The second term arises from the massive scalar field. Using the same logic as in the four
173
dimensional case, we would like to predict
.), (.9a
f/f)2
9l
+ C2
2M3R —.
via )
(167)
In these equations I have assumed that the power like piece dominates in (150), otherwise we would just reproduce the four dimensional result. We can see that when we try to solve for R using the first two equations, we won't be able to drop the term which goes as M2R, since it's not small compared to the log term. There are more unknowns than equations and we have no predictive power. Notice that this shortcoming is not related to calculability, but to the UV sensitivity of the result. Even if you knew the "beta function" exactly it would not matter, you still would need to know the values of the masses of the GUT particles to accurately predict the low energy value of sin2 9w •
3.5.
Orbifolding
As I mentioned at the beginning of this lecture, the phenomenological interest in extra dimensional theories was resurrected when it was realized that the D-brane techniques discovered in string theory [60] could be useful for model building. The world volume theory of a D-brane can be described by a gauge theory which lives on the brane. Thus, D-branes allow for a natural way of "trapping" fields onto a surface (these fields won't have KK modes). In this way one can make the space as large as one wants without worrying about introducing phenomenologically meddlesome light KK modes. However, we are not free to restrict gravity to the brane, since by general covariance, all stress-energy couples to the graviton, and thus gravity must propagate in the "bulk" of space-time. Let us explore the effective field theories of theories with D-branes. If all the fields of our theory are trapped to the D-brane, then there really isn't anything interesting going on, since the theory is just a normal four dimensional theory up to the scale of quantum gravity.v So to make things interesting we will allow at' least some of our fields to traverse the bulk, and discuss the description of the theory in terms of an effective field theory [61]. Let us assume, for the moment, we are working in a flat V
I will assume here that we are talking about D3 branes which have four dimensional world volumes.
174
spacetime. w Then in order to keep the KK modes sufficiently massive to conform to accelerator limits, we must also compactify the space with a radius at least as big as a TeV. The scenario I shall consider here is a compactification on the five dimensional manifold M = MAx
S1/Z2
(168)
where the Z2 defines an equivalence class via a reflection across the S1. Note that Z2 acts (K) non-trivially on both the coordinates and the fields K:
(x,y)^(x,-y)
(169)
K :
$ - • R{k)&
(170)
where R(k) furnishes a representation of the Z2. I have chosen the circle to have radius R and coordinatized it in the two dimensional plane (x, y). The points y = 0 and y = TTR are fixed points meaning they are left invariant under the action of the Z2 and are singular. The Einstein equations require that we place a singular sources of stress energy at those points, so that is where we must put a pair of D-branes. Such singular spaces are technically not manifolds, and are instead called orbifolds. The thickness of the Dbranes are of order the string scale, so as far as our effective field theory description is concerned, they are infinitely thin in the limit where E/Ms goes to zero. How would the force law in this space differ from the case of the pure S1 we previously discussed? We would expect that at long distances the force laws should also be identical, as long as the massless spectra are the same. Suppose we allow for our charged scalar to propagate into the bulk, as in the previous case. There is a simple way to use our result for S1 to calculate the vacuum polarization on S1/Z2. First we must decide how the bulk fields transform under the Z2. If we want the massless spectrum to be the same then we must choose the field to be even under the Z2 otherwise the theory would posses no zero mode. We then KK decompose the field in the usual way
* = £ *™w-jm> n— — oo w
v
<171>
I n general the brane tension itself will curve the space, but we will assume that the tension is small enough so as not to significantly perturb the theory away from Minkowski space.
175
and 0 > z < n coordinatizes the fifth dimension. We then further decompose the field as $ = $+ + ; $ _
(172)
where
*-' = 7^1 ( $ ( n ) ( a ; ) " $ ( - ) ^ ) s i n ( f ) •
(173)
Then on S\ the action can be written as
s=o^r 5X5
f
r
2
a
£ y ** (»„*S°) + ( ^ v - ^ (($in))2 + ($^)2)
• (174)
n=l'
Orbifolding by Zi eliminates the $ _ component so that we may write 7T51 = 7 T S i / Z 2 + 7T ( _ ) ,
(175)
where 7P ) is the contribution from $_ . Recall that in the case of Si, we found a finite result, and we concluded that the UV divergence from the zero mode must have cancelled with the pole from the KK sum. But since 7r(~) = 7f(+) (where the bar indicates that we don't include the zero mode) we can see that for the orbifold we no longer expect a finite result since 7TSV.Z2 = nsi - ^ ( ^ ( + ) +
7r(
~ ) ) = 2 ^ 1 + 271"0'
(176)
and we know that the KK mode sum (W^+' + n^) is divergent, since its divergent piece is minus the zero mode divergence. At first this may seems rather mysterious given that we would not expect that a non-local operation like a change in boundary conditions could affect a local structure like a UV counter-term. How can the local bulk physics be sensitive to what's going on at the boundaries? The answer is, it is not, the divergence corresponds to a local divergence on the boundary. Introducing the boundary allows for the existence of new counter-terms, since we can now write down localized interactions on the boundary, SB
f d5x(5(z)L0
+ S(z - R)LR)
(177)
where La and Ln are the most general actions allowed by the symmetries of the theory.
176
Returning to our calculation, we find
**'*"¥* + ^4
+C
>-ijt^rt-
(178)
As I said previously we would expect that at long distances one cannot distinguish between S1 and S1 /Z2, so we can immediately read off the q dependence of irs1/z2 from (178), when qR /z3
= \<12)s^
- 9 ^ 2 ln(g2/M2) = - ~
m(
(179)
and in fact this agrees with the expansion of the result we found previously in (150). Note again that' we do not expect constants to agree as these are local short distance contributions. The \ divergence can be subtracted by putting a FF counter-term on the boundary. Before closing this section I would like to discuss a common mistake you might find in the literature. Suppose we have a theory on S1/Z2, where we have a certain set of fields ($0) which are localized to a boundary at one boundary at z = 0, another set $ # on the second boundary which we will place at z = R, and finally yet another set which propagate in the bulk $ 3 . At low energies we may write down an effective field theory consisting of the low energy modes of $0 and QR and a few {NKK) of the low lying KK modes of the bulk fields,x such that dAxLEFT
= I d*x Y, (M*o) + mR) •^
+ L($5)).
(180)
n=0
At this point I have not allowed for interactions between the various fields.y Whose effective theory is this? An observer in the bulk? Or on one of the boundaries? The answer is that the question is not well posed. At low energies we cannot probe distances short enough to talk about positions in the fifth dimension. The action correctly describes the physics for all the observers. The Lagrangian will correctly describe all low energy experiments. However, the possible experiments which can be performed will depend upon the location of the observer, because an observer on the brane at z = R will not have access to a beam of particles which are interpolated x
T h e number of modes we choose to keep is fixed by the maximal energy allowed in the theory. y Strictly speaking this isn't possible since they must interact at least gravitationally given that the fields are sources of stress energy. Such interactions terms will be described in the effective theory via higher dimensional (i.e. d > 4) operators which are suppressed by the Planck scale.
177
by $ 0 and visa versa. In fact, matter on the brane at z = R will just look like dark matter for an observer on the brane at z = 0. 3.6. Classical
Divergences
as UV
Ignorance
Let me now utilize an extra dimensional theory to emphasize one of the key ideas that I have tried to convey in these lectures. The UV divergences we find in field theory calculations are not exotic, nor do they imply the theory is sick. They just tell us that our theory has limited predictive power, because they do not correctly describe the UV. To hammer this point home, I would now like to give an example where the UV divergences show up at the classical level [63,62]. Suppose I have a six dimensional theory with a 3-brane located at the origin, around which I impose a Z% symmetry, so that the space is M 4 x R2 jZi. Strictly speaking for arbitrary brane tension, the true geometry is conical [64] and not a flat orbifold, but I can always fine tune the tension so as to get a solution to Einstein's equation which yields a Z^ flat orbifold. To simplify matters I will consider a simple theory of a real scalar field in Euclidean space such that 5 = |ci6xQ(^)
2
+ i m V + A04 + ^ ( x ) i m ^ .
. (181)
where for simplicity I have only included a mass term on the brane. Now let's calculate the two point function for . Ignoring for the moment the brane mass term we find d4k
GoM
f
feikq-(y-y')
d2
= I T( 2HT TZ) 7/ T ^ *~ ' - * ' ' (2TT) 4
2
_|_
eikq-(y+y')\
T^—2q
k2 +
2
"
(182)
Exercise 3.4: Prove equation (182). To calculate the complete two point function we include insertions of the brane mass so that G(x,x')
= G0(x,x1) -ml +m4b f d6zd6z'G(x,
f
d6zG0(x,z)52(z)G0(z,x') z)52(z)G(z, z')52(z')G{z',
x') + ...
(183)
Since we have four dimensional Poincare symmetry it is simpler to work in the mixed momentum configuration space representation of the Greens
178
function Gk(w), where w are the transverse coordinates. Then Gk(w, w') = G°k(w, w') - m2G°k(w, 0)G°k(0, w') +m^(»,0)G2(0,0)G»(0,»') + ... .
(184)
Notice that G°(0,0) is divergent. This divergence arises as a consequence of the fact that we have approximated the brane as being infinitely thin. But we know this is not a problem, we simply parameterize our ignorance of the correct brane structure in a local counter-term.
Exercise 3.5: Show that in the MS scheme the beta function for mi, is given by ^ <
= <.
d/i
TT
(185)
Suppose we knew the true structure of the brane, and that it can be described by some profile F(w), which in the limit where the UV scale (which well call the Planck mass) goes to infinity approaches a delta function, lim
F(w) = \m2b82(w).
(186)
This information can then be encoded in the action S=
fd6x
Q (dA
.
(187)
Then the divergent integrals gets damped out by F and the result is finite. We have one less counter-term to fix and thus increased predictive power, but only because we claim to know the underlying theory. Of course, there is an even simpler example of UV divergences arising classically. The total energy in the Coulomb field of a charge particle diverges when the electron is treated as point-like. 3.7. Effective
Field Theory in Curved
Space
Adding curvature to the space-time manifold forces one to carefully inspect many of our starting assumptions in building a quantum field theory. For instance, even the notion of what is a particle must be reconsidered [65] since if the space-time does not admit a time-like killing vector there is no notion of a conserved energy. However, things simplify considerably as the space-time becomes more symmetric. So we will limit our discussion to a
179
specific set of theories which posses four dimensional Poincare symmetry. These theories have preferred vacua around which to quantize. Such spacetimes have metrics which can be written as ds2 = dz2 + g{z)-qtl,l/dx'1dxv,
(188)
for simplicity I've chosen to work in five dimensions, generalization to higher dimensional spaces is straightforward. Such space-times are often called "warped". We can use quantum field theory on this, as well as any other, manifold as long as the curvature scale is small compared to the Planck scale. Much, though not all, of our flat space effective field theory intuition carries over into curved space. Most importantly, the counter-term structure of the theory does not change since for momentum much shorter than the curvature the space looks flat.z So, as in flat space, we write down all possible counter-terms which are consistent with the symmetries of the theory under consideration. However, since momentum scales are now dependent upon the coordinate in the fifth direction, the momentum at which the loop expansion breaks down will depend upon the choice of observable. Let me specialize to a particular space which has received much attention in the past few years. Namely, five dimensional anti-deSitter space compactified on S1 jZ-i. anti-deSitter space is a maximally symmetric space with negative cosmological constant [66], and its compactification on Sl/Z2 was used in [67] to solve the hierarchy problem. The warped metric can be written in the conformally flat form ds2 = — ^ {dz2 + •nia/dx'idxv)
(189)
where k is the inverse radius of curvature. The two branes necessary to account for curvature singularities are positioned at z = 1/T (the "TeV brane") and z = 1/k (the "Planck brane"). T is taken to be near the TeV scale while the bulk cosmological constant (A) is related to the curvature scale via fc2 =
~ (190) V ; 24M 3 where M is the five dimensional fundamental scale, i.e. it is the scale in front of the Einstein term in the action. Note, that A will not be the four dimensional cosmological constant, nor will M be the effective Planck z
Here I'm ignoring counter-terms involving powers of the curvature which will in general be generated.
180
mass. This theory, in conjunction with a Goldberger-Wise stabilization mechanism [75], solves the hierarchy problem, and is sometimes referred to as RSI. A simple way to see this is to note that if the standard model fields are localized to the TeV brane, then the masses of the fields are naturally of order MT/k (see [51]), while gravity remains weak. Let us study the low energy effective field theory derived from this model. Now we would like to ask, what observables are calculable in an effective field theory? If we perform a KK decomposition, the action looks like
S= fd*xY,Hp,n), ^
(191)
n
where the fields 4>T (4>P) are fields localized to the TeV (Planck) brane, and 4>n are the KK modes of the bulk fields. The KK masses correspond to zeros of Bessel functions (see [51]), and the lowest lying masses are near a TeV (note that there is a vanishing effective cosmological constant. This only comes about after fine tuning a linear combination of the brane tensions.). Prom a low energy view point this looks like a theory with an effective compactification scale of a TeV, since the first KK mode has a mass of that order. So if we dimensionally reduce to a four dimensional effective theory, we will have, in general a set of zero modes and a tower of KK modes. Additionally we could have zero mass (m
S = ^ J d5x^FMNDFMN
(192)
where A is a dimensionful quantity of order 1/M in our normalization where the gauge field kinetic term is accompanied by a factor of 1/g2. The contribution of this term to the zero mode gauge field two point function will
181
be of the form U(q2) = Xk^,
(193)
so we can see explicitly that we have no hope of calculating the gauge field zero mode two point function for <j,2much larger than T2.
Exercise 3.6: Prove equation (193). Be sure to remember that the gauge potentials are the components of a one-form, so its index is naturally down.
At first this result is rather disheartening, since it appears to mean that extra dimensions with size of order the TeV scale have no hope of reproducing the four dimensional prediction for the weak mixing angle. Is there no way to have higher dimensional physics which is accessible at the LHC without the loss of this beautiful prediction [76]? The answer is yes, and it is through the magic of AdS. 3.8. Unification
in AdS 5
Suppose we calculate the low energy (q
<
l 48TT 2
VH&)
—r / dxx\Jl
2 16TT >TT2 Jo
— x2\a.N
where
\nN(p) = -\n\iv{jplT)kv{p/k)-iv{p/k)kv{p/T)\, iv(z) = (2-v)lv(z)
+
kv{z) = {2-v)Kv{z)-zKlJ_l{z),
(195)
zlv-1(z), (196)
and v = \Jk + m2/k2. Let's study this result in some detail. We first notice the UV pole A. This pole is identical to the pole we found in flat space orbifold, and can be subtracted away by a boundary counter-term. This fact also tells us that the ji dependence in (195) determines the running
182
of the boundary couplings, one of which lives at z = 1/T (AT) the other at z = l/k (Afc). In the MS scheme, the effective dimensionally reduced coupling looks like g2(q2
= - 2 +A f c (/i)95
+ 16TT
2
1
A T (/i)
(jl_\
1
96TT2
n
\4kTj
/ dxx v 1 — x2 In N Jo
(197)
where R = ln(fc/T)/fc, is the effective compactification radius. The first three terms result from the tree level bulk gauge coupling and boundary couplings, respectively. The remaining terms encode all the information about the KK modes. In flat space this term would be linearly sensitive to the bulk mass so in AdS we would expect that for m larger than the curvature scale fc we would again have linearly sensitivity. The interesting case is m < k. Using the expansions 2+u
Iz\u
r
„,
0N
-,
(198)
i > n ) ( 2 ) [i+ °(*') + - ] , r("), 2 "©""[ l + 0(z ) + ---}
(199)
for p ^C T, we have InJV(p)~-ln
k
i
(200)
T
and thus, for m ^ 0 1 g2(q2)
- -J2 + A fe(M)
9l
XT(n)
+analytic in q2.
1 96TT2
i (-£-] n
\4kTJ
48?r2
In
y2
(201)
This is a remarkable result, in that the five dimensional vacuum polarization looks just like what we would expect from a four dimensional calculation. The result (201) is only logarithmically sensitive to the masses, so there is hope that a GUT theory with a large mass scale (by large I mean large compared to the TeV scale, but still smaller than k) will still lead to the correct value of the weak mixing angle.
Exercise 3.7: Show that in the limit where m 3> k the inverse coupling is linearly sensitive to the mass.
183
Let us try to get a better understanding of how the logarithmic sensitivity to the symmetry breaking scale arises. It seems rather strange that we can make a prediction for a lower energy observable in terms of high energy parameters (the symmetry breaking scale m) when the observable under consideration is not well defined at scales above T. Clearly we can't get the result (201) from an effective field theory calculation where we run the zero mode correlator down from the scale m. Instead of tracking the zero mode correlator we will calculate another observable, calculable at higher scales, which matches onto the zero mode correlator at the scale T [68]. This other observable is the Planck brane gauge field correlator, which is defined as the gauge field two point function with its end point on the Planck brane. This correlator is calculable up to the scale k. In fact, the cut-off scale for any correlator restricted to a hyper-surface of fixed z is 1/z. The tree level Planck brane correlator, in mixed momentum coordinate space representation, can be written in the Az = 0 gauge, as (here r)^ is the flat Euclidean metric) £>£"(*, z') = rTDq(z,
z') + ^Hq{z,
z'),
(202)
where we won't need the form of Hq. If we take both end points on the Planck brane (z = 1/fc) one finds (for q> T) 'xe^iA^x,
l/k)Av(0,
l/k)) cx -^rrV^
+ ••-,
(203)
whereas in the case where only one end-point is on the Planck brane (this is needed for the one loop calculation) we find [69] D(zl/k)_kz
K,{qz)I0{qIT)+KQ{qlT)I0{qz)
At q
184
since it depends only upon the geometry. So that is a first step in understanding the result (201), but the real challenge in understanding the logarithmic mass dependence. From a four dimensional perspective this term looks like it's coming from a heavy scalar which decouples at its mass scale (compare to the result in section 3.4). In terms of the Planck brane correlator we can understand this from the following remarkable fact. All of the KK modes with masses much less than k, save for one, are localized to the TeV brane while the external gauge boson lines in the self energy die off exponentially fast as one moves away from the Planck brane. So the net contribution of the KK modes to the symmetry breaking effects is suppressed relative to contribution of a single mode. The modes with masses larger than k contribute in a symmetric way and thus only affect the unmeasurable counter-term. The one special mode has a mass near the value of the bulk mass. In the massless case this mode is the zero mode, which is flat, while in the massive case there is a "magic mode" which has support near the Planck brane [77]. Let us see how this happens quantitatively. In the limit T < A / ^ < k
Dg(z,l/k)
K^f'-
<205>
we see the exponential suppression of the gauge field propagator. This suppression will kill the contribution of the TeV brane localized KK modes. However, one must be careful to note that there is regulator dependence in this statement. A more precise statement would be that the non-analytic momentum dependence generated by the KK modes is exponentially small. I will refer the reader to reference [68,69,74] for more details. For the case of gauge fields and fermions, the suppression of the KK modes does not follow this argument, as they are not in general TeV brane localized. In fact, the contributions from these modes are only logarithmically suppressed. That is, they look like they contribute to the two loop beta function. This can be seen from studying the non-decoupling calculation for gauge field [73]. But given that these effects are of the same order as the usual unknown threshold effects, they won't change the prediction for the weak mixing angle [74]. To understand this behavior from a nondecoupling argument one can use arguments based on AdS/CFT, but these are beyond the scope of these lectures. At this point I should also mention that the gauge fields differ from scalar fields in another important way. The one loop calculation for a gauge field with mass mx gives a contribution of
185
the form 777
ir(q2) oc ln(^ff-) + . . .
(206)
so that the effective mass is parametrically enhanced [74], though numerically, this effect is small unless we are willing to tolerate a large hierarchy between k and rrixLet's now consider the effects of the higher dimensional operators on our prediction for the weak mixing angle, as they could perhaps ruin our predictive power. Any higher dimensional operator which is insensitive to the symmetry breaking masses will obviously not affect the prediction. These higher dimensional operators would all correspond to some universal contribution to the couplings and thus be irrelevant for the difference which determines the mixing angle. It is also possible that there are higher dimensional operators such as got an expectation value. However, since m
Acknowledgments I would like to thank the TASI organizing committee, Howie Haber, Ann Nelson and especially K.T. Manhanthappa for organizing yet another fruitful TASI meeting. I would also like to thank the UCSD theory group for its hospitality, as well as Iain Stewart and especially Walter Goldberger for comments on the manuscript.
186
References 1. The basic ideas behind effective field theory go back to Wilson's original work. The intuition gained by understanding the renormalization group in position space in terms of block spins makes it well worth the time to study this topic. I suggest the following review to the interested reader. K. G. Wilson and J. B. Kogut, Phys. Rept. 12, 75 (1974). 2. M. E. Peskin and D. V. Schroeder, "An Introduction To Quantum Field Theory,", Reading, USA: Addison-Wesley (1995) 3. S. Weinberg, "The Quantum Theory Of Fields. Vol. 2: Modern Applications", Cambridge, UK: Univ. Pr. (1996) 4. H. Georgi, "Weak Interactions And Modern Particle Theory," Menlo Park, USA: Benjamin/Cummings (1984) 5. T. Appelquist and J. Carazzone, Phys. Rev. D 11, 2856 (1975). 6. For a discussion see G. Sterman, "An Introduction To Quantum Field Theory,", Cambridge, UK: Univ. Pr. (1993) 7. S. Kamefuchi, L. O'Raifeartaigh and A. Salam, Nucl. Phys. 28, 529 (1961); R. E. Kallosh and I. V. Tyutin, Yad. Fiz. 17, 190 (1973) [Sov. J. Nucl. Phys. 17, 98 (1973)]. 8. The use of the OPE in DIS is discussed in [2]. The B decay calculation is discussed in [18], 9. H. Georgi, Nucl. Phys. B 361 (1991) 339. 10. J. Wess and B. Zumino, Phys. Lett. B 37, 95 (1971). 11. E. Witten, Nucl. Phys. B 223, 422 (1983). 12. E. Witten, Phys. Lett. B 117, 324 (1982). 13. E. D'Hoker and E. Farhi, Nucl. Phys. B 248, 59 (1984), Nucl. Phys. B 248, 77 (1984). 14. J. Goldstone and F. Wilczek, Phys. Rev. Lett. 47, 986 (1981). 15. For a review and references, see P. F. Bedaque and U. van Kolck, arXiv:nuclth/0203055. 16. N. Seiberg, Phys. Lett. B 390, 169 (1997) [arXiv:hep-th/9609161]. N. Seiberg, Phys. Lett. B 388, 753 (1996) [arXiv:hep-th/9608111]. 17. A. J. Buras, arXiv:hep-ph/9806471. 18. A. V. Manohar and M. B. Wise, Cambridge Monogr. Part. Phys. Nucl. Phys. Cosmol. 10, 1 (2000). Also see M. Luke, these proceedings, as well as M. Neubert, Phys. Rept. 245, 259 (1994) [arXiv:hep-ph/9306320]. 19. H. Georgi, Phys. Lett. B 240, 447 (1990). 20. The theory was originally formulated in, W. E. Caswell and G. P. Lepage, Phys. Lett. B 167 (1986) 437. The formulation discussed in these lectures was given in [21]. For a review see [22]. NRQCD in the context of production and decay was discussed in [23]. 21. M. E. Luke, A. V. Manohar and I. Z. Rothstein, Phys. Rev. D 61, 074025 (2000) [arXiv:hep-ph/9910209]. 22. A. H. Hoang, arXiv:hep-ph/0204299. I. Z. Rothstein, arXiv:hep-ph/9911276. 23. G. T. Bodwin, E. Braaten and G. P. Lepage, Phys. Rev. D 51, 1125 (1995) [Erratum-ibid. D 55, 5853 (1997)] [arXiv:hep-ph/9407339].
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189 66. S.W. Hawking and G.F.R. Ellis, "The Large Scale Structure of Space-Time", Cambridge, Uk: Univ. pr. (1975). 67. L. Randall and R. Sundrum, Phys. Rev. Lett. 83, 3370 (1999) [arXiv:hepph/9905221]. 68. W. D. Goldberger and I. Z. Rothstein, Phys. Rev. Lett. 89, 131601 (2002) [arXiv:hep-th/0204160]. 69. A technique for calculating loop diagrams in higher dimensional spaces, including warped spaces, can be found in the appendix of, W. D. Goldberger and I. Z. Rothstein, arXiv:hep-th/0208060. For results including higher spin fields see [73]. A method of calculating vacuum bubbles is given in [70]. 70. W. D. Goldberger and I. Z. Rothstein, Phys. Lett. B 491, 339 (2000) [arXiv:hep-th/0007065]. 71. J. M. Maldacena, Adv. Theor. Math. Phys. 2, 231 (1998) [Int. J. Theor. Phys. 38, 1113 (1999)] [arXiv:hep-th/9711200]. S. S. Gubser, I. R. Klebanov and A. M. Polyakov, Phys. Lett. B 428, 105 (1998) [arXiv:hep-th/9802109]. E. Witten, Adv. Theor. Math. Phys. 2, 253 (1998) [arXiv:hep-th/9802150]. 72. E. Witten, remarks at ITP Santa Barbra conference, "New Dimensions in Field Theory and String Theory". S. S. Gubser, Phys. Rev. D 63, 084017 (2001) [arXiv:hep-th/9912001]. N. Arkani-Hamed, M. Porrati and L. Randall, JHEP 0108, 017 (2001) [arXiv:hep-th/0012148]. R. Rattazzi and A. Zaffaroni, JHEP 0104, 021 (2001) [arXiv:hep-th/0012248]. 73. K. w. Choi and I. W. Kim, Phys. Rev. D 67, 045005 (2003) [arXiv:hepth/0208071]. 74. W. D. Goldberger and I. Z. Rothstein, arXiv:hep-ph/0303158. 75. W. D. Goldberger and M. B. Wise, Phys. Rev. Lett. 83, 4922 (1999) [arXiv:hep-ph/9907447]. 76. A. Pomarol, Phys. Rev. Lett. 85, 4004 (2000) [arXiv:hep-ph/0005293]. L. Randall and M. D. Schwartz, Phys. Rev. Lett. 88, 081801 (2002) [arXiv:hep-th/0108115]. 77. K. Agashe, A. Delgado and R. Sundrum, Nucl. Phys. B 643, 172 (2002) [arXiv:hep-ph/0206099]. 78. K. Agashe, A. Delgado and R. Sundrum, Annals Phys. 304, 145 (2003) [arXiv:hep-ph/0212028]. W. D. Goldberger, Y. Nomura and D. R. Smith, Phys. Rev. D 67, 075021 (2003) [arXiv:hep-ph/0209158]. M. Carena, A. Delgado, E. Ponton, T. M. Tait and C. E. Wagner, arXiv:hep-ph/0305188. L. J. Hall, Y. Nomura, T. Okui and S. J. Oliver, arXiv:hep-th/0302192. Z. Chacko and E. Ponton, arXiv:hep-ph/0301171.
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MICHAEL LUKE
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BOTTOM QUARK PHYSICS A N D THE HEAVY QUARK EXPANSION
MICHAEL LUKE Department of Physics University of Toronto 60 St. George Street Toronto, Ontario Canada M5S1A7 E-mail: [email protected]
These lectures were given at TASI-02 on the subject of b quark physics. Contents 1
Introduction
194
2
Precision Measurements and Flavour Physics 2.1 b Quarks as Probes of New Physics
194 198
3
From Quarks t o Hadrons 3.1 Effective Field Theory 3.2 m\y > fi > rrn,: Four-Fermi Theory Generalized 3.2.1 Semileptonic decay 3.2.2 Nonleptonic b -> c decay 3.2.3 b -> S7 decay 3.3 n < mb: HQET 3.4 Inclusive Decays and the OPE
203 207 208 213 214 214 216 221
4
Applications 4.1 Inclusive Decays: Corrections to the Parton Model 4.1.1 Total decay widths 4.1.2 Differential widths and spectra 4.1.3 Vuf, and the shape function 4.2 Exclusive Decays and Symmetries 4.2.1 B -> D*tv and Vcb 4.2.2 B -> J/iiKs and sin 2/3
227 227 228 229 231 234 234 237
5
Conclusions
239 193
194
1. Introduction In these lectures I hope to give you an introduction to bottom quark physics, and a feel for the theoretical issues which are involved. Because of time constraints, I cannot cover all aspects of the field, so I have chosen to focus on a few key ideas. The most important is the idea of "factorization". This is a word which is used to mean many different things in particle physics, but here I use it in the very general sense of separating long and short-distance physics. This is important for B decays, because much of the theoretical progress in the subject relies on being able to separate the short-distance physics at scales 1/mw and 1/mj, which determines the quark-level process (and for which perturbation theory is a useful tool) from the complex longdistance physics of hadronization. Effective field theory will be introduced as a tool which makes this factorization automatic. The outline of these lectures is as follows: First, I will discuss the use of precision measurements of b quark transitions to probe physics at or above the weak scale. I will then discuss the extent to which 6-flavoured hadrons may be used to probe the interactions of b quarks, and then look at some important applications. Finally, I will briefly discuss some current directions of research. Throughout the lectures I will use the decay b —> sj to illustrate the concepts, since a complete understanding of this decay requires almost all of the theoretical machinery I will discuss. 2. Precision Measurements and Flavour Physics As you have heard in many lectures at this school, there are many good reasons to believe that the standard model is not the correct description of nature at scales above a few hundred GeV or so. A strongly-inter acting symmetry-breaking sector and supersymmetry in one of its many forms are perennial favourites for new physics, as are the more recent ideas of large extra dimensions and Little Higgs models. All of these theories conjecture new particles at the 100 GeV-few TeV scale. Unfortunately, to date no experimental evidence for such particles has been found. Historically, there have been two ways to discover new particles experimentally. The first is to build ever-larger colliders which can directly create more and more massive particles. The second is to perform precision lowenergy measurements, which corresponds to looking for new physics via virtual effects. While the first is usually preferable, because it's easiest to study particles if you can actually make them, important complementary information can be found by following the second path. This is the course
195
being pursued at the B factories at SLAC and KEK, as well as by many other precision low energy experiments, and has has a number of important successes in the past. Here are a few examples: (1) Virtual c quark loops contribute to K°—K° mixing via the diagrams in Fig. 1. From the measured size of the mixing, Gaillard and Lee [1] were able to predict that mc had to be less than a few GeV.
Kc
>u, c, r>
d Figure 1.
\ K°
s
K°\
*u,c,t
d
Kc
+
s
Quark-level diagrams contributing to K° — K° mixing.
(2) Similarly, virtual t quark loops contribute to B° — B° mixing. In 1986, the ARGUS and UA1 experiments measured a rate for this process that was much larger than expected [2], implying a top quark mass greater than 50 — 100 GeV. This was the first evidence for a heavy top quark. (3) Precision electroweak measurements (mostly at LEP) determined rrit = 169l 1 8 1 2 GeV [3], making it fractionally one of the best known quark masses, before it was even discovered. Indeed, the constraints from precision measurements puts strong constraints on a variety of new particles at the few TeV scale, and has even been dubbed the "little hierarchy" problem - the fact that there appears to be a sizeable hierarchy between the Higgs mass in the Standard Model and the scale of new four-fermi operators. It is straightforward to calculate the Standard Model contribution to flavour transitions at the quark level. Since there are many more possible transitions than free parameters, one could therefore in principle compare with experiment and either discover discrepancies or place constraints on models of new physics. However, there is a problem: when comparing with experiment the theoretical predictions often have large theoretical errors arising from the difficulties in relating the underlying quark process to the observed hadronic process (denoted by the black blobs in Fig. 1). Even purely leptonic interactions can be plagued by hadronic uncertainties
196
through loop effects, as the next example illustrates. Dealing with this issue will occupy much of these lectures. Example: g — 2 of the muon An instructive example of the importance of understanding low-energy QCD is provided by the recent measurement of the anomalous magnetic moment of the muon by the E821 experiment at Brookhaven [4]. As is well known, there was initially great excitement in the community due to a reported 2.6 a discrepancy between theory and experiment: (af - alxp) x 10 11 = 426 ± 150 exp ± 67 t h .
(1)
where a^ = \ (g — 2). This was seen as a possible signal of new physics, such as the supersymmetric contributions such as those in Fig. 2. Unfortunately, the discrepancy was reduced to a little over one standard deviation when a sign error was found in the theoretical calculation [5]. Nevertheless, it is worth considering what the theoretical limitations on this prediction are, to determine what level of discrepancy would be required to be a convincing signal of new physics.
Figure 2.
A supersymmetric contribution to g — 2 of the muon.
The bulk of the contribution to g — 2 arises from QED diagrams, which have been calculated to a quoted precision of ± 3 x 1 0 - 1 1 . Electroweak corrections give a much smaller contribution, but with a comparable error. These are both reliable because they are purely perturbative calculations. Unfortunately, even with muons, low-energy hadronic physics creeps in, via the vacuum polarization graph in Fig. 3(a). Fortunately, while this graph is nonperturbative, the hadronic contribution to the photon vacuum polarization may be measured experimentally via e+e~ annihilation to hadrons, as well as hadronic r decay, via a dispersion relation [6]. (This is discussed in more detail in a different context in Section 3.4.) Unfortunately, there is also a "light-by-light" scattering contribution, shown in Fig. 3(b), which cannot be extracted from data. This contribution is therefore typically estimated using models of hadrons, not from QCD [7].
197
A recent estimate [5] of this contribution to a p is (80 ±40) x 10 - 1 1 , but the size of this error is controversial. The only completely model independent analysis is based on chiral perturbation theory, where there is a contribution from the unknown coefficient of an operator which could easily be several times larger than this uncertainty: a£ L (had) = (l3±gg + 31C) x H T 1 1
(2)
where C is a nonperturbative parameter which is expected, on dimensional grounds, to be of order 1. However, a value of 2 or 5 is equally natural; even the original discrepancy could be fit with C ~ 10. Fitting the experimental result gives C = 7± 6
(3)
(where we have added the uncertainties in Ref. [8] in quadrature).. Most
>
\
^ (a)
>
>
^
^
>
(b)
Figure 3. Contributions of (a) vacuum polarization and (b) light-by-light scattering to the muon anomalous magnetic moment. The blobs contains low-energy quarks and gluons and so are strongly interacting.
importantly, the contribution of C is roughly the size of the proposed new physics contributions. What this means is that it is extremely hard to convincingly extract signs of new physics from this measurement. Since we are currently unable to calculate C from first principles, we are forced to rely on models of low-energy hadronic physics. Certainly, some models are demonstrably better than others [5], but the agreement between theory and experiment is good enough that without a model-independent treatment of the hadronic physics it is very difficult to convincingly argue that a discrepancy of this order could be a sign of new physics, rather than a failure in our understanding of low-energy QCD. Furthermore, it is difficult to see how this theoretical error could be reduced in the future without a fully nonperturbative calculation. The moral of this story is that, when searching for new physics in precision experiments, we need precise and model-independent predictions with
198
quantifiable error estimates. If hadrons are involved anywhere, this necessarily involves being able to systematically deal with low-energy QCD. 2.1. b Quarks
as Probes of New
Physics
Before looking for deviations from the Standard Model, we need to understand it. As indicated in Fig. 4, we observe a complicated pattern of flavour transitions, which follow a well-defined hierarchy. In the Standard Model, this pattern arises because of the hierarchy of the CKM matrix,
(
Vud Vus Vub Vcd Vcs Vcb Vtd Vt. Vtb
+ h.c.
(4)
where
O(l) O(X)
o(x32) o(\ ) FCNC Figure 4. Model.
The relative magnitudes of quark transition amplitudes in the Standard
(I A A3 Vud Vus Vuf>\ Vcd Vcs Vcb r.- A 1 A2 3 \A A2 1 Vtd Vu Vtb J
^ C KM
(5)
and A = sin^c = 0.2205 ± 0.0018 is the Cabibbo angle. Since A is small, it is useful to treat is as an expansion parameter. This allows us to write a particularly useful parameterization of the CKM matrix due to Wolfenstein [9]: 1 VcKM KA\\l
¥
-A -p-ir))
A 1 . £ -AX
AXi(p-ir1y AX2 | + 0(A4).
(6)
199
A is known at the ~ 1% level, A = 0.83 ± 0.03 is known (due to measurements I will later discuss) at the ~ 5% level, and p and r\ have larger uncertainties. A huge number of quark transitions is therefore determined by these four parameters (in addition to the quark masses, gauge couplings, and gauge boson masses), a few of which are shown in Fig. 5. The standard model is therefore extremely predictive, and the goal of the B factories and other precision low-energy experiments is to test these predictions as redundantly as possible.
s
Figure 5.
*
Some flavour-changing transitions.
Transitions between quarks of different charge, such as b —> civg. and b —> cud, are dominated by tree-level graphs in the SM and many of its extensions, and therefore simply reflect the CKM hierarchy. At one loop, however, things get more complicated. The decays denoted "FCNC" are flavour-changing-neutral current decays, in which the transition is between quarks of the same charge. These decays are forbidden at tree level in the SM, but can arise through loop diagrams such as those in Fig. 6, and so are related in a complicated way to the various SM parameters. (In addition, the GIM mechanism [10] ensures that they are also forbidden in the limit of degenerate intermediate quarks.) Hence, the amplitudes for FCNC processes are suppressed in the SM, and so new physics at the few hundred GeV scale may contribute at a comparable level to these decays, as illustrated schematically in Fig. 7. Thus, rare b decays provide a sensitive probe of the structure of flavour physics at high energy scales. Since there are many such transitions determined by a small number of parameters in the standard model, one can place strong constraints on (or discover!) new physics by studying as many transitions as possible.
200
fY U
(a)
(b)
(c)
Figure 6. FCNC processes in the standard model. Note that some four-fermion transitions, such as b —+ uud (which in turn determines the hadronic process B —> TTTT) receive comparable contributions from tree-level and one-loop "penguin" amplitudes, because the tree level amplitude is suppressed by the small CKM element Vub. This "penguin pollution" is a serious impediment to the theoretical prediction for such decays, as will be discussed in Section 4.2.2.
IT Figure 7.
A contribution to b —> S7 from new physics.
The preceding arguments hold for transitions involving any of the quarks; however, b quarks are particularly good probes of short-distance physics. There are a number of reasons for this, both theoretical and experimental, but one of the most important is that there are a number of observables that can be cleanly predicted in the SM, despite the complications of hadronization. As we will discuss in these lectures, many of these arise because the b quark is heavy: rrn, ~^> AQCD, SO b decays are shortdistance compared with the QCD scale; However, clean observables are not restricted to the b system; for example, some rare kaon decays (such as K ^ -KVV) are also very clean theoretically, and are also being intensely studied [11]. Let me make a couple of additional comments on the CKM matrix: (1) CP Violation: VCKM has a single nontrivial phase (i.e. a phase that cannot be absorbed in a field redefinition), which in the Wolfenstein parameterization is put into Vub = \Vui>\e~'17 and Vtd = \Vtd\e~l/3 (these two parameters are not independent in the SM). This phase violates charge-parity (CP) invariance of the theory, and in the SM is responsible for the observed CP violation in the K and B systems. In the presence of CP violation, the rate for a
201
(p.v)
(0,1)
Figure 8.
The unitarity relation (7) in the complex plane.
decay and its CP conjugate may differ: T(B —> / ) ^ T(B —> / ) , and so measurement of CP conjugate rates may be used to determine the phase. CP violation is interesting for a number of reasons. First of all, it is interesting in its own right, in particular because the observed universe is highly CP-asymmetric: there is far more matter than antimatter. Hence, CP violation is necessary to explain the observed state of the universe [12]. For our purposes, however, it is interesting because (i) the SM is highly predictive (there is only one phase in the CKM matrix, and so all CP violation depends on the same phase) and (ii) the CP invariance of QCD may be used to get clean hadronic measurements of this phase, and hence to test the CKM picture (as will be discussed in Section 4.2.2). Note that the presence of a single nontrivial phase is a general feature of a 3 x 3 unitary matrix, so the weak interactions with three generations naturally violate CP (and hence CP violation is not a mystery, as is sometimes stated). (2) The Unitarity Triangle: It is customary to express all of this graphically. The unitarity of VCKM leads to the relation
vudv:b + vcdv;b + vtdv;b = o.
(7)
Since each CKM element is a complex number, this relation may be drawn as a triangle the complex plane, as illustrated in Fig. 8. The length of the base of the triangle has been scaled to 1 by dividing each side by |VcdKfc| = sin^clKbl, and the apex is at the point (p, rj) as defined in Eq. (6). The angles (3 and 7 are the CKM phases denned in Eq. (6), and a = TT — j3 — 7. This construction is known as the "unitarity triangle" (UT). There are two other such triangles, arising from the two other unitarity relations, but the relation in
202 1.5
* * * « & U 0.5
sin 2 ^
ivjv, -0.5
-0.5
0.5
P
Figure 9. The experimental constraints on the unitarity triangle from different processes, and the best fit (as of summer, 2002).
Eq. (7) has the further property that each term is of order A3, so the sides of the unitarity triangle are of comparable length, while the other triangles have two sides much longer than the third and so are almost flat. This in turn means that the sides and angles of the UT should be measured with comparable precision to obtain useful constraints. Note that in the absence of CP violation, the UT would collapse to a line. Hence, by performing only CP conserving measurements of the sides of the triangle, one may determine the CP violating phase in VCKMThe UT provides a convenient way to plot constraints on the SM from different experiments, as illustrated in Fig. 9. Note, however, that it can be misleading as a guide to new physics: two measurements of a given parameter may be redundant in the SM, but in the presence of new physics are as interesting as measurements of two different parameters. For example, the CP violation in both B —* J/i^Kg and B —>• (f)Ks is proportional to sin 2/9 in the SM. However, in the presence of new physics which contributes to the b —> sss transition but not to b —* ccs this need no longer be true,
203
and so both measurements are important. Thus, the goal is not simply to measure the sides and/or angles of the unitarity triangle, but rather to measure as many different quark-level transitions as possible, to look for deviations from the SM predictions. Currently, all determinations of sides and angles of the UT are consistent with one another within the errors (see Fig. 9). 3. From Quarks to Hadrons In the last lecture we discussed how precision measurements of b quark transitions provide a sensitive probe of New Physics. Unfortunately, we can only do experiments on hadrons, not free quarks and gluons, and a simple quark transition like b —> cud can give rise to many complicated hadronic process, such as B —> DTT, B —> Dp or B —> Dinnrir, etc., all of which are completely incalculable analytically. The problem, of course, is that even though the decay of a 6 quark is a short-distance process, occurring over a typical distance scale r ~ 1/rrib, hadronization is by its very nature long-distance (r ~ 1/AQCD) and nonperturbative.
SJmn
^WD Figure 10.
An incalculable mess.
Fortunately, a Feynman diagram like that in Fig. 10 is in fact a terrible representation of the physics of B decay. If you were to actually calculate this diagram, you would find loop integrals containing wildly disparate scales: mw (the scale of the short-distance physics underlying the decay), mi, (the typical energy release of the decay) and AQCD (the scale of hadronization). Some of the physics would be perturbative, and some nonperturbative. The short-distance physics would be independent of the complications of hadronization, whereas the long-distance physics would be insensitive to many of the details of the short-distance physics (just as fluid mechanics is insensitive to the structure of the proton). Thus, a naive dia-
204
grammatic analysis obscures much of the important physics of the process, and the corresponding simplifications. Example: b —> sj We can illustrate this with an instructive example. Suppose we were particularly sheltered theorists who had never heard of hadronization, but wanted to calculate the rare FCNC process b —> sj in perturbation theory. As discussed in the previous section, because this decay arises at the loop level in the SM, new physics at the weak scale can be competitive with the SM rate, and so it provides a sensitive probe of weak-scale physics. We therefore need to be able to calculate the rate in the SM, where it gets contributions from the diagrams in Fig. 11.
b
,^%
^ y ^ v w ^ », ^ y (b)
(a)
(d)
(c)
Figure 11. Feynman diagrams contributing to b —> S7 decay in the SM at one loop. The contribution from intermediate u quarks is negligible.
Since we know about perturbation theory and infrared divergences we know we have to include graphs with both real and virtual gluons to have an infrared safe quantity. The decay is dominated by the t quark loop, and we find at 0(as) [13] TFmb
r(6 -+ x3l)
1287T
4
4a«
VM
b
w
+3
log-
(8)
7T
where [14] F2{x) = -:
+
4rr
4 0 - l) 2 1
2z-l
+ 4 (x - l ) 2
2{x 1 3 2(x-l)3
2 (x - l ) 4 2 (x - 1)
logs,
log a; (9)
Xs denotes s or s+gluon, c is a non-logarithmic term, and I have neglected small terms suppressed by powers of m^/m 2 ,. When we tried to compare with experiment, we would discover that nobody had measured the decay rate for the quark level process b —> Xsj. However, we could argue that the final state quarks and gluons have to
205
hadronize to something, so the appropriate thing to compare this to would be the sum of the decay rates to all final states containing a strange quark and a hard photon (K*j, K**j, Knj, etc.). Such decays are called inclusive decays, to be distinguished from exclusive decays to a specific hadronic state. The measured inclusive branching fraction is [15] Br(B - • Xsj)
= (3.3 ± 0.4) x 10" 4
(10)
Comparing this with the theoretical prediction in Eq. (8) is, however, problematic. At tree level, Eq. (8) gives to a branching fraction of about 1.3 x 10~ 4 , or almost a factor of three smaller than observation. Before we get too excited about having seen evidence for new physics, we notice that the one-loop correction to this result is huge, because it contains a term proportional to logm1/m\. This provides an enhancement by about a factor of seven, which counteracts the as/-7r suppression from the loop. Indeed, the oneloop correction is more than a factor of two larger than the tree level result! There is also a large scale uncertainty in this result: evaluating as(fj,) at fi = m,b increases the predicted branching fraction by almost an order of magnitude to 1.2 x 10~ 3 , or about four times the observed rate, while choosing fj, = raw lowers the prediction to about 7 x 1 0 - 4 . Both raw andTOf,are relevant scales in the problem, so there is no obvious way to choose between them. The difference is formally a two-loop effect, but the corresponding uncertainty in the branching fraction is huge. Perturbation theory is therefore showing no signs of converging: two-loop, one-loop and tree level effects are all about the same size. Clearly, we have a problem if we want to do precision physics. At the moment we have no idea whether the experimental result is in accord with the SM. Despite the fact that as(mb)/ir is less than 0.1, perturbation theory is failing badly because of these logarithmic enhancements. Nor will this problem stop at two loops - at n'th order in perturbation theory we will find terms proportional to a™ log"{m%/m\). It looks like we can only predict the decay rate to within an order of magnitude, much less the ~ 10% precision required to compete with the experimental result, and there is no way we can use this mode to search for new physics. Physically, this makes no sense. The scale m\, is large enough for perturbation theory to be valid, and the scales raw and rat are completely irrelevant to a decaying b quark - a b quark can't resolve anything smaller than its Compton wavelength, so as far as the b quark is concerned, it is decaying via a point interaction. The poor behaviour of perturbation the-
206
ory must just be an artifact of the fact that we are not calculating things sensibly. A sensible theory of b decay should never contain logarithms of mt/mi, or mw/'mb in matrix elements. Furthermore, we have been rather glib in comparing the quark-level process with the inclusive hadronic process. The decay of a b quark to free quarks and gluons is not the same as the decay of a B meson to all possible hadrons:
(1) The initial states are different. A b quark bound in a hadron is not the same as a free b quark - it is interacting with a complicated state of strongly-interacting quarks and gluons, and so is not at rest. In addition, there are dynamical light quarks and gluons in the initial state. As a practical matter, the parton-level rate in Eq. (8) is proportional to m\. Since this factor arises from phase space, we might be tempted to replace this with m% for B meson decay, which changes the prediction by about 50%. With no good reason as yet to choose one over the other, this is a large source of uncertainty. (2) The final states are different. If we were to measure the inclusive hadronic invariant mass spectrum, we would find broad peaks at the masses of all the possible kaon resonances superimposed on a continuum from multiparticle hadronic states, while the parton level prediction has a peak at ms followed by a perturbative tail from gluon and light-quark bremsstrahlung. Since the partons must hadronize to something, we may argue that if we average both distributions over a large number of resonances the two results should be equal (this idea is called "quark-hadron duality"), but how many resonances is enough? Should this argument also work for semileptonic D decay, which is dominated by only two resonances (the K and K*)l
Both of these problems arise because of the interplay of physics at the decay scale rrn, with the hadronization scale AQCD- Furthermore, in the limit mi, —> oo (or, more precisely, AQco/mb —• 0) both should vanish: an infinitely massive b quark is unaffected by collisions with light degrees of freedom, the difference between rrib and TUB is of order AQCD and so a subleading effect, and in the limit m;, —> oo there is a always a large number
207
of hadronic resonances to average over. Thus, we expect
r(l?->
J2
Xsl)=r(b^
Yl
Xsl)+0(AQCD/mb).
(11)
S99,sqq,...
However, on dimensional grounds these corrections should be at the 10% level, and as our phase space argument above indicates, may be much larger. If we really want to use B decays to look for new physics, we need a systematic way of calculating AQCD/TO& corrections. 3.1. Effective
Field
Theory
The two problems raised in the previous examples (AQCD/""ib corrections and large logarithms in perturbation theory) look different, but both have a common origin: the presence of multiple scales in the problem mw,
rnt >
mb »
AQCD-
(12)
To make progress, we need a tool which systematically disentangles the scales. This is the whole purpose of Effective Field Theory (EFT). Since Ira Rothstein has lectured extensively on EFT's at this school [16], I will be brief in my treatment and just stress the essential features [17]. The idea behind EFT is that the theory you use to calculate physics at a given scale /n should only contain degrees of freedom relevant at that scale. Anything happening at shorter distance scales — virtual heavy particle exchange, high momenta in loops — can't be resolved by external particles with momenta < fi, and so is replaced in the EFT by local interactions. Short-distance physics is therefore removed from the dynamics and simply enters in the values of the coefficients of the local operators in the effective Lagrangian. This is the same reasoning behind the multipole expansion in electrostatics: if we have a distribution of charges of size r^ and are interested in physics at distances of order r ~ ro away, we need to know the precise charge distribution. On the other hand, at distances r » ro, it is much more efficient to expand the theory in powers of TQJT. At leading order, we only need to know the total charge to determine the potential. If we wish to be more accurate, we can include the dipole moment, quadrupole moment, etc, systematically expanding the true result in powers of ro/r. It is important to stress that EFT's are useful whether or not we know the "full" (short-distance) theory. Even if we had a theory of quantum gravity, we wouldn't use it to calculate projectile motion. Similarly, the
208
fact t h a t we have a renormalizable theory of the weak interactions doesn't mean t h a t this is the appropriate theory to calculate low energy (/i
3 . 2 . mw
> fj, > nib:
Four-Fermi
Theory
Generalized
A familiar example of at E F T is four-fermi theory, the E F T of the weak interactions at scales much less t h a n mwSuppose we were interested in nonleptonic b —> c decay. At leading order this is determined by the diagram in Fig. 12(a). However, all external momenta in this graph are of order nib, while the mass of the W boson is mw 3> nib, so according to the rules of E F T it should not b e present in t h e theory. This is easy enough t o implement: since the m o m e n t u m transfer q through the W boson is
(13)
where ipL = | ( 1 — 75 )V'- This amplitude may be reproduced to arbitrary order in q/mw in an E F T with no W boson, but a series of higher-dimensional operators: 4GFycb £ S M —• -Ceff = £ Q C D + Q E D
1 m
2
w
-cLj^bLD2
V2 (dL^^uL)
cL-yfJ,bLdLJtj,uL (14)
This procedure of determining the coefficients of the operators in the E F T
Figure 12. For momentum transfers q <S mw> the W boson may be integrated out of the effective theory and replaced by a series of nonrenormalizable operators.
209
by demanding that the full and effective theories give the same physics is called matching, and the W (along with the Z, higgs and t quark) is said to have been "integrated out" of the theory. In principle the EFT has an infinite number of terms, but for q2
c
(a) Figure 13.
\c
(b)
Radiative correction to b decay in the full and effective theories.
calculate (because it contains four propagators, two of which are massive), and is a complicated function of mi, and mw- By power-counting, it is UV convergent. The effective theory graph is quite a bit simpler to calculate, but by power counting is UV divergent. This extra divergence looks worrisome, but it shouldn't be: the EFT is designed to get the physics right only at scales \i mw the W propagator softens the high-momentum behaviour of the loop integral. Since the EFT isn't supposed to get the physics right at momenta > mw, it is not surprising that it is divergent. (Both graphs are also IR divergent.) The EFT graph therefore requires additional renormalization. The result is then scheme-dependent, so it is not clear that it is telling us anything useful. However, the full theory graph will contain logarithms of the form V ^ l n ^ -
(15)
where the pj's are the external momenta in the graph, in addition to infrared divergent logarithms of the gluon mass (if we choose to regulate the theory this way). These are the large logarithms which we encountered in
210
the previous section, and which can spoil the convergence of perturbation theory. Since the EFT gets the physics of the external states right, it must reproduce these logarithms, and so must contain terms of the form
J^ln-^-
(16)
Pi • Pi
where c^ • = Cij + 0 ( 1 / T O ^ ) , and the renormalization scale /J, must appear in this way on dimensional grounds. In addition, the infrared divergences arise from low-momentum physics, so are identical (at leading order in 1/m^) in the two theories. Thus, as advertised, at one loop the EFT correctly reproduces the lowenergy physics (the logarithms) of the full theory. Of course, there are other important, non-logarithmic terms, but in the EFT these do not arise from loop graphs, but rather are contained in the coefficients of operators in the effective Lagrangian. These must be included for consistency, because before we can consistency calculate radiative corrections in the EFT, we must also calculate the radiative corrections to the matching procedure in Fig. 14; in other words, we must adjust the coefficients of the four-fermi operators in the EFT to ensure that matrix elements are the same in the two theories up to 0(as, l / m | ^ ) . This is illustrated schematically in Fig. 14. In this case, we find that we must add an additional operator to the effective Lagrangian,
Figure 14.
One-loop matching.
02 = cLTa^bLdLTalfluL.
(17)
whose coefficient is just given by the difference of the one-loop calculations in the full and effective theories. Thus, the difference in the finite pieces of the graphs goes into the coefficient C2(M)- Furthermore, to work consistently to this order in a3, matrix elements of O2 in the effective theory are only evaluated at tree level.
211
In general, then, the effective Lagrangian may be written £eff = £QCD+QED + J2
C
^)°i
(18)
i
where the 0;'s denote operators renormalized at the scale /J,, and the Cj(/Lt)'s are determined by matching to a given order i n a s . Note that in order for this whole procedure to work, it is crucial that logarithms of external momenta (as well as infrared divergences) are identical in the full and effective theories; otherwise, the difference between the graphs in the two theories could not be written as the matrix element of local operators. Tree-level matrix elements of local operators can only contain powers of external momenta (from derivatives), not logarithms. At this point it may seem that we've gone through a long song-anddance for no apparent reason. The EFT graphs may be simpler than the full theory graphs, but we have to calculate both to do the matching conditions. So what have we gained? (1) We have explicitly factorized the long- and short-distance physics, with /i serving as the factorization scale. The long-distance physics (the logs of fj?/pi • pj) is contained in the matrix elements of the operators Oi, while the short-distance logs of mw/fJ- are contained in the coefficients in the effective Lagrangian. Note that the longdistance physics is process-dependent (differing for different external states) whereas the short-distance physics is incorporated into the parameters of the theory, and so is independent of the particular process being studied. (2) Because the matching conditions are independent of the external states, we may calculate them using any external states we choose. In particular, if we are working at leading order in 1/m^, we may calculate the matrix elements in both theories between external states with zero momentum, which will greatly simplify the calculations. Both theories will give infrared divergent results, but the infrared divergences will cancel in the matching. (3) Finally, since the renormalization scale [J, is arbitrary, we can eliminate large logarithms in matrix elements by simply choosing to renormalize the theory at /x ~ ra^. Note that there appears to be a tradeoff here: choosing /z ~ rrn, makes perturbation theory work well for matrix elements, but poorly for the matching conditions, since the coefficients Cj(/i) in Eq. (18) will contain the large logarithms. However, the renormalization group gives us the best of
212
both worlds: we can calculate the matching conditions in the EFT renormalized at /J, ~ mw, where perturbation works well for the Cj's, and then use the renormalization group equations (RGE's) to lower the renormalization scale to \i = m,b, where we calculate matrix elements. Let me elaborate on that last point. Matrix elements of the effective Hamiltonian are physical, and so independent of the unphysical renormalization scale fji. Thus, /i-f
{CiMOi)
=0
(19)
and so
A ^ M = I>iCiG")
(20)
j
where d/d/i is the total derivative, including the change in the coupling
SO): d
d
_. . d
«" = ^ = A i S i
+
<21>
-
and /3o = —(11 — 2n//3) for n / light quark flavours. The anomalous dimension matrix is defined by
(22)
l,i = Zjk ( ^ ^ ) where Zij relates the bare and renormalized operators, 0^are = ZijOj.
(23)
The RGE is solved by diagonalizing the anomalous dimension matrix; for the simple case where there is only one operator 0(/z), the solution is 70
where the anomalous dimension of the operator O is 7oas(/u)/47r. Expanding this out gives C{fi2) =
1
-.
4TT
7ou In — + '
">
2
'
32^
70(70 + 2/30) In — + . . . >^'»
' - ™ -
C(m).
M2
(25)
213
and we see that the complete series of leading logs of ^2/^1 has been summed by the RGE and put into the coefficient function C{^2)The procedure is then straightforward. We calculate the matching conditions at the high scale fi = mw, where perturbation theory works well. Then, using the RGE, we evolve the coefficients from fs = mw to \i ~ mb. The large logarithms are summed to all orders in the Cj(/i)'s, and matrix elements at /i — mb contain no logarithms of m\/mw. Note that the leading logs in Eq. (25) correspond to matching at tree level, running at one loop and taking matrix elements at tree level. Including the one-loop matching corrections and one-loop matrix elements corresponds to including terms of order a " + 1 In™ [ixjiii- This is the same order as the terms summed using the two-loop running, and so both must be included to work consistently at this order. Thus, as a general rule, the matching conditions and matrix elements must be evaluated at one lower order in as than the anomalous dimensions of the operators. 3.2.1. Semileptonic decay Let me illustrate this with a few examples. In the SM, there are many flavour changing operators in the effective theory below mw, but for a given process one only deals with the appropriate subset of operators. Semileptonic b —> c (or b —> u) decay is the simplest: at leading order in 1/mw, the only operator is 0(/i) = cL^bLPe^eL.
(26)
M
Since C7 (1 — 75)6 is a partially conserved current in QCD (because chiral symmetry is only softly broken by the quark masses mc and mi,), this operator requires no additional renormalization, and so does not run below the scale mw- Thus, C(mb) = C(mw)
= -^Vcb
and there are no large logarithms of mb/mw to sum. The final result is r(6 ^ XJLv) = ^^f"
(27)
in the semileptonic b decays
f(mc/mb)
+ 0(as)
(28)
where f(x) = l-8x2 and the 0(as)
+ 8x4-...
terms do not contain any logarithms of mw-
(29)
214
3.2.2. Nonleptonic b —> c decay Nonleptonic decays are considerably more complicated. The simplest nonleptonic decay, the Afe = 1, Ac = 1 (i.e. b —> cqq) transition, is mediated at tree level by the operator Oj = cL^bLdLltluL
(30)
where
C K ) = -~^vcbv:d.
(31)
However, under renormalization, diagrams such as that in Fig. 13 (b) require a counterterm with a different colour structure: On = CL^^dLjT^ul
(32)
where i and j are explicit colour indices. (On is just a convenient linear combination of Oi and O2 from the previous discussion.) The corresponding RGE is therefore a matrix equation M
d ( din) \ dM \Cnfjj))
=
a ^ /711 7 i 2 \ ( C 7 (/i) \ 4TT U i 722J [Cuifj,))
(33)
where, at one loop, 7 n = 722 = - 2 , 712 = 721 = 6.
(34)
Since 712 and 721 are nonzero, 0 / and On mix under renormalization. The solution to Eq. (33) is
CiMn) = \
as(mw)\ as(fi) J
23
,
fas{mw)\ V «*(M) /
(35)
Thus, even though Cn{n~iw) = 0 at tree level, it is induced for \i < mwUsing as(m\v) = 0.12 and as(mb) = 0.22, we find Ci{mb) = 1.11, Cn{mb)
= -0.26.
(36)
3.2.3. b —> S7 decay The Ab = 1, As = 1 FCNC decay 6—> S7 is more complicated still. The diagrams in Fig. 11 (a, b) match onto an operator in the EFT of the form 07 ~ SL^baF^
(37)
whose matrix element determines the decay rate at tree level. However, Or mixes under renormalization with a whole slew of other operators. For
215
Figure 15. O2 and Or mix under renormalization. This is a two-loop graph, but it contains only one strong loop and so contributes to the leading order anomalous dimension matrix.
example, the operator 6L7' 1 CLCL7 M SL requires Or as a counterterm from the graph in Fig. 15. It turns out that a total of eight operators are required [18]: AC
HeS =
(38)
—lvtbv;sJ2cl(^)oi
where 0 1 = {CLf3l^bLa)
(SLaJ^Cip)
,
q=u..b °5=
^2 (sLotJ^bLa) (<7fl/37/^ii/3) . q=u..b
Or = ^ n i t i I a / ' W F ,
02 = {cLal^bLa)
(s L/3 7 M C L/3 )
q—u..b °& = J Z i^Lal^Lp) q=u..b
08 = -^mhsLa<j»vT^bR!iGlv
(QRPl^QRa)
(39)
and the en's and /3's are colour indices. At leading order in as only C2(mw), Cr(mw) and Cg(mw) are nonzero, but the other operators mix with these under renormalization, so one needs an 8 x 8 anomalous dimension matrix to obtain C-j{rrn,)- Furthermore, the calculation is complicated considerably by the fact that the mixing of O2 — 0% with Or and 08 first occurs at the two-loop level, through diagrams like that in Fig. 15. Since there is only one QCD loop in these diagrams, they contribute to the anomalous dimension matrix at leading order. The leading order (LO) RGE's for b —> Xsj were first solved by Grinstein, Springer and Wise [18] in the simplified approximation in which the mixing of O3 — Oe with Or and 08 was neglected. With this simplification,
216
they found as{mw) as(mb) _
C7(mw)
-
232
+ 513
-C8(mw)
_ / as(mb) \ '' as(mw)J
us(jnb) V as(mw)J
(40)
The complete anomalous dimension matrix was later completed by Misiak [19]; the results differ from this by about 10%. Resumming the leading logs in this way increases the tree level theoretical prediction in Eq. (8) by roughly a factor of three, bringing the prediction into agreement with the measurement, Eq. (10). However, there is still about a 25% theoretical uncertainty in this prediction, just by varying the renormalization scale by about a factor of two about /i = mb- For precision physics, this uncertainty is still disturbingly large, and to reduce it the next-to-leading-order (NLO) calculation is required. The complete NLO calculation of b —> Xsj is probably the most involved QCD calculation ever performed. There have been many papers over the past ten years to get to the current state of the art [20]. The complicating factor is that, since the leading order RGE equations involve two-loop diagrams, the NLO' calculation requires three-loop anomalous dimensions, as well as two-loop matching conditions and matrix elements. The current state of the art gives [20] Br(& -+ Xsl)f™l6
GeV
= (3.57 ± 0.30) x UT 4 .
(41)
Comparing this with the experimental result, Eq. (10), we see that as long as the l/m& corrections relating this to the branching fraction for B —» Xsj are small, there is little room for discovering new physics in this decay. Since the NNLO calculation requires four-loop calculations, it is unlikely that the theoretical error in this prediction will be able to be signficantly reduced anytime soon. 3.3. n <mb:
HQET
In the parton model calculation of inclusive B widths no scales below mj appear in the calculation, so it is sufficient to work with the effective fourfermi theory discussed in the previous section, renormalized at /J, = nibThis is a consequence of the fact that at leading order in l/nib the decay is entirely determined by short (r < l/mb) distance physics. On the other hand, decays to specific final states (exclusive decays) do get contributions
217
from long distance physics, since that determines the details of hadronization. For example, to calculate the exclusive process B —> if* 7 instead of the quark level process b —> X s 7 , we need to know the matrix elements of all eight operators (39) in the effective Hamiltonian (38) between a B meson and a if*, which are all determined by long-distance physics. Thus, in keeping with the EFT philosophy, we would like to work in an EFT which factorizes the physics of the parton-level decay at [i = rrib from the physics at lower scales. Since the long-distance contribution to exclusive decays is nonperturbative, it's not clear that switching to an EFT will be particularly useful, since EFT doesn't give us any new tools to solve strongly coupled theories. However, there are still reasons to switch to an EFT description: (1) The EFT allows us to identify simplications (in particular, approximate symmetries) which (in some cases) still allow us to say something about the hadronic physics. (2) The only way to calculate nonperturbative physics is to use lattice gauge theory. It is extremely inefficient to attempt to simulate a wide range of energy scales on the lattice, since the lattice spacing must be very fine (to simulate short distances correctly) and the lattice must be large (to fit the long-distance physics in). It is much more efficient to take care of the short-distance physics analytically, and only simulate the long-distance physics on the lattice. EFT does this automatically for us: an EFT renormalized at fj, can be matched smoothly onto a theory with a lattice spacing a ~ l//i. I won't have much to say about lattice gauge theory in these lectures, so I will instead focus on the first point. In general, factorizing physics at TOh from lower scales in B decay is a much trickier procedure than simply integrating heavy particles out of the theory, since in a given decay there are partons carrying large (~ TO;,) momenta (such as the constituents of the if* in B —> if *7 decay), so there may be both hard (~ mj,) and soft (~ AQCD) momentum transfers in the same process. This should be contrasted with four-fermi theory, where no external momenta are of order mw • It is not obvious how to unravel the physics at these different scales, making it complicated to construct the appropriate EFT. I will say a little bit at the end of these lectures on the general problem, but in this section I will discuss an important case where we do not have to worry about such subtleties. "Heavy Quark Effective Theory" [21-23] (HQET) is the EFT for a single
218
heavy quark in which all hadronic momentum transfers are much smaller than mi,. This is much simpler than the general case because the scale mj never enters the dynamics, so there is no complicated dynamical unraveling of scales required. This is the appropriate theory for semileptonic B to D decay, for example, at least in the region of phase space in which the D recoils slowly. However, it is not appropriate for B —> K*~f decay, since in the limit mj, —> oo the recoiling K* carries energy of order mb/2, which is not
p£ = mbv" + k»
(42)
where v^ is a fixed four-velocity (usually chosen to be the four-velocity of the meson, although any velocity differing from this by at most 0(AQCD/mb) will do). Since all momentum transfers are soft, scattering in the EFT can only change the residual momentum k^. The EFT can then be obtained by expanding amplitudes in powers of k^/nibThis is easy to do: for example, expanding the b quark propagator in powers of k^/mb gives j) + mb _ mbf + mb p2 — m2 + it 2m,bV-k + ie
| Q /fc"\ = TO
\
b/
/ ! +A 1 | \ 2 Jv-k + ie
which can then be inverted to give the kinetic term in the EFT: ZHQET
= bviD-vbv + o(-^-j.
(44)
Note that the propagator includes the projector \(l+f), which projects out the upper 2 (particle) components on the b field, so the EFT is a theory of 2 component spinors bv. The bv fields have also been defined with a large phase exp(imbv • x) relative to the QCD field, so that derivatives acting on the bv's pull down factors offc^instead of p%. The Feynman rules for HQET are shown in Fig. 16. Let me make some comments: a
Although we do integrate out b quark loops.
219
1
12VV K£>
XL
v-k Figure 16.
Feynman rules for HQET at leading order.
(1) Working for simplicity in the rest frame of the meson, v^ = (1,0,0,0), we see that the propagator is just the nonrelativistic energy k° of a static quark, and that it couples only to the electric field (via the scalar potential A0). Such objects had actually been extensively studied before the advent of HQET - they are called Wilson lines, and are simply static colour charges. Indeed, this is exactly what a heavy quark looks like in the rriQ —> oo limit: since all external momenta are negligible compared withTOQ,interactions can't change the 4-velocity of the heavy quark, so in its rest frame it is just a static source of colour. (2) The leading order HQET Lagrangian has no reference either to mj, or to the spin of the quark. Physically, this is because of the previous comment: as long as TUQ is much greater than any external momentum, the mass of the heavy quark is irrelevant to its dynamics - it's just a static source. Furthermore, the magnetic moment of a particle is inversely proportional to its mass, so the colour field of an infinitely massive quark only depends on its total charge, not its spin. This simple observation has profound implications, because it means that HQET has an enhanced symmetry compared with QCD: for nh heavy flavours, long-distance QCD is symmetric under the interchange of either spin state of any of the heavy flavours with the same four-velocity. Hence, the Lagrangian for HQET with n/i heavy flavours with the same four-velocity, nh
£ = ^
'
hiiViD • vhiiV
(45)
i=l
has an SU(2n/») spin-flavour symmetry. This will prove very useful shortly. Note that the symmetry exchanges quarks of the same velocity, not momentum. (3) We can easily go to higher orders in 1/mb by matching Green's functions [24,25]. For example, the three-point quark-quark-photon
220
vertex can be expanded using the Gordon decomposition , .q.v°'a'' ^ 2rrib
'{p + p'Y 2rrib = v-Au'u+
—
2mq
(k + k') -Au'u + i-^-u'tr^A^u. 2rrib
(46)
The first term arises from the leading order kinetic term in the Lagrangian; the second two arise from dimension five operators which can be added: £HQET
= bviD -vbv + -— (bv{iD)2bv + CV(/x)-6„cr • GbvJ
+0(l/ml).
(47) 2
The first correction term is just the k /2m kinetic energy term, while the second corresponds to the chromomagnetic moment of the b quark, and Cpirrib) = 1. This term is the leading operator which breaks the heavy quark spin symmetry. Weak Currents In addition, we can match weak currents between the full and effective theories. At leading order in as and l/m&, this is trivial. For example, the weak b —> c current matches onto the current CM6„7"(1 -
7 5 )c,
C(mb) = 1 + 0(as)
(48)
in HQET. However, unlike in the full theory, this operator requires additional renormalization in the EFT. From the diagrams in Fig. 17, the current has an anomalous dimension [21,22]
(49)
™=-6 and so
CM
/•
/
NN
=(w)
-6/25
c(mt)
(50)
But now we must be careful - this gives the correct running between \i = mb and fx = mc. But below // = mc, we now restrict all momentum transfers to be < mc, which means we must match onto a new EFT where the c quark is treated as heavy as well, £ = bviD • vbv + cv'iD • v'cv> + 0(l/m(, C)
(51)
221
Figure 17.
Diagrams contributing to the anomalous dimension of a heavy-light current
in HQET.
and the current is C(v • v', Ai)6„7/i(1 - 75)
(52)
The anomalous dimension for this current is [26] lT=ih(y-
v'r(v • v') - 1)
3TT2
(53)
where In (w + \/w2 - 1)
r(w) = 2
y/w
—1
V
(54)
/
and so nf i \ C(vv,(i)=[
(as{mb)\ — r \as(mc)J
fas(mc)\ C(mb), rT\ aa(n) J
n<mc.
(55)
Note that at when t> = v' (v • v' = 1), the anomalous dimension JT vanishes, and the heavy-heavy current doesn't run. This is because of the enhanced symmetry I mentioned earlier: when v = v', the Lagrangian (51) has an 577(4) spin-flavour symmetry which interchanges the spin components of both heavy flavours. The current bvTcv (for any T) is a generator of this symmetry. It is therefore related to a conserved charge, and so its anomalous dimension in HQET vanishes. 3.4. Inclusive
Decays and the
OPE
For general inclusive decays, we can't naively use HQET. Because the final state particles typically carry large (0(rrib)) momentum, it is not true that all the hadronic momentum transfers are soft. However, as we have argued on physical grounds, at leading order in A Q C D / ^ 6 , B meson decay is identical to free b quark decay. The argument depended on scale separation, so it is not surprising that we should be able to formulate the problem in the EFT language. The trick is that because the final state particles are highly
222
energetic, through a bit of cleverness in the complex plane we can relate the physical decay amplitude to an amplitude in which the decay products off their mass shells by 0(mb). In this region they may be integrated out of the theory just like a massive W boson, and we may treat the rest of the problem in HQET. This trick is called an operator product expansion (OPE), and its use long predates b physics. e+e~ —> hadrons and r decays In a classic 1975 paper, Poggio, Quinn and Weinberg [27] showed that the ratio = w
g(e+e--»
hadrons + X)(s) a{e+e~ -^n+n~ + X){s)
(where X denotes additional photons and fermi-antifermion pairs) was calculable in perturbation theory. It is instructive to go through their argument to see how it applies to the case of B decays. The optical theorem tells us that R(s) is proportional to the imaginary part of the photon self-energy (see Fig. 18) R(s) =
12TT Im
n ( s + ie)
(57)
where I P V ) =i|rf4xe^-(0|T^M(x)^M(0)|0).
(58)
Independent of perturbation theory, we know the analytic structure of
Im||vW\J
|/W\/]«£w rPvWNC^" 1 X
Figure 18.
The optical theorem.
H(q2): it is analytic everywhere except along the real axis for q2 > 4m^, where there is a branch cut due to real intermediate states, as shown in Fig. 19(a). (There are also isolated poles due to positronium bound states, but these are not important for our discussion.) Near the branch cut there are resonances which are sensitive to the details of long-distance QCD, so perturbation theory breaks down; however, in unphysical regions far from the cut we expect perturbation theory to be valid.
223
-*-x A WWWWAIV
(a) Figure 19. The analytic structure of n ( q 2 ) in the q2 plane. The wavy line denotes the branch cut due to real external states, (a) The smeared ratio for e+e~ —> hadrons is determined by II(q 2 ) in the unphysical region, (b) The phase space integral along the path C\ for hadronic r decay may be deformed to the path C2, in the unphysical region.
Of course, we are interested in the physical region. However, Poggio et. al. pointed out that if we average the value of R over a region of size A, Cauchy's theorem allows us to relate this to U(q2) in the unphysical region, where there are no hadrons: R(s') , , 1 A r (59) R(s,A) 2 2 da' = — [U(s + iA) - U(s - iA)}. (s' - s) + A 2i 7T JO This shouldn't come as a surprise. For q2 » A Q C D annihilation is a shortdistance process. For a fixed physical value of q2 the cross-section might be sensitive to long-distance physics (since q2 could be sitting just above or below a threshold), but if we average over a number of states these effects should cancel out, leaving us with a purely short-distance process. This idea is called "global duality", and it suggests that perturbation theory should be appropriate for sufficiently smeared observables. (The definition of "sufficient" here can be problematic, but in general it means that many resonances are averaged over.) This argument only strictly holds in the q2 —> 00 limit. To see how corrections arise, let's study this in a bit more detail. In the physical region, n(q 2 ) is nonlocal: the intermediate hadronic state is on shell, and so can travel a long distance. On the other hand, in the unphysical region the final-state quarks, like the virtual W in four-fermi theory, are far offshell, and so can be integrated out of the EFT and replaced by a series of local operators, as illustrated in Fig. 20. This procedure is called an operator product expansion (OPE) — morally, it's the same as integrating heavy particles out of the EFT, but in this case the intermediate particle isn't necessarily massive, just constrained by the kinematics to be far offshell.
224
Im v/vf
Vv^ + vA/^<|)jiA^+...
= 'vxgyv
Im (^ <<§•
££
%
Figure 20. The operator product expansion (OPE) for IT(g 2 ). The sum is over all operators with the quantum numbers of the vacuum. Oo is the identity operator, and Oi =Ga^G^va.
Just as in an EFT, the matrix elements of operators of increasing dimension are suppressed by powers of AQCD/? 2 > and so for q2
(60)
i
where the Oi's include all operators with the quantum numbers of the vacuum, and as in an EFT, the C^s are calculated order by order in perturbation theory. The leading order operator (denoted Oo in the figure) is the unit operator. Since its matrix element receives no radiative corrections, its coefficient is given by the imaginary part of the perturbative vacuum polarization, which is just the parton-level cross-section. Thus, as advertised, at leading order the OPE simply reproduces the parton model. The leading correction arises from the operator 0\ = G'\vG^va, whose coefficient is obtained by calculating the T-product of two currents in which two soft gluons are also emitted, and expanding the result in powers of the external gluon momenta. By dimensional analysis, the matrix element of 0\ is proportional to A Q C D , so the leading corrections to the parton model are suppressed by A Q C D / S 2 . This technique has many applications. In addition to e + e~ annihilation, it can also be easily extended to hadronic r decays [28], T —> vT + hadrons
(61)
which has been used to extract as(mT) with high precision. In this case the smearing is automatically done by the phase space integral, as illustrated in Fig. 19(b). The phase space integral of the imaginary part of II(s) from
225
s = 0 to s = m2 (weighted by the appropriate kinematic functions) is equivalent to the weighted contour integral of II(s) along the curve C±. C± may be deformed to C2, which lies almost entirely in the unphysical region, and so the OPE may be applied to the integrated rate. OPE for B decays For B decays similar arguments hold, although in this case the initial state contains a B meson rather than the vacuum [29]. This is in fact not much of a complication, since the QCD vacuum is also nonperturbative and complicated, and in both cases the hadronic physics enters in the matrix elements of local operators. In addition, in both cases the matrix element of the leading operator is normalized, so that the leading order calculation reproduces the parton model. As with r decays, the phase space integral over the lepton momentum in B decays performs the smearing automatically. As in Eq. (58), the decay rate is proportional to the imaginary part of the T-product of two weak currents. We can then decompose this into different Lorentz structures: T V
"
=
j{B\Tjt{x)jU^\B)e-^x^x
" 2 ^
= -g^T^q2, v
v»v"T2{q2, q • v) - ie^^vaq0T3(q27q
q-v)+ 2
+q»q Ti(q ,q
v
2
• v) + {v»q + v"q»)T5(q ,q
• v).
• v) (62)
Because the heavy meson rest frame defines a vector v^, the TVs are functions of q2 and q • v instead of just q2. For fixed q2, the analytic structure of the Tj's in the q • v plane is shown in Fig. 21. The left-hand branch cut corresponds to physical intermediate states for the decay process, while the right-hand cut corresponds to intermediate states containing a q quark and two b quarks (which is related to scattering, not decay). The differential decay rate is given by the integral over the left-hand cut,
dT~ / Jo ~ /
Im Im T^L^
T^L^diP.S.) d(RS.)
JCi
-
226
are free to deform the contour C\ into C2, which lies far from the resonances. We then perform an OPE for the intermediate state, as illustrated in Fig. 21. In addition to integrating out the intermediate hadronic state, in performing
q-v\ W M ws/VQ/ywyst <st-
\
VWVWSr
C2
Figure 21. The analytic structure of T^v. may be deformed to the path Ci.
The phase space integral on the path C\
the OPE for B decay we also switch to HQET, since the matrix elements of the resulting operators are to be evaluated at a renormalization scale below rrib. Explicitly, for semileptonic ( ) - » « decay, the forward matrix element of T^v between free quarks with momenta pb = m^v + k is (rribV — q + k)2 + ie
= {mbva - qa), + (mbv - q)2 + ie pHV
hvPhhv (iribV — q)2 + ie
0(hP/mb)
0{k^/mb) (64)
where P^ = \{mbva - qa){v"gatx + v»gav - vag^ + iea^xvx)Note that in the expansion, the external current q is taken to be of order nib, while fcM ~ AQCD- Taking the imaginary part converts the denominator to a delta function, —Im 7T
{mbV
2
q)
5 ({mbv - q)2) .
(65)
ie
The axial piece of the matrix element of bvPjJ3v vanishes in B-mesons, so at leading order the imaginary part of the nonlocal T-product is given by the matrix element of the local operator Oo
(66)
with coefficient given by Eq. (64). This operator is a symmetry current of the theory — it just counts the number of b quarks in the state — so its
227
matrix element is fixed by symmetry. With the conventional normalization of states, {B\hvhv\B)
= 2mB.
(67)
Thus, just as for e+e~ annihilation and r decay, the leading term in the OPE simply reproduces the free quark process, as calculated in perturbation theory. At subleading order, the only dimension four operator is hvD^hv. By Lorentz invariance, its matrix element must be proportional to ^ , so (B\bvD»bv\B)
= v»{B\bvv • Dbv\B).
(68)
However, from Eq. (44), the leading order equation of motion in HQET b is v • Dhv = 0, so the matrix element vanishes at this order. Thus, there are no corrections to the parton model at 0{l/rrn,)\ We will discuss higher order corrections in the next section. Note that one immediate implication of the OPE is that inclusive decay rates for ^-flavoured mesons are determined by parton-level kinematics, not hadron kinematics. In particular, the rate for B —> Xs-f decay is therefore proportional to ml, not mB, since the latter would introduce a 1/mb correction to the parton rate, which we have seen is absent. 4. Applications In the last few sections I introduced the main theoretical tools that allow us to factorize long and short-distance physics. Now we proceed to some applications. 4.1. Inclusive
Decays:
Corrections
to the Parton
Model
We saw in the previous section that up to 0(l/rrib), inclusive B decay rates are the same as that of free b quarks to quarks and gluons. To obtain higher accuracy, we must include in the OPE operators of dimension five, of which there are only two. Their matrix elements are conventionally denoted as Ai =
2mB
-^(B\bv(iD)2bv\B)
A2 = -^—(B\gbva^G^bv\B). 6ms
(69)
b A n important, but sometimes unappreciated, feature of the equations of motion is that they may always be applied at the operator level, even though the quarks in hadronic matrix elements are not on shell. See Ref. [30].
228
(In another convention, these matrix elements are denoted fj% = — \\ + ... and p?G = 3A2 + . . . , where the ellipses denote terms suppressed by powers of l/mb.) These are the same operators which appear in the HQET Lagrangian Eq. (47), and correspond to the kinetic energy of the heavy quark in the meson, and the chromomagnetic energy. Since the latter is the leading operator which violates spin symmetry, it may be determined from the experimentally measured B — B* mass splitting, A2 - i ( m | . - m2B) + 0(AQCD/mb)
~ 0.12GeV 2
(70)
while Ai cannot be simply related to meson masses. Thus, up to 0 ( A Q C D / T O ^ ) , inclusive quantities may be expressed in terms of only two unknown parameters, mb and Ai. From the HQET Lagrangian (47), the meson and quark masses are related via Ai+j3A 2 —-2 \-... (71) 2m{ so mb may be exchanged for A, the energy of the light degrees of freedom in the meson. At the price of introducing even more parameters, one may continue to expand to higher orders in 1/m;,. At 0 ( l / m | ) there are two more operators, whose matrix elements are defined as 1_ • D){iD»)bv\B) Pl = 7^-(B\bv(iDfl)(iv 2mB mB = mb + A
Pi = -T-^-e a ^*(B|6„(i£> a )(t£> M )(i£> /3 )7 4 7 5 6„|B). (72) orris In addition, there are the T-products of l/mb corrections to the HQET Lagrangian with the operators (69), for a total of six unknown quantities. For most quantities, expressions are available up to 0(l/mjj), although uncertainties at this order mean that the l/m^ terms are typically used to estimate theoretical uncertainties. 4.1.1. Total decay widths For example, for b —> s-y we have, at tree level [31], T(B - Xsl)
=
^r^\VtbVtsf\C7(rnb)\2
1m\
2ml
(73) The A2 term, which is known, reduces the rate by about 2%, while the Ai term is less than a 1% effect for reasonable values of Ai. The l/mb
229
corrections are therefore very small for this decay, and do not appreciably shift the prediction (41). The semileptonic B —> Xciv decay width is proportional to the CKM matrix element Vcb- The result is [29] T(B - • Xcev) = r 0
1 as fo{rhq)+ ^—2/i(m„Ai,A 2 ) + A0(mq) —
(74)
where G%ml\Vcb\"
r
1927T3
fo(mq) = l-8rh2q
+ 8mq - m8q - 24mq l n m , ,
/i(m„Ai,A 2 ) = Xi (l - 8ml + 8mq - m8q - 24mqlnmq) 2
(75) 8
+A 2 ( - 9 + 24m - 72m* + 72mq - 15m q - 72m^lnm g ) and AQ contains the one-loop" radiative corrections [32]. Hence, with a precise determination of A and Ai, a precise determination of Vcb is possible. Finally, setting mc = 0 in this expression, we obtain the rate for B —> Xu£v, T(B - •
G2Fml\Vub\
Xuev)
1
, Ax -9A2 , / 2 5
2
2\
as
1927T3
(76) 4.1.2. Differential widths and spectra We can construct an OPE not only for total decay rates, but also for differential rates and therefore spectral moments. There has been much recent interest in such quantities because they allow the nonperturbative parameters A, Ai (and, in principle, higher order terms) to be determined experimentally. This not only allows the rates in the previous section to be predicted with high accuracy, but consistency between different observables provides a stringent test of the whole OPE picture for inclusive decays. Some quantities of particular recent interest include the first moment of the photon energy spectrum in B —> Xsj decay [31,33], the first moment of the hadronic energy spectrum in B —> XJLv decay [34], and various weighted spectral moments of the charged lepton spectrum in B —> Xc£p decay [35]. Working to 0(\jmff), each of these observables constrains a linear combination of A and Ai. For example, the results of several recent CLEO analyses are shown in Fig. 22. This analysis obtained [36] { xs) mh
= 4.82 ± 0 . 0 7 B ± 0.11 T GeV,
Ai = -0.25 ± 0.02,sT ± 0 . 0 5 s y ± 0.14 T GeV 2
(77)
230 1630802-010
0
rr-.ro, | i i r.i | i n I [ i M i [ I I i I | I I I I | I i I I | i i
0 t h Moment of Lepton Energy
%v \\\
-0.05 -0.10
St
-0.15 1 a i Moment ^ % . of Lepton Energy $-0.25 l St
1 Moment of Photon Energy (b—sy)
=£-0.20
1 s t Hadronic _ Mass Moment
-C-0.30 -0.35
1a Total Experimental Ellipse
-0.40 -0.45 -0.50
11
0
0.1
J
1 1 1 '
0.2
1 1 1 1 1 i>
0.3 0.4 0.5 A (GeV)
Figure 22. Experimental constraints on A and Ai, from between the different measurements.
~
r I I l W l ' 1
0.6
0.7
I I
0.8
[36]. Note the consistency
(IS)
where the "IS" mass myb ' is an example of a short-distance b quark mass, which is preferred for practical purposes to A because of the poor behaviour of perturbation theory when the quark pole mass is used in calculations. 0 Global fits to a variety of spectral moments have also been performed by various groups, using expressions up to 0(1/ml), which give comparable results [38]. Combining these values with the experimental measurement of the semileptonic B —> Xc width gives [36] |Vcb\ = (40.8 ± 0.5 ± 0.4 ± 0.9) x 10~ 3
(78)
where the first error is from the uncertainty in the total semileptonic width, the second from mb and Ai, and the third an estimate of higher order corrections. c This is a fascinating side issue which I have no time to discuss here. See Ref. [37] for further discussion.
231
4.1.3. Vut, and the shape function In principle, one could use Eq. (76) and the values of rrn, and Ai from the moment fits to determine Vub from the total inclusive semileptonic b —> u width, with a theoretical uncertainty at the ~ 5% level. However, because of the ~ 100 x background from b —> c decays, the inclusive b —> u rate can only be measured in restricted regions of phase space where the charm background is absent. In these restricted regions, the usual OPE breaks down. This is easiest to see by instead considering the analogous situation in inclusive b —> Xs^f decay. Suppose we were interested not in the total rate but instead in the differential rate dTjdE1 This is in fact an important experimental quantity, since one has to impose a hard cut on the photon energy E1 > 2.2 GeV to eliminate background from b —> c decay, and it is important to know how much of the b —• sj this cuts out. Unfortunately, the OPE gives us little information. At tree level the decay is purely twobody, so the photon energy spectrum is just a delta function at E7 = m&/2. At higher orders in the OPE it doesn't look much better - higher order terms are proportional to derivatives of delta functions at the same point, so this spectrum makes no sense unless we integrate it over some region of E1. Radiative corrections populate the region E < rrib/2 because of gluon bremmstrahlung, but the resulting theoretical spectrum does not have peaks due to the various hadronic resonances. This is an example of quark-hadron duality failing if the rate is not sufficiently inclusive. But how large a region must we integrate over? The OPE tells us. The denominator T-produce of the currents for b —> Xs-i is (mbV + k — q)2 +ie
(mbV — q)2 + 2k • (mbV — q) + k2 + ie
where fcM is the residual momentum of the b meson, defined in Eq. (42). Writing q^ = xm(,nIJ'/2, where nM is a light-like vector, and x = 1 corresponds to the maximum photon energy at the parton level, this becomes D=
;
;
;
(80) 2
1 — x + k • n+ (1 — x)k • n + k + ie (where k = k/rrib). There are three possible regions: (1) l - i > A Q C D / W 6 : This is the usual assumption for the OPE. In this region, the denominator in (80) may be expanded in powers of k^/rrib, and the result matched onto local operators.
232
(2) 1 — x < A Q C D / 7 ? T | : In this region, there is no sensible 1/mb expansion of the denominator, and the OPE breaks down. This is physically reasonable: in this region, the invariant mass of the final hadronic state is constrained to be of order AQCD, and so the decay is dominated by a small number of resonances, and one would expect an inclusive description to fail. Indeed, in this region the leading term of the OPE is the same size as all the other terms in the denominator, and so the expansion in powers of 1/rrn, doesn't converge. (3) 1 — x ~ AQCD/«"16: In this region, the maximum invariant mass of the final state is of order {h-QCD'mb)1'2 3> AQCD, and one would expect an inclusive description to be valid. However, in this region k • n and 1 — x are both of order AQCD , and one cannot treat the k • n term as a perturbation. This is often known as the shape function region. [39] In the shape function region, the usual OPE fails, but we may still expand the denominator in powers of AQCB/iribD =
; + • • • =^ ImD = -7rJ(l -x + k-n) + ... (81) 1 — x + k-n + ie A delta function containing the residual momentum is not related to the matrix element of a local operator; instead, it is given by the function f(u) = (B\b5(u> + ib-n)b\B).
(82)
This is a strange-looking object, but it looks less peculiar if we perform a Fourier transform in u> and look at it in position space, f(t) =
B\b(0)Pexp
n • A(t')dt' b(t)\B) (83) o where the P denotes path-ordering, and t measures the distance along the light-like direction nM: b(t) = b(tn/mb). This is a nonlocal operator where the b and b fields are separated by a light-like displacement, and the pathordered exponential is there to restore gauge invariance. (This is technically known as a light-like Wilson line.) In n-A = 0 gauge, the function f(u>) gives the amplitude for the light-cone component of the residual momentum fcM to be UJ. f(u>) is therefore the light-cone distribution function of the heavy quark residual momentum; it is often referred to as the "shape function". (The shape function was actually originally defined as an integral of f(u), but this nomenclature seems to have stuck.)
233
Thus, at leading order in AQCD/'^ib the nonperturbative contribution to the photon spectrum in b —> Xsj is given, in the region 1 — x ~ AQCD/W&: is determined by the light-cone distribution of the heavy quark residual momentum. [39] At this stage we don't seem to have gained anything. However, the function f(u) is universal. Consider, for example, the shape of the hadronic invariant mass spectrum in b —> Xu£p decay. In this case, D=
, ^ , s 0 + 2k • (v - q) + k2 + ie
(84)
where gM is the momentum transfer to the leptons and So = (v — q)2 is the parton-level hadronic invariant mass. d If SQ
; + ---=>lmD = -n6(s0 + k-n) + ... (85) so + k • n + ie Comparing this with Eq. (81), we see that the hadronic invariant mass spectrum is determined in the region s 0 ^ A Q C D " ^ by the same function that determines the shape of the photon spectrum near the endpoint of the b —> Xsj. A similar argument shows that the rate for B —> Xulv in the region Ei > rrib — AQCD is given by the same distribution function. More precisely, at leading order in 1/rrib the various spectra are determined by convoluting f(uj) with the appropriate kinematic functions: l^(B^Xsl)
= 2f(l-2E7)
^ ^ - (B - • Xulv) r dEi ~ ( B ^ r dsH
+ ...
=2 [ 8(1 - 2Ee - u>)f(w) dw + ... J
xutv) = f j
^ ^ - I 4B * ) w
(86)
_ % ) / ( w _ A) d, + . . .
This is useful phenomenologically for determining the CKM mixing angle \Vub\- As mentioned in Section 4.2.1, the semileptonic width for b —> u decay is only easily experimentally accessible in restricted regions of phase space, such as Eg > (m2B — m2D)/2mB, or SJJ < m2D [40], where background from b —> c decay is absent. Because m2/m2 ~ AQCD/«T-6, this d
T h i s differs from the physical hadronic invariant mass ( m j « — q)2; however, we won't worry about that complication here.
234
corresponds in both cases to the shape function region. The shape function f(ui) may then be measured in B —> Xsj decay and then used to predict either the charged lepton or hadronic invariant mass spectrum in semileptonic B —> Xu decay to obtain a theoretically clean measurement of \Vub\. At subleading order in 1/rrib the universality is broken, and new subleading shape functions are required, introducing theoretical uncertainty into the extraction of \Vub\. The expansion has been carried out systematically to 0(l/rrib), while large 0 ( 1 /ml) effects due to annihilation graphs has also been considered [41]. 4.2. Exclusive
Decays and
Symmetries
4.2.1. B - • D*£u and Vcb In Section 3.3 I discussed the fact that HQET has a spin-flavour symmetry which is not manifest in QCD. We made us of this in Section 3.4, where we used the fact that the operator bvbv is a generator of that symmetry, and therefore its matrix element is fixed (matrix elements of symmetry generators are related to conserved charges). However, heavy quark symmetry gives us much more information. Any operator of the form hi,vThjtV
(87)
for any Dirac structure T and heavy flavours i and j is a generator of the symmetry, and so has an absolutely normalized matrix element. Physically, this just corresponds to the fact that exchanging the heavy quark in a meson with another heavy quark of arbitrary spin has no effect on the light degrees of freedom in the meson. This is true because, as we have discussed, in the rriQ —> oo limit a heavy quark simply behaves as a static source of colour charge, and the so the light degrees of freedom are insensitive to the mass and spin of the quark. The most important phenomenological application of this symmetry is to semileptonic B —> D/D*lv decays. Isgur and Wise [22] showed that heavy quark symmetry implies that all matrix elements of the form (H} (v')\hitV>ThjiV\Hj(*)(v)) (where i?j(*) denotes either the pseudoscalar (£) or B) or vector (D* or B*) meson with a heavy quark of flavour i = c, b) could be related to a single universal function: {Hf\v')\hi,v>rhj,v\Hf\v))
~ £(v • v').
(88)
£(v • v') has been dubbed the "Isgur-Wise function". It has the additional
235
property that, because of the absolute normalization discussed in the previous paragraph, (89)
£(1) = 1-
This means that all form factors for semileptonic B —» D and B —> £)(*) decay are known at the kinematic point where the c quark moves with the same velocity as the c quark (i.e., the q2 transfer to the leptons is maximal.) We can use the EFT machinery to calculate the corrections to this prediction. Quite generally, we can write the differential decay rate dT (B dw
D^ePe)
48TT 3
\Vbc\2(mB
- mD.)2m3D.(w
Aw m2B — 2wm,BV(iD* • (mB - mD*)2 W+ l
1 m2 D*
+ 1)V™2 - 1 \F(w)
(90)
where F(w) = £(w) + 0(as) +
0(l/mCib)
(91)
and w = v • v'. The complicated w dependence just comes from the phase space integrals. Radiative Corrections: These arise from perturbative corrections to matching conditions, as well as running of the bTc current below fi = rrib, and so are completely calculable. The one-loop running calculation has already been discussed in Section 3.3. From our previous results, then, we get the leading perturbative correction to Eq. (90), C7M(1 - -y5)b
as(mb)\ Ois{mc)J
6/25
cv-fil
~ l5)bv
(92)
and so 6/25
\as(mc)J
0 «
+ 1
log" mc/mb)
+
0(l/mb,c).
(93)
At next order, the current must be matched to 0(as) accuracy, and the running performed to two loops (resumming all terms of order Q,n+i \ogn mc/mb). This calculation has been done [42]. For practical purposes, however, logmc/mb is not very large, and so it is not necessary to sum terms of order a™ logmc/mb to all orders. Instead, one typically uses a fixed order calculation. This therefore includes terms of order a2 which are not enhanced by powers of log mc/mb, but neglects terms of 0(al) and higher.
236
Power Corrections: Just as the Lagrangian has corrections corresponding to higher dimension operators, so the weak current matches in the EFT onto additional operators. For general v and v', the only operator at 0(l/mc) is
cv,il?^(l-"/5)bv and so at 0(1/mc)
(94)
we need the matrix element
(95)
There are three possible form factors required to describe the matrix elements of this operator in HQET. Furthermore, additional 1/m.Q corrections arise due to T-products of 1/TTIQ operators in the effective Lagrangian and the leading order current, (D(*\v')\T{cv,YK,Zi)\B{v)).
(96)
However, at zero recoil (v = v') things simplify dramatically: both of these corrections vanish, although each for a different reason. • The T-product in (96) vanishes by the Ademollo-Gatto theorem when v = v'. The theorem states that the symmetry breaking corrections to matrix elements of an approximate symmetry current are second order in the breaking terms in the Lagrangian. • The matrix element (95) can be related, when v = v', to the matrix element of the operators cvD • vbv, which vanishes by the equations of motion. Thus, we have the nice result that the leading nonperturbative effects to the absolute normalization of the Isgur-Wise function at zero recoil are of order AQCD/W;?, n ° t A-QCi>/mc- [25] This immediately raises this prediction to a reasonable level of precision, since corrections of order A Q C D / T O ^ are expected to be of order 5%. Putting everything together, we find cvi\
i , (i)as(mb)
. (2) (as(mb)\
,
F(l) = 1+??V—-—+VA ( —-—J = 0.960 ± 0.007 + S1/m2 +0(a3s,
3
3
,
+^+OK,l/<)
l/m^c)
(97)
where ryA and ryA' are perturbatively calculable, and <5i/m2 refers to the incalculable 0(l/m2c b) corrections. There have been a number of attempts to estimate the size of these corrections from quark models, sum rules and
237
other methods. Combining the results in the literature gives the rough estimate [43] h/m? = -5-5 ± 3%
(98)
F ( l ) = 0 . 9 1 ±0.04.
(99)
which gives the result
Combining this with the world averaged measured value of dT/dw at the endpoint gives the value [44] \Vcb\ = (41.9 ± l . l e x p t ± 1.8theory) x 10" 3 .
(100)
Note that this is not as precise as the result from inclusive decays, Eq. (78). However, it is based on a completely different approach, both theoretically and experimentally, and so the agreement between the two results within errors is a nontrivial check on the validity of the 1/rrn, expansion. 4.2.2. B -> J/ipKs
and sin 2/?
I have been focusing in these lectures on the heavy quark expansion and its application to decay rates, etc. in CP conserving quantities, but I must say something about CP violation. The B factories were designed, after all, to study the SM picture of CP violation and while I cannot give this subject its due, I would like to at least mention it [45]. It also provides a nice illustration of another situation where we can use symmetry considerations to make model-independent predictions for hadronic processes. In this case, the relevant symmetry of QCD is CP. Consider charged B decay to some final state / . The amplitude for this decay may be written as Af = (F\H\B+) = Y, AkeiSkeilfk.
(101)
k
The sum over k corresponds to the sum over different amplitudes (or operators) which contribute to the decay. In general, such amplitudes are complex, since there may be real intermediate states, as shown in Fig The phase ipk is the weak phase which is present in the weak Hamiltonian itself. Similarly, the amplitude for the CP-conjugate process B~ —> / may be written
Af = (f\H\B~) = Y,Akel5ke-^. fc
(102)
238
The important thing is that, because QCD is CP-conserving, the strong amplitude is unchanged in the CP-conjugate process. The only change in the amplitude arises because the CP-violating weak phase flips sign. This immediately can give rise to CP-violation in the rates, since the different amplitudes can interfere, and so we can have \Af{2 7^ \A%. The problem is that in general, this interference depends on the values of the strong amplitudes Aket5k, which in turn depend on long-distance QCD. For most decays, then, it is difficult to translate measured CP violation into a determination of the weak phases J/ipKs, which proceeds dominantly via the b —> ccs process. (There is also a "penguin" diagram which contributes to the decay, but it is proportional to the same weak phase, so the argument still holds.) Since it is dominated by one amplitude, it does not appear that we will get any CP violation, since CP violation requires the interference of at least two amplitudes. However, there is another amplitude hidden in the process: the B° can oscillate into a B°, which can then decay, and this mixing carries a relative phase (due to Vtd m the Wolfenstein parametrization) which can interfere with the decay. Thus, we really do have two interfering amplitudes, and the relative phase between them is /3. More precisely it is easy to show that
r(B»(t)^^s) + r(B«(t)^^,)
H
v >\ >
and thus this measurement provides a clean measurement of sin 2/3. The current world average, from Belle and Babar, is [15] sin 2/3 = 0.79 ±0.14.
(104)
This is shown in Fig. 8 along with the other experimental constraints, and is clearly in good agreement with the SM expectation. A similar analysis holds for the rarer decay B —> sss. Since in the SM in the Wolfenstein parameterization, neither b —-> ccs nor b —-> sss contains a weak phase, both measurements measure the phase /3 in mixing. However, since the second process occurs at the loop level in the SM, it is easy to cook up models of new physics which contribute a large phase to the b —> sss amplitude, so consistency of the measured /3 from the two processes provides an important test of the SM. Indeed, there has been recent excitement over a possible
239
discrepancy between these measurements [46]: sin 2/3(5 - • Ka) = - 0 . 9 6 ± 0 . 5 0 t ° ; ^ (BELLE) sin 2f3(B -> 4>KS) = —0.19±g;|g ± 0.09 (BABAR)
(105)
However, the experimental errors are high enough that the discrepancy with Eq. (104) cannot be considered more than a tantalizing hint at present. Unfortunately, it is not so easy to come up with decays which are dominated by a single amplitude like this. For example, we could try a similar trick with b —> uus to try to predict CP violation in B —> TTTT. In this case, the relative phase between the amplitudes is 7 + /3 = 7r — a, so one could hope to use this decay to determine sin 2a. However, as shown in Fig. 6(c), this process receives contributions from both the tree-level b —> uus amplitude and a penguin amplitude. The latter is a one-loop process, but since it is a strong loop there is no reason for it to be suppressed, and it contributes with different weak and strong phases. Thus, without knowing the relative amplitudes between the tree and penguin processes, we can no longer make a model-independent determination of a in this manner, and more complicated and/or less theoretically rigorous approaches must be used.
5. Conclusions In these lectures I have discussed some of the most important applications of the heavy quark expansion to B decays. This is a large and rapidly evolving field. The applications I have discussed are mostly "classic", in that the theory was developed a number of years ago and is well understood. Recently, there has been much work attempting to further exploit (generalized) factorization in B decays, and to make use of the resulting simplifications, in particular to better understand nonleptonic decays. As an important example, several years ago, Beneke, Buchalla, Neubert and Sachrajda [47] proposed a generalized form of factorization applicable to two-body nonleptonic decays. Within their framework, they showed that so-called naive factorization, in which matrix elements of two currents are written as the product of matrix elements of the individual currents, arises modelindependently from QCD leading order in l/mb and as. This claim has generated much interest in the field, both phenomenological and theoretical, as the validity of the arguments has been heatedly debated. More recently, an EFT description which encompasses the kinematics of these decays, dubbed Soft-Collinear Effective Theory (SCET) has been developed [48]. Since the
240
kinematics are much more complicated t h a n in H Q E T — with hard, soft and collinear momenta possible — the E F T is more complicated, and (I believe) not completely understood. However, much progress has been made in understanding the interplay of the various scales, and this is an active and rapidly-evolving field.
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35. M. B. Voloshin, Phys. Rev. D 51, 4934 (1995); M. Gremm, A. Kapustin, Z. Ligeti and M. B. Wise, Phys. Rev. Lett. 77, 20 (1996); M. Gremm and I. Stewart, Phys. Rev. D 55, 1226 (1997); 36. R. A. Briere et al. [CLEO Collaboration], arXiv:hep-ex/0209024. 37. M. Beneke, Nucl. Phys. B 405, 424 (1993); M. E. Luke, A. V. Manohar and M. J. Savage, Phys. Rev. D 51, 4924 (1995); M. Neubert and C. T. Sachrajda, Nucl. Phys. B 438, 235 (1995). 38. C. W. Bauer, Z. Ligeti, M. Luke and A. V. Manohar, Phys. Rev. D 67, 054012 (2003); M. Battaglia et al, eConf C0304052, WG102 (2003) [Phys. Lett. B 556, 41 (2003)]. 39. M. Neubert, Phys. Rev. D49 (1994) 3392; D49 (1994) 4623; I.I. Bigi et al, Int. J. Mod. Phys. A9 (1994) 2467; T. Mannel and M. Neubert, Phys. Rev. D50 (1994) 2037. 40. V. Barger, C. S. Kim and R. J. Phillips, Phys. Lett. B251, (1990) 629; A.F. Falk, Z. Ligeti, and M.B. Wise, Phys. Lett. B406 (1997) 225; I. Bigi, R.D. Dikeman, and N. Uraltsev, Eur. Phys. J. C4 (1998) 453; R. D. Dikeman and N. Uraltsev, Nucl. Phys. B509 (1998) 378. 41. C. W. Bauer, M. E. Luke and T. Mannel, arXiv:hep-ph/0102089; Phys. Lett. B543 (2002) 261; M. Neubert, Phys. Lett. B 543 (2002) 269; A. K. Leibovich, Z. Ligeti and M. B. Wise, Phys. Lett. B539 (2002) 242. 42. D. J. Broadhurst and A. G. Grozin, Phys. Lett. B 267, 105 (1991). 43. P. F. . Harrison and H. R. . Quinn [BABAR Collaboration], "The BaBar physics book: Physics at an asymmetric B factory," SLAC-R-0504 . 44. http://www.slac.stanford.edu/xorg/hfag/semi/summer02/summer02.shtml 45. I. I. Y. Bigi and A. I. Sanda, Cambridge Monogr. Part. Phys. Nucl. Phys. Cosmol. 9, 1 (2000). 46. K. Abe et al. [Belle Collaboration], arXiv:hep-ex/0308035; B. Aubert et al. [BABAR Collaboration], Phys. Rev. Lett. 89, 201802 (2002). 47. M. Beneke, G. Buchalla, M. Neubert and C. T. Sachrajda, Phys. Rev. Lett. 83, 1914 (1999); Nucl. Phys. B 591, 313 (2000); arXiv:hep-ph/0007256. 48. C. W. Bauer, S. Fleming and M. E. Luke, Phys. Rev. D 63, 014006 (2001); C. W. Bauer, S. Fleming, D. Pirjol and I. W. Stewart, Phys. Rev. D 63, 114020 (2001); C. W. Bauer and I. W. Stewart, Phys. Lett. B 516, 134 (2001); C. W. Bauer, D. Pirjol and I. W. Stewart, Phys. Rev. Lett. 87, 201806 (2001); C. W. Bauer, D. Pirjol and I. W. Stewart, Phys. Rev. D 65, 054022 (2002); M. Beneke, A. P. Chapovsky, M. Diehl and T. Feldmann, Nucl. Phys. B 643, 431 (2002);
^ _ ^
m
Y
^ ^ .
SALLY DAWSON
^
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THE TOP QUARK, QCD A N D N E W PHYSICS
S. DAWSON* Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA [email protected]
The role of the top quark in completing the Standard Model quark sector is reviewed, along with a discussion of production, decay, and theoretical restrictions on the top quark properties. Particular attention is paid to the top quark as a laboratory for perturbative QCD. As examples of the relevance of QCD corrections in the top quark sector, the calculation of e + e - —> it at next- to-leading-order QCD using the phase space slicing algorithm and the implications of a precision measurement of the top quark mass are discussed in detail. The associated production of a it pair and a Higgs boson in either e + e - or hadronic collisions is presented at next-to-leading-order QCD and its importance for a measurement of the top quark Yukawa coupling emphasized. Implications of the heavy top quark mass for model builders are briefly examined, with the minimal supersymmetric Standard Model and topcolor discussed as specific examples.
Contents 1
Introduction
246
2
W h o N e e d s a Top Quark? 2.1 Anomaly Cancellation 2.2 b Quark Properties 2.3 Precision Measurements
247 248 249 251
3
Top 3.1 3.2 3.3
253 253 258 259
Quark Properties Hadronic Production Weak Interactions of Top Top Quark Decay
"This work is supported by the U.S. Department of Energy under contract number DE-AC02-76-CH-00016. 245
246
3.4 3.5 3.6 3.7 3.8
W Helicity in Top Quark Decay Top Production and Decay Single Top Production Measurements of Mt Top Spin Correlations
261 263 264 264 266
4
T h e Top Quark as a Q C D Laboratory 4.1 NLO QCD Corrections to e+e~ -> it 4.2 0(as) Corrections to e+e~ -> it 4.3 Real Contributions : 4.4 Threshold Scan in e+e~ - • it 4.5 tth Production in e + e ~ and pp Collisions 4.6 tth at an e + e ~ Collider 4.7 pp -»• tth at NLO 4.8 Collinear Singularities and Phase Space Slicing 4.9 Mass Factorization
5
Heavy Top as Inspiration for Model Builders 5.1 EWSB and the Top Quark Mass 5.2 Supersymmetry and the Top Quark 5.3 mSUGRA 5.4 Charged Higgs Decays 5.5 The Top Squark 5.6 The Top Quark and Dynamical Symmetry Breaking
6
Conclusions
267 267 269 271 275 277 278 280 281 283 285 286 289 •. . . 291 294 295 295 296
1. I n t r o d u c t i o n Long before its discovery in 1995, 1 the top quark was regarded as an essential ingredient of the Standard Model (SM) of particle physics. Its existence, and many of its properties, are determined by requiring the consistency of the S t a n d a r d Model. These requirements (discussed in Section 2) specify the couplings of the top quark to the SU{2>) x SU{2)L X U(1)Y gauge bosons and many of the top quark properties: • O*
= - I e I
Hem
3 I °I
• Weak isospin p a r t n e r of b quark: T | = • Color triplet
• spin-1
|
247
The top quark mass, which is measured at CDF and DO to be Mt = 174.3 ± 5 . 1 GeV,2 is not predicted in the Standard Model, but is restricted by precision electroweak measurements. 3 ' 4 In order to confirm that the observed top quark is that predicted by the Standard Model, all of the properties listed above must be experimentally verified by direct observation. Section 3 contains a discussion of the measurements of top quark properties at the Tevatron and surveys the improvements expected at future colliders. Recent reviews of top quark physics can be found in Ref. 5. In Section 4, we discuss the top quark as a laboratory for perturbative QCD. At the top quark mass scale, the strong coupling constant is small, as(Mt) ~ 0.1, and so QCD effects involving the top quark are well behaved and we expect a perturbation series in as to converge rapidly. The process e+e~~ —• it provides an example of QCD effects in top quark production and the next-to-leading-order (NLO) QCD corrections are described in detail using the phase space slicing (PSS) algorithm. A knowledge of these higher order QCD corrections is vital for extracting a precise value of the top quark mass from the threshold behavior of the e+e~ —> it cross section. As a further example of the role of QCD effects in the top quark sector, we consider the associated production of tih and the implications for measuring the top quark Yukawa coupling. Finally, in Section 5, we discuss the importance of the top quark mass in model building. Since the top quark is heavy, it is expected to play a special role in elucidating the source of fermion masses. We begin by discussing the significance of the top quark in supersymmetric models, and finish with a brief discussion of the top quark and models with dynamical symmetry breaking.
2. W h o Needs a Top Quark? In this section we discuss some of the reasons why the top quark was believed to exist even before its experimental discovery. These considerations fall into three general categories: theoretical consistency of the Standard Model gauge theory (anomaly cancellation), consistency of b quark measurements with SM predictions, and consistency of precision measurements with the SM. We then turn to a discussion of top quark production and decay mechanisms at the Tevatron and the LHC. The particles of the first generation of fermions, along with the Higgs doublet, are shown in Table 1, with their gauge quantum numbers. The third generation is assumed to follow the same pattern, with the left-handed
248
top and bottom quarks forming an SU(2)i doublet with hypercharge, Y = | . The right-handed top and bottom quarks are SU(2)L singlets. Our normalization is such that: Qem =T3+Y
(1)
with T 3 = ±\. Table 1. Fermions in t h e first generation of t h e Standard Model and their SU(3) X SU(2)L X U(1)Y quantum numbers. Field {uL,dL) UR,
dn, (yL,eL) eR H
2.1. Anomaly
SU(S) 3 3 3 1 1 1
SU(2)L
U{1)Y
2 1 1 2 1 2
1
I 1 ! 2
-1
1 2
Cancellation
The requirement of gauge anomaly cancellation 6,7 puts restrictions on the couplings of the fermions to vector and axial gauge bosons, denoted here by V* and AM. The fermions of the Standard Model have couplings to the gauge bosons of the general form: L~g ^ T ^ ^ A ^
+ gv^T^^V^,
(2)
where Ta is the gauge generator in the adjunct representation. These fermion-gauge boson couplings contribute to triangle graphs of the form shown in Fig. 1. The triangle graphs diverge at high energy,
T^-Tr^iT^T^J-^^,
(3)
where r\i = ^ 1 for left- and right-handed fermions, ipL,R = | ( 1 T 75)V'This divergence is independent of the fermion mass and depends only on the fermion couplings to the gauge bosons. Such divergences cannot exist in a physical theory, and must somehow be cancelled. The theory can be anomaly free in a vector-like model where the left- and right-handed particles have identical couplings to gauge bosons and the contribution to Eq. (3) cancels for each pair of particles. From Table 1, however, it is
249
K"
Vr
'www
Figure 1. Generic Feynman diagram contributing to gauge anomalies. The fermion loop contains all fermions transforming under the gauge symmetry, a, b, c are gauge indices.
clear that the Standard Model is not vector-like. The anomaly, Tabc, must therefore be cancelled by a judicious choice of fermion representations under the various gauge groups. The only non-vanishing contribution to the anomaly in the Standard Model is from T,Tr[Y{Ta,Tb}},
(4)
a
where T are the SU(2)L generators and the sum is over all fermions in the theory. Equation (4) vanishes for the hypercharge assignments given in Table 1. Note that the anomaly cancels separately for each generations of fermions. The cancellation of the gauge anomalies in the Standard Model for the third generation therefore requires that the b quark have a T3 = | partner with electric charge Q\m = § | e |, and hypercharge Y* = Q\m — T|. The partner of the b quark is by definition the top quark. Since anomaly cancellation is independent of mass, a priori, the top quark mass could be anything. 2.2. b Quark
Properties
Many of the experimental properties of the b quark require that it be a T36 = - \ particle with Qbem = - § and Y 6 = \. The coupling of the b quark to the Z boson can be tested to check if it has these quantum numbers. The SM fermions couple to the Z boson as, L=-
9 T 4 cos 6w
Ri(l + J5) + HI - 75) AZ* ,
where Ri
-2QiSin
Ow
(5)
250
Li = 2T3i - 2Qi sin2 0W ,
(6)
and 6w is the electroweak mixing angle. The experimental value of Rhad is sensitive to Qlm, a(e+e Rhad
—> hadrons)
(7)
=
At a center-of-mass energy, y/s = 2m),, Rb depends sensitively on the b quark electric charge, SRhad(2mb) = Nc(Qb)2 + 0(as),
(8)
where Nc = 3 is the number of colors. The experimental measurement, 8 SRhad(2mb)
= 0.36 ±0.09 ±0.03
(9)
is in good agreement with the theoretical prediction from Qbem = — \ , verifying the SM electric charge assignment of the b quark. Similarly, the SU(2)L quantum numbers of the left- and right- handed b quarks are probed by the decay rate for Z to bb quark pairs. If the b did not have a top quark partner, it would be an isospin 0 particle ( T | = 0) and the decay width would be dramatically different from that of the SM. The decay width is given in terms of the left and right- handed couplings of the Z to the bb pair, 9 r(Z^bb) = ^^(Ll+Rl).
(10)
If the b quark were an isospin singlet, then the decay width would be changed, T(Z -> bb)T3 =~i _ 1 + 4Q b sin 2 0W + 8Q 2 sin 4 9W T(Z^bb)Ts=° ~ 8Q2bsm4ew
13.
The measurement of the Z —> bb decay width excludes the T36 — 0 hypothesis for the b quark. 3 The measured b couplings, combined with anomaly cancelation, require that the b quark have a T | = |2l ', color triplet, fermion partner: this is the top quark.
251
Figure 2. Feynman diagrams for gauge boson self-energies which give contributions proportional to M t 2 .
2.3. Precision
Measurements
Before the top quark was discovered, an approximate value for its mass was known from precision measurements, which depend sensitively on the top quark mass. At tree level, all electroweak measurements depend on just three parameters: the SU(2)L X U(1)Y gauge coupling constants and the Higgs vacuum expectation value, v = 246 GeV. These are typically traded for the precisely measured quantities, a, GF, and Mz- All electroweak measurements at lowest order can be expressed in terms of these three parameters. Beyond the lowest order, electroweak quantities depend on the masses of the top quark and the Higgs boson. A typical example of the role of the top quark mass in precision measurements is the calculation of the p parameter, M ._ w ~~ M§ cos2 Bw _ Aww{0) AzziO) M^ Ml '
H
( U [
'
where Ayv is defined by the gauge boson 2-point functions, Ol£V = Avv(p2)
+ Bvvp^p"
.
(12)
At tree level, the p parameter in the Standard Model is exactly one, but at one loop it receives contributions from gauge boson, Higgs boson, and fermion loops. The largest corrections are those involving the top quark loop. For simplicity, we compute only the corrections proportional to M t 2 , which are found from the diagrams of Fig. 2. The 2-point function of the Z boson is given by,
i""z =--N m
~
"c\4cwJ
W
J
(2n)n[den\
(is)
252
Winter 2003 Measurement
//-vmeas r > f i t w r r m e a s
Pull
-3-2-10123 -0.16
4«^ r! {m 7 ) mz [GeV]
0.02761 +0.00036 91.187510.0021
0.02
r z [GeV]
2.4952 ± 0.0023 41.540 ±0.037
-0.36 1.67
20.767 ± 0.025 0.01714 ±0.00095 0.1465 ±0.0032
1.01 0.79
° L [nb] R, A o,i M
fb
A(Pt) Rb Rc
0.21644 ±0.00065 0.1718 ±0.0031 0.0995 ±0.0017
c
A,°b C
0.0713 ±0.0036
Ab
0.922 ± 0.020
0.670 ± 0.026 Ac 0.1513 ±0.0021 A,(SLD) sin2e^tf(Q(b) 0.232410.0012
i
•
a
-0.42
.
0.99 -0.15
i
-2.43 -0.78 -0.64
•
0.07 1.67 0.82
^™ •i
m w [GeV]
80.426 ± 0.034
1.17
••
r w [GeV] mt [GeV] sin26w(vN)
2.139 ±0.069 174.3 ±5.1
0,67
•
0.227710.0016
(3w(Gs)
-72.83 + 0.49
0.05 2.94
— —
U.I £-.
1
-3-2-10123 Figure 3. Fits to electroweak data. The bars show the pull from the Standard Model global fit.3
where cw = cos 9w and , T"" = Tr[(k + Mt)Y(.RtP+ den={k2-M?}[(k+P)2-M?},
+ LtP-)(k-p
+ Mt)Y(RtP+
+ LtP-)] (14)
and P± = ^(1 ± 75). The coupling of the Z to the top quark is given in Eq. (5) and p is the external gauge boson momentum. Shifting momentum in the integral, k' —> k — px, and using Feynman parameters, the integral in n = 4 — 2e dimensions becomes, i^zz
... 2 f dnk' ( -ig \4cw [4k' 2 ( - l + l){R2t+Ll)
-N,
f1 ,
+ (k'2+p2x{l-x)-Mf)
l6LtRtM?}g^ 2^2
(15)
253
where we retain only those terms contributing to Azz{0)portional to M 2 is,
The result pro-
The analogous result for the W two-point function is,
A
™V
=
g2Nc(A*\\f2(i
,l
V-2^\M)
Mt
\t
+
V-
{17)
Combining Eqs. (16) and (17), the contribution to the p parameter which is proportional to M 2 is found from Eq. (II), 1 0 2 g
JVcM2
Sp
-64^M*;-^v^-
_GFNCM? ( 8 )
Experimentally 4 p = 1.00126±o;ooi44 and so an upper limit on the top quark mass can be obtained from Eq. (18). Many of the precision measurements shown in Fig. 3 are sensitive to M 2 and so a prediction for the top quark mass can be extracted quite precisely by combining many measurements. In fact, precision measurements were sensitive to the top quark mass before top was discovered at Fermilab! The agreement between the direct measurement of the top quark mass in the Fermilab collider experiments and the indirect prediction from the precision measurements (as shown in Fig. 4) is one of the triumphs of the Standard Model. It is interesting to note that a similar limit on the Higgs boson mass from precision measurements gives Mh < 193 GeV at the 95% confidence level.3 Precision measurements depend logarithmically on the Higgs mass, and so it is much more difficult to bound the Higgs mass in this manner than it is to restrict the top quark mass. Increases in the experimental precisions of the top quark mass and the W boson mass in Run II measurements at the Tevatron will provide an improved bound on the Higgs boson mass, SMh/Mh ~ 4 0 % . n In the next sections, we discuss the discovery of the top quark at Fermilab and the experimental exploration of the top quark properties, both at Fermilab and the LHC and at a future high energy e+e~ collider. 3. Top Quark Properties 3.1. Hadronic
Production
The top quark was discovered in 1995 at Fermilab in pp collisions at \/S = 1.8 TeV.1 This data set (called Run I) consists of an integrated luminosity of
254
Top-Quark Mass CDF
[GeV]
•—
176.1 ±6.6
D0
172.1 ±7.1
Average
174.3 ±5.1 170.7 ±10.3
LEP1/SLD —U—
LEP1/SLD/mw/rw 125
150
175
mt
177.5 ±9.3 200
[GeV]
Figure 4. Measurements of the top quark mass at Fermilab (CDF and DO) and indirect predictions from precision measurements (LEP1, SLD and M\y).3
L ~ 125 pb~x. Both CDF and DO are currently rediscovering the top quark in the Run II data set. Run II will produce roughly 500 clean top quark events for each inverse femtobarn of data and so precision measurements of many top quark properties will be possible. In hadronic interactions, the top quark is produced by gluon fusion and by qq annihilation as shown in Fig. 5, 99
tt
qq
it.
(19)
The hadronic top quark production cross section, CTH, at the Tevatron, pp —> it, (or pp —• it at the LHC) is found by convoluting the parton level cross section with the parton distribution functions (PDFs), <7H(S)
= Sjj
/ fi(Xi,
[l)fj(x2,
fjL)&ij(xiX2S,
fi),
(20)
where the hadronic center of mass energy is S, the partonic center of mass energy is s = xix2S, the parton distribution functions are fi(x, fi), and the parton level cross section is &ij(s,fi). The parameter fi is an unphysical
255
' ^smrnmc-fY-
t
+
+ /
rjirinrirwsOTir'Figure 5. liders.
8-
1
„/
'flaaaaasa.
g'
Feynman diagrams contributing to top quark pair production in hadron col-
renormalization/factorization scale. To 0(a2s), the sum in Eq. (20) is over the qq and gg initial states shown in Fig. 5. It is useful to parameterize the parton level cross section (to next-toleading order in as) as, Vij{s,n)
Mi
u °ij(p) + Hjas{lA + h2i:jas(n) log( -^2
h
At lowest order, 0(a2s), the parton level cross sections are
,0 99
h
=
*PP
192
-p{p2
(21)
12
+ 16p + 1 6 ) l o g ( [ ± | ) - 28 - 31p (22)
o,
where 4Mt2
/3 = y r
(23)
The threshold condition is p = 1 and so &ij vanishes at threshold. To any given order in as(p), the hadronic cross section an must be independent of p, daH d log p/
= 0.
(24)
256 Applying this restriction to Eq. (21) yields 0=
2-/(x 2 ,At) + / ( x i , / i i j — 2~ d log p? d log p? da(x1x2S,p) f(xi,p)f(x2,p)'9 log p2 -dx\dx2
}a(x1x2S,p,)dx1dx2 (25)
The scale dependence of the PDFs is governed by the Altarelli-Parisi evolution functions, Pij (z): dfi(x,n)
f1 dy
as
Ix .
(26)
y Similarly, the scale dependence of the strong coupling constant, a s (/z), is governed by the evolution of the QCD /3 function. das(n) d log p2
2
(27)
-60o;;
where 6o = -—12™lf , and nif is the number of light flavors. At nextto-leading order the scale dependence of the various terms must cancel, yielding a prediction for ft.2-,
* & - *
Anboh^ (p) - Sfc J
dzh°kj ( j j Pki (z) - Sfe J
dzh°k (A Pk M (28)
Using the explicit forms of the Altarelli-Parisi evolution functions, combined with Eq. (22). we find12 h2qq- = — 2TT
16?rP
81
^ ( m )
7T
h2gg
*{*(•
~~ 2n 192
1-/3
+ \hUP) (l27 - 6nlf + 4 8 1 o g ( ^
59p2 + 198/9 - 288 log
1 + /3 1-/9
+12p( p 2 + 16/0+16 )
h2qg
4 /?( 7449/02 - 3328/0+724 ) [> - 12h°gg(j>)]og[ -^ ~15 4p 32 1+ 0 = 3— 14/92 + 27/o - 136 log p{2-p)9l{(3) 8 4 1-/3 ~
(1319/32 - 3468/0 + 724
(29)
257
where we have defined,
»c»-*(^)-*(^) + ^)-™,(V) The quantities ft1 • can only be obtained by performing a complete nextto-leading order calculation 12 ' 13 and analytic results are not available, although a numerical parameterization is quite accurate. The strong coupling evaluated at the top quark mass is small, as(Mt) ~ 0.1 , and so a perturbative expansion converges rapidly. At energy scales significantly different from the top quark mass, there are also large logarithms of the form (a3 log(-pj5-))ra, which can be summed to all orders to obtain an improved prediction for the cross section. 14 The inclusion of next-to-leading-logarithm effects reduces the scale dependence of the cross section to roughly ±5%. 1 5 Figure 6 shows the Run I CDF and DO measured top production cross sections and top quark mass compared with two theoretical predictions. The Run I it cross sections are, 16 * « = 6.5il;Ip6 <7tI = 5 . 9 ± 1 . 7 p 6
CDF DO.
(31)
The curves labelled NLO+NLL include some of the logarithms of the form (as log(-gj-))™ and the agreement with the experimental results is clearly improved from the NLO prediction alone. An updated theoretical study including next-to-leading-logarithm resummation gives the prediction atl(VS
= 1.8 TeV) = (4.81 - 5.29) pb for Mt = 175 GeV,
(32)
with the range in the prediction corresponding to Mt/2 < fi < 2Mt-15 The agreement between the predicted and the experimental production rates implies that the top quark is a color triplet, since that rate would be significantly different for a different color representation. At the LHC, the subprocess gg —> it dominates the production rate with 90% of the total rate and the cross section is large, (JLHC ~ 800 pb, yielding about 108 it pairs/year. Because of the large rate, the LHC can search for new physics in top interactions. The large rate also has the implication that top production is often largest background to other new physics signals. A detailed understanding of the top quark signal is therefore a necessary ingredient of new physics searches.
258
S = 1.8 TeV, pp MRSR3
a. CDF NLO NLO + NLL, A=2
180
140
160
1B0
200
m t (GeV) Figure 6. Fermilab Tevatron Run I cross sections for pp —» it. The dashed line is the NLO prediction, while the solid lines incorporate the next-to-leading logarithms.14
3.2. Weak Interactions
of Top
T h e weak interaction eigenstates are not mass eigenstates, 9 S,=d,»,6*7"(l - l5)VtgqW+ 2 ^
+ h.c.
(33)
and so the interaction of the t o p quark with a W boson and a light quark is proportional to the C K M mixing element, Vtq. Measurements of the decay rates of the top quark into the lighter quarks therefore translate directly into measurements of the C K M mixing angles. T h e unitarity of the C K M m a t r i x and the restriction to 3 generations of fermions gives a limit on Vtb from the measured values of | Vub | and
I Vcb |:4 1 = | Vub
vcb |2 +1 vtb
(34)
giving, 0.9991 < | Vtb \< 0.9994
(35)
If there are more t h a n 3 generations, however, Eq. (34) is no longer valid and there is almost no limit from unitarity on VtbUnitarity in the three generation model can be tested by measuring the ratio of the rate for top decays to the b quark t o the rate for top decays to
259
Figure 7.
Feynman diagram for top decay to Wb.
lighter quarks, 17 R •tb
T(t - • Wb) T{t - Wq)
| Vu, | Vu | 2 + | Vu ? + I Vt6 | 2 = 0.94+°Ji
(CDF)
(36)
This quantity can be measured at Fermilab by counting the number of tagged 6's in a top quark event. Since the b quark lives approximately 1.5 ps, the b quark will travel 450 fim before decaying. The secondary b vertex can then be measured with a silicon vertex detector. Assuming unitarity of the CKM matrix, the denominator of Eq. (36) is 1, and the measurement can be interpreted as a measurement of Vtb, I Vtt |= 0.97±°;&
(37)
consistent with Eq. (35). If there is a fourth generation of quarks, (t',b'), the charge — | b' is experimentally restricted to be heavier than mt — Mw,4 and so the top quark cannot decay into the b'. Then the denominator of Rtb need not equal 1, and the measurement implies only | Vtb \»\ Vta |, | Vtd |The direct measurement of Vtd in single top production will be discussed in Section 3.6. 3.3. Top Quark
Decay
The top quark is the only quark which decays before it can form a bound state. This is due to the very short lifetime of the top quark, rt - 5 x 1CT25 sec ,
(38)
260
compared to the QCD time scale: TQCD ~ 3 x
10" 2 4
sec .
(39)
This is very different from the b quark system, where the b quark combines with the lighter quarks to form B mesons, which then decay. Since | Vtb | ~ 1, the dominant decay of the top quark is t(p)^W»(Pw)
+ b(P'),
(40)
The lowest order amplitude is shown in Fig. 7 and is, A0(t -> W»b) = --JL=utf)rf(l
-
7 5 )«(p)
.
(41)
Squaring the amplitude, and summing over the W polarization vectors, Ee"(p w )e*"(p w ) = - < T + ^ # , M w gives the amplitude-squared, M2 A0(t - Wb) | 2 =
(42)
(43)
The total decay width is
T0(t -+ Wb) ={^)~J\
Mt - Wb) |2 (dPS2) .
(44)
The factor of ^ is the average over the initial top quark spin, and the phase space factor is,
M? - Ml
{dFS
* = I 87rM sjf • 2
(45)
t
Including the CKM mixing, the final result is then
1 „ ^ „,^( -f) ( 1+ ^ ) . ( «, r (1 )H 1 Higher order QCD corrections can be calculated in a straightforward manner and yield a precise prediction for the decay width, 9,18 T(t - • bW+) = | Vtb | 2 1-42 GeV.
261
3.4.
W Helicity
in Top Quark
Decay
The helicity structure of the top quark decays is interesting because it yields information on the V — A nature of the tbW vertex as is clear from Eq. (41). The top quark is produced through the weak interactions as a left-handed fermion (neglecting the b quark mass), which has spin opposite from the direction of the top quark motion. If the W is produced as a longitudinal W (helicity 0), then its momentum and polarization vectors are (see Fig. 8):
Pw = (Ew,0,0,\pw eY =
|)
~(\Pw\,0,0,Ew)
Mw ^
K
\Mt J
'
The amplitude for a top decaying into a longitudinal W is then,
This gives the result that the decay of the top into longitudinal W s is, 777
r(t - bwL) ~ -j-.
(49) w
The decay of the top quark into a positive helicity W, h = +, is forbidden by angular momentum conservation since a massless b quark is always left-handed, while the heavy top can be either left- or right-handed. This is illustrated in Fig. 9. CDF has measured the decay of the top quark to a right handed W and finds a result consistent with 0, 19 BR(t -+bWR)=
0.11 ± 0.15 .
(50)
For a transversely polarized W, the polarization vector is given by, ef = -^=(0,l,±z,0).
b ** Figure 8.
t •
(51)
|
W
v A A / V W W W
Spin vectors for longitudinally polarized W's produced in the decay t —+ Wb.
262
t
W
•
v/VWWWW
forbidden Figure 9. Spin vectors for positive helicity (right handed) W's produced in the decay t —> Wb. Conservation of angular momentum forbids this decay.
b •* Figure 10.
t W • v/wwwvw Spin vectors for transverse helicity W's produced in the decay t —> Wb.
This gives a decay rate, T(t - • bWT) ~ g2mt .
(52)
Comparing Eqs. (49) and (52), it is clear that the rate for top decay to transverse W's is suppressed relative to that for decay to longitudinal W's,
-TO
—
r(t - bwL) T(t -> Wb)ToT 2M,2,
1+ = 70.1% .
(53)
Can we measure this? The W helicity is correlated with the momentum of the decay leptons: hw = + gives harder charged leptons than hw = -. A preliminary measurement of the polarization of the W's in top decay yields, 19 F 0 = 9 1 ± 3 7 ± 1 3 % CDF,
(54)
consistent with Eq. (53). With 2 / 6 " 1 of data from Run II at the Tevatron, a more definitive (~ 5%) measurement of F0 will be possible.20
263
3.5. Top Production
and
Decay
The Tevatron and the LHC produce predominantly it pairs, which decay almost entirely by t —> Wb (since | Vtb |~ l ) , a pp-+tt->
W+W~bb.
(55)
It is useful to classify top quark events by the decay pattern of the pair:
W+W~
• Di-lepton: Roughly 5% of the it events fall into this category,
W~ -> l~v .
(56)
Since Br(W -> ev) + BR(W - • \iv) ~ 22%, the 125 pb~x at Run I of the Tevatron produced ~ 625 it events which yielded ~ 0.22 x 0.22 x 625 ~ 30 di-lepton events. These events are clean, but suffer from the small statistics. • lepton + jets: This class of events comprises roughly 30% of the total, W± -> l±v,
WT - • jets .
(57)
Lepton plus jet events are fully reconstructable and have small backgrounds. Assuming at least 1 b quark is tagged, CDF observed 34 such events in Run I, with an expected background of 8.22 In Run II, with 2 / 6 " 1 , there should be 1000 lepton+jets events/experiment. This increase from Run I results from higher luminosity, the higher cross section at vS = 2 TeV compared to \/S = 1.8 TeV, and upgraded detectors with improved capabilities for 6-tagging. This channel produced the most precise measurement of the cross section for top pair production. The combined average from CDF and DO at Run I is, a(pp -+ it) \jg=1A
TeV=
5.9 ± 1.7 pb
(58)
• All jets: 44% of the events fall into this category. This class of events was observed by both CDF/DO at Run I, with large backgrounds. a
Recent reviews of the experimental issues involved in top quark physics can be found in Ref. 21.
264 Table 2. Contributions to single top production at the Tevatron and the LHC. &qb
(Tqq Oqb
Tevatron, Runll 2.1 pb .9pb .1 pb
LHC 238 pb 10 pb 56 pb
Figure 11. Feynman diagrams contributing to single top production at the Tevatron and the LHC.
3.6. Single Top
Production
Single top production can provide a precise measurement of Vtb- The dominant production processes at the Tevatron are the t channel W exchange and the s channel qq annihilation shown in Fig. 11. At the LHC, the subprocess gb —> tW is also important. The contributions (to NLO QCD) of the various subprocesses at the Tevatron and the LHC are given in Table 2 23,24 ^yj Qf these processes are proportional to | Vtb |2Single top production was not observed in Run I, and both experiments set limits on the production rate. At Run II of the Tevatron, and at the LHC, single top production can be observed with a rate roughly half that of the tt rate and will lead to a measurement at the Tevatron of,
with 30 fb 1 of data. With 2 / 6 accuracy of | ^ |~ 13%. 20 3.7.
Measurements
1
, the Tevatron Run II should achieve an
of Mt
Mt is a fundamental parameter of the Standard Model, but the value is not predicted (except indirectly from precision measurements such as the p parameter discussed earlier). In the Standard Model, a precise value of Mt is important primarily for its role in precision electroweak observ-
265
ables and for the prediction for the Higgs boson mass extracted from these measurements. 25 In extentions of the Standard Model, such as those discussed in Section 5, a precise knowledge of Mt plays a critical role in defining the parameters of the theory. The top quark mass was measured in Run I at the Tevatron and the most accurate value of Mt comes from the single lepton+jets channel, 2 Mt = 176.1 ± 4.8 (stat) ± 5.3 (syst)
CDF
= 173.3 ± 5.6 (stat) ± 5.5 (syst)
DO .
(59)
In the lepton plus jets channel there is one unknown parameter (the longitudinal momentum of the v coming from the W decay) and three constraints from the reconstruction of the W masses and the requirement that the reconstructed masses of the top and anti-top be identical, Mt = Mj. The best value of the top quark mass found from combining all channels at the Tevatron is, Mt = 174.3 ± 5 . 1 GeV .
(60)
The systematic error is dominated by the uncertainty on the jet energy scale, ±4.4 GeV
CDF
±4 GeV DO .
(61)
With increased data from Run II, the mass measurement at the Tevatron will be improved to SMt ~ 3 GeV with an integrated luminosity of 2 fb~l. Figure 12 shows the impact of a precise measurement of the top quark mass combined with a W mass measurement for the prediction of the Higgs boson mass. Clearly, for the precision measurements of the W and top masses to be useful, the experimental error on both measurements needs to be comparable. The Higgs mass prediction extracted from precision measurements can be compared with the direct measurement of the Higgs mass at the LHC and the consistency of the Standard Model tested. Tevatron Run II with 2 / 6 - 1 should achieve accuracies: 8MW ~ 27 MeV 5Mt ~ 3 GeV ,
(62)
yielding a prediction for the Higgs mass with an uncertainty of25 ^ ~ 4 0 % . Mh
(63)
266
80.6
—i
1
1
1
1
1
1
1
1
r
— LEP1,SLDData • - LEP2, pp Data
80.5-
68% CL
> CD
£ 80.4 5 E 80.3
mH [Ge
114 ' 130 150
80.2
170
190
mt [GeV] Figure 12. Implications of precision measurements of Mt and Myy on the indirect extraction of the Higgs boson mass. The Standard Model allows only the shaded region. 3
The LHC will have large statistics on top quark production due to the large rate and can use the lepton+jets channel with 10 fb^1 of data to obtain a measurement 26 SMt ~ 0.07 GeV .
3.8. Top Spin
(64)
Correlations
When pair produced in either e+e~ or qq collisions, the spins of the it pair are almost 100% correlated. 27 The top decays before the spin can flip, and so spin correlations between the t and t result in angular correlations among the decay products. At threshold, the it cross section is s-wave, and the e+e~ or qq spins are translated directly to the it spins. The incoming q, q have opposite helicities, since they are interacting through a gauge interaction and the it spins aligned along the beamline. At high energy, v ^ > > mt, the top mass is irrelevant and the t,i must have opposite helicities. Finding the correct basis for intermediate energies is subtle, however.
267
e~(p)
t^)
/ e+(q) Figure 13.
4.
1 , z
/
\ X
t(q>)
Lowest order Feynman diagram for e + e - —> it.
The Top Quark as a QCD Laboratory
The production and decay mechanisms for the top quark in both e + e~ and hadronic collisions are well understood and so the top quark system provides an ideal laboratory for studying perturbative QCD. This section contains a pedagogical treatment of the 0(as) next- to- leading order (NLO) QCD corrections to the process e + e~ —> ti.2S This process is particularly simple since the QCD corrections affect only the final state. The calculation is performed using the two cut-off phase space slicing (PSS) algorithm for evaluating the real gluon emission diagrams. 29 ' 30 The total rate for e+e~ —> it tests our understanding of perturbative QCD and additionally the threshold behavior of the total cross section can be used to obtain a precise measurement of Mt, along with information on as(Mt) and the iih Yukawa coupling. An understanding of the QCD corrections are crucial for these interpretations. As a second example of the role of QCD in top quark physics, we consider the associated production of a it pair along with a Higgs boson, h. The iih Yukawa coupling is a fundamental parameter of the theory and probes the mechanism of mass generation, since in the Standard Model, 9tth = —• The processes e+e~ —> iih and pp(pp) —> iih provide sensitive measurements of the top quark- Higgs coupling and can be used to verify the correctness of the Standard Model and to search for new physics in the top quark and Higgs sectors. The importance of the QCD NLO corrections to iih production is discussed at the end of this section. 4.1. NLO QCD Corrections +
to e+e~ —• it
The Born amplitude for e e~ —> it proceeds via s— channel 7 and Z exchange, as shown in Fig. 13. The dominant contribution at yfs = 500 GeV
268
is from 7 exchange and so for simplicity we will neglect the Z contribution here. (The complete 0{as) result, including the contribution from Z— exchange can be found in Refs. 28 and 30.) The electron mass can also be neglected. The lowest order amplitude is:
ALO{e+e~ - t t ) = ^ ^ ^ ) 7 ^ b W ) 7 ^ ( < / ) % , s where i, j are the color indices of the outgoing top quarks. It is simplest to work in the center-of-mass frame of the incoming e + e~ pair, where the 4— momenta of the particles are given by: p=
^(1,0,0,1)
q=
^(1,0,0,-1)
p> = ^ ( l , O , / ? s i n 0 , / 3 c o s 0 ) q' = ^ ( 1 , 0 , - / 3 sin 0,-/3 cos 0) with s = (p + q) 2
*-Vi-^
(«)
and 6 is the angle of the top quark emission with respect to the electron beam direction. The threshold condition is defined by j3 = 0. It is straightforward to find the tree level amplitude-squared in n = 4—2e dimensions, (summed over colors), ALO
\2= 64ir2a2Q2tNc 2(1 -e)+P2(cos2
0-1)
(66)
The color factor is Nc = 3. The corresponding total cross section is:
vLo = YsJ\ALO\2(dPS)2. Integrating over the two body phase space,
(67)
269
*(PO
Figure 14. Vertex correction to e+e —> tt. The 7*" denotes an off-shell photon which is connected to the initial e + e - state.
and including a factor of | for the average over the initial electron spins, gives a total rate, <JLo{e+e- - • it) = -Jj—\ OZ7TS 4
f
dcos9 \ ALO | 2 + 0(e)
J_i
The 0(e) terms which we have neglected will be important in computing the 0(as) QCD corrections. The cross section vanishes at threshold, (3 —> 0, and decreases with increasing energy, a ~ K This has the obvious implication that the higher energy an e + e~ collider, the larger the required luminosity. 4.2. 0(as) The 0(as) butions:
Corrections
to e+e — —• it
corrections to e+e~ —> it contain both real and virtual contri-
ONLO = CLO + ^virtual
+ Vreal-
(69)
The one-loop virtual contribution consists of a vertex correction and a wavefunction renormalization of the top quark leg. The vertex correction is shown in Fig. 14 and can be parameterized as: Averts = eQev(q)j„u(p) -V(p',
q')
(70)
with,
r„(p',g') = {~eQt)5i3g2sCF[N}u{p') ^
+ ^
v(q>)
(71)
270
where the normalization is, [N] =
1
fA-np?
16^2
\Mf
r(i + c)
(72)
and
N?
(73) 2Nr_ is the color factor for the vertex diagram. The parameter /x is an arbitrary renormalization scale. Explicit calculation gives the results, 28 ' 30 CF
1
A,
is A/3 = log
1-/3 1 + /9
B= - 1
3/SA^
+2Li2
- A | + 2A/3log
/ ^
2/9 1 + /3
2TT2
1 + /3
+
e The di-logarithm is denned by, (74) ! > ^ The 0{as) contribution to the virtual cross section is the interference of the vertex correction with the lowest order result. (The A term of Eq. (71) does not contribute to the interference with the lowest order result when the electron mass is neglected). Li2(x) =
^vertex
1 as /47r;r 2 2TT V Mt
J{dPS)2AL()Avertex CFT{1 + e)aLO
El
B fin
(75)
Note that the 0(e) terms from the lowest order amplitude and from the 2particle phase space combine with the — term to give a finite contribution 10 (Jyertex •
The wavefunction renormalization for the top quark must also be included in the virtual correction, as shown in Fig. 15. This diagram gives
271
t(P')
t(q') Figure 15. Feynman diagram for top quark wavefunction renormalization.
a multiplicative correction to the lowest order rate. Using the on-shell renormalization scheme, (76)
az = 26Ztt
where 5Zt,
dp2
\P
~Mt
4HMr;^ r ( 1 + e ) U + 4
(77)
The total virtual contribution is thus: 28 ' 30 0~VIRT = ^vertex + <J Z «LO^T(l '2n
4.3. Real
+ e)CF{
^
(-3 + Be)--4
+ Bfi.
(78)
Contributions
The virtual singularities of Eq. (78) are cancelled by those arising from real gluon emission, e + e~ —> tig. A sample diagram of this type is shown in Fig. 16. The real gluon emission diagram of Fig. 16 contains a fermion propagator on the outgoing top quark leg of the form: 1 (p'+pg)2-M?
_ 1 ~ 2p'-pg
~ 2E'Eg(l
1 - /3cos0)'
where E' and Eg are the energies of the outgoing top quark and gluon, 8 is the angle between them, and (3 = \/l — ^ 2 • Because of the massive top quark, /3 ^ 1, and this diagram has no collinear singularity as 6 —> 0. There is, however, a soft singularity as the energy of the gluon vanishes, Eg —> 0.
272
Figure 16.
Sample Feynman diagram for real gluon emission in the process e + e ~ —* tig.
The soft singularity is regulated using a technique called phase space slicing (PSS). 30 This method divides the gluon phase space into a soft gluon plus a hard gluon region: Cereal = CTsoft + &hard •
(79)
The soft region is defined by the energy of the emitted gluon, Eg<Ss^,
(80)
while the hard region is the remainder of the gluon phase space. Eg>6.^.
(81)
The separation into hard and soft regions depends on the arbitrary parameter 53. Individually, asoft and a hard depend on the cutoff, 5S, and the independence of the sum is a check on the calculation. S3 must be small enough that terms of 0(SS) can be neglected, while large enough to prevent numerical instabilities, a hard is finite and can be evaluated numerically using standard Monte Carlo techniques. In the soft gluon regime, the cross section can be evaluated analytically. The three body phase space is evaluated in the soft limit in the rest frame of the incoming e + e~: p + q = p' + q'+pg,
(82)
giving a gluon energy of TP
_
I
S
~
S
P'<,
(p' + q'f.
(83)
273
The 3-body phase space in the soft limit can be separated into a 2 body phase space factor times a soft phase space factor:31 dn~xt)'
f { PSh
"
dn~xa'
f
"/ ^
J 2 M S F
| !
"
T ( P
+ «"!'-«'-»>
(84)
x/^S^-
For the soft limit, we set the gluon momenta pg = 0 in the delta function and the phase space then factorizes: (dPS)3(soft) = (dPS)2 J
2
f^_i
•
(85)
The phase space is most easily evaluated by choosing an explicit representation for the gluon momentum, pg = Eg(l, ...,sin#i sin9 2 , sin61 cos#2, cos#1).
(86)
yielding,32 dn-1Pg=d\pg\\pg\n-2dSln-2 = dEgEg2'2 sin™"3 01d01 sin™"4 02d62dnn^4
(87)
where, 27r(n-3)/2
(88)
°"-4 = ~Yr^3y-
The integrals can be performed analytically to find the three body phase space in soft limit: 32 (dPS)s(soft) = (dPS)2(^y
(J^jI^±dPSaoft
%/J
dPSsoft = - ( ~ j
f '
2
dEgEg~2t
sin 1 " 26 QxdQx sin" 26 02d62 . (89)
Note that the soft phase space depends explicitely on the cut-off 6S through the limit on the Eg integral.
274
The soft phase space must be combined with the amplitude for e + e tig in pg —> 0 limit: i-soft
e2QeQt s
-9sTAv{q)llxu{p)
"{'£
x u(p
+ Pg + Mt) _M ^ ^ (-
F
9sTA
'}v(q>) (90)
_P -Pg
Q 'Pg.
where we have set pg = 0 in the numerator. The approximation of retaining only the most singular contributions is known as the eikonal (or double pole) approximation. 33 The soft limit of the amplitude always factorizes in this manner into a factor multiplying the lowest order amplitude. The soft amplitude-squared is A soft | 2 = g2sTr(TATA)
| ALO | 2 *«2 Mi
Mi .
2
- -2Mf 2
(P'-Pg)
(
+
(91)
P'-PgQ'-Pg.
giving the cross section in the soft limit, b
J | ASOft |2 (dPSHsoft)
(92)
The problem reduces to evaluating the soft integrals. A typical integral is 1
jsoft
;dPS3(Soft) .
(P' -Pg)2
(93)
Since the phase space and the amplitude squared are Lorentz invariant, we are free to chose any convenient frame. We use pg = Eg(l, ...sin6*i sin#2;sin#i cos62,cos9{) p' =
(94)
^(1,0,0,0)
and the soft integral is, jsoft
_
W^dPS'{soIt) s/l
- "
dEJnEl-2e 9^g
7T\4 b
As always, the factor of 1/4 is the spin average.
2e
e ™ iL--zee1d8 a 1JO sm sm-'„:„-2£. d02
275
sE\)) ( 1 - /? cos 6»02 Ye+^g(5s)
M2
+
^A0)+O(8s,e)
(95)
The complete set of soft integrals relevant for this process is given in the appendix of Ref. 34. The soft cross section is
/V
r(l-e)
Csoft
Us ^LO-^—CF
Cfi
~Uog(Ss)-^A0-2^+p
9i
(96) •Cfi e where B€ = —Ce so the sum of virtual plus soft contributions is finite. The finite contribution, C/j„, depends on 6S and is given by 30
'to'
Li2 The complete 0(as)
s
2(3
, 1 + /3,
J
r(l-2e)
A2
-f + log(5s)Ap
0(SS)
(97)
result for e+e~ —> it is then,
&NLO = &LO + 0'virtual + &soft + CT/iard •
(98)
The combination (Jsoft + Phard is independent of6s, while the \ poles cancel between cr so / t + (JvirtUaiThis example, e + e~ —> ii, is simple enough that it could have been done analytically without the approximation of phase space slicing.28 Phase space slicing, however, is useful for more complicated examples and also to construct Monte Carlo distributions beyond the leading order. 4.4. Threshold
Scan in e + e
tt
The energy dependence of the cross section for e + e~ —> it at threshold depends sensitively on the parameters of the top sector. The location of the peak depends on Mt, while the height of the peak depends on Tt- The problem is that the location of the peak as a function of center-of-mass energy shifts at higher orders and there is a large renormalization scale dependence to the prediction as seen in Fig. 17. 35 At threshold, (3 —> 0, the theory possesses two small parameters: as and j3 and the cross section diverges at 0(aa),9 (j(e4
tt)
1/3^0 = C r L O
1
2nas 3/?
(99)
276
/
/
/
/
N
v/
/ ' /
f ' ~
//
*-., "" - -
\C""-»-^L
f
/ / / /
s
=:/--.
<*, ^
—— ====
~^-^- .r-^^-"" Figure 17. Cross section for e + e _ —> it as a function of the center-of-mass energy in GeV using the pole mass definition for the top quark mass at lowest order ( dot-dashed), NLO (dashed), and NNLO (solid). The vertical axis shows tt)/avt, where Opt = 47ra 2 /3s. The three sets of curves at each order represent the variation with the renormalization scale /U, (jj, = 15, 30, and 60 GeV).37
This divergence as /? —> 0 is known as a Coulomb singularity and nonrelativistic quantum mechanics can be used to sum the leading contributions in powers of i . 3 6 Beyond lowest order in as, the predictions are quite sensitive to the precise definition of the top quark mass. The most straightforward definition is the pole mass appearing in the top quark propagator, D{p)
(100) Mt
\Pole - S ( p )
The pole mass, Mt \poiei is measured by reconstructing the four-momenta of top quark decay products. This definition of the top quark mass is, however, uncertain by QCD hadronization effects, O(AQCD)- These effects connect the top quark with its decay products. Even more disturbing is the fact that the location and height of the peak in the e+e~ —> it cross section change significantly between lowest order, NLO, and NNLO as seen in Fig. 17. A better definition of the top quark mass is the short distance 15 mass: 37 - 38 m i s = Mtt \pole
l
9 '
(101)
277
/'~'N
/
\
/
V
' /
4*—s.
' / ft /^
/ /
/ / //
/ // /
K*- - ^
"^
X^-*-^^ •
-
=
^-
/
: _ __ •
W '/
?s?= ^ ~~ Figure 18. Cross section for e + e - —> it as a function of the center-of-mass energy in GeV using the IS mass definition for the top quark mass at lowest order ( dot-dashed), NLO (dashed), and NNLO (solid). The vertical axis shows <x(e+e~ —> tt)/apt, where &pt = 47ra 2 /3s. The three sets of curves at each order represent the variation with the renormalization scale /i, (fi = 15, 30, and 60 GeV). 3 7
Using this mass definition, the cross section for e+e —> tt peaks at 2niis and the location of the peak does not shift at NLO or NNLO, as seen in Fig. 18. The NNLO result still has roughly a 20% scale uncertainty. It is estimated that by scanning the threshold dependence of the cross section for e+e~ —> tt a measurement with the accuracy 8Mt ~ 200 MeV can be made with an integrated luminosity of L = 50 fb^1. This is to be compared with the expected accuracy of the LHC measurement, SMt ~ 1 — 2 GeV.26 A precise measurement of the top quark mass is of interest primarily because of its implications for electroweak precision measurements. 25
4.5. tth Production in e+e
and pp
Collisions
In many models with physics beyond the Standard Model, the Higgs coupling to the top quark is significantly different from that in the Standard Model. In the Standard Model, the fermion-Higgs Yukawa couplings are, L
=
-9ffhipiph
(102)
278
where 9ffh = ~f
•
(103)
Since the top quark is the heaviest quark, it has the largest Yukawa coupling to the Higgs boson. It is important to measure this coupling accurately in order to verify that the Higgs mechanism is the source of fermion masses. The Higgs boson couplings to the lighter quarks can be tested by measuring the Higgs decays to fermion pairs. This is not possible, however, for the top-fermion coupling. Only for very heavy Higgs bosons (M/j > 2Mt) is the decay h —> it kinematically accessible, and the branching ratio of the Higgs into it pairs is significantly smaller than that into gauge boson pairs. It does not appear feasible to measure guh through this channel. The most promising prospect for measuring the top quark Yukawa coupling is ith production in e+e~ and pp (or pp) collisions. 4.6. iih at an e + e -
Collider
+
At an e e~ collider, iih associated production occurs through s-channel •y and Z exchange, as shown in Fig. 19. The contribution from Z exchange is a few percent of the total rate and will be neglected here. With this assumption, the rate is directly proportional to gfth. In order to interpret the cross section as a measurement of guh, however, it is necessary to include the QCD corrections which are potentially of the same order of magnitude as new physics effects which may change gtth from the SM prediction. The NLO QCD result includes virtual and real gluon corrections on the outgoing t, t legs. The calculation follows that of the previous section for e + e~ —> it, although in this case the virtual
Figure 19.
Contributions to e + e
tth at lowest order.
279
e+e - > tth
Mh=110GeV Mh=130GeV
500
600
700
800
900
1000
Vs (GeV) Figure 20.
NLO rate for e + e
—> tth as a function of center of mass energy.'
calculation also includes box diagrams. These corrections have been found in Refs. 39 and 40 and can be parameterized as K
&NLO <JLO
l + as(n)F(s,Mt2,M£)
.
(104)
The parameter \i is an unphysical renormalization scale and, at this order, the only dependence on JJL is in the running of as(fi). For Mh = 120 GeV, and /j, ranging from Mt to y/s, K(sfs = 500 GeV) ~ 1.4 - 1.5 K(y/s = 1 TeV) ~ 0.8 - 0.9 .
(105)
The NLO results for tth production in e+e~ collisions are shown in Fig. 20. The rate is quite small and has a maximum value for ^fs ~ 700 — 800 GeV for Mh in the 120 GeV region. In order to measure the top quark Yukawa coupling in e+e~ collisions, we consider the final state tth -» W+W~bbbb
(106)
The events are then classified according to the W decays, in an analogous manner to that discussed in Section 3.5 for top quark decays. Since
280
the event rate is quite small, it is advantageous to combine hadronic and semi-leptonic W decay channels. 41 ' 42 At \/s = 800 GeV , an integrated luminosity of L = 1000 / 6 _ 1 , can measure the coupling to an accuracy of41 ^
~ 5.5% .
(107)
9tth
At a lower energy, i / i = 500 GeV, the rate is considerably smaller and even with an integrated luminosity of L = 1000 / 6 - 1 , the precision is quite 42
poor, Sgtth
20% ,
(108)
gtth
for Mh = 120 GeV.
4.7. pp —• tth at NLO The QCD corrections to pp —> tt/i (or pp —> iift) can be calculated in a similar fashion to those for e + e~ —> ii/i. In the hadronic case, however, the calculation is complicated by the existence of strong interactions in the initial state. In this section, we discuss the treatment of initial state singularities in hadron collisions. The ingredients of an NLO calculation for pp or pp —> tih are: 34,43 • &virtual'- This contains all of the one-loop diagrams contributing to the process. Finite diagrams, or diagrams which contain only ultraviolet singularities, can be evaluated numerically.44 Diagrams which contain infrared singularities must be evaluated analytically. • asoft: This contains the soft singularities arising from both the initial and final state soft gluon radiation in the limit pg —> 0. The calculation is analogous to the discussion of Section 4.3. • Chard'- This is the contribution from real gluon radiation when the gluon has an energy above the soft cut-off. In contrast to the e+e~ example of Section 4.3, this contribution has a singularity when the initial state quark or gluon is parallel to the radiated final state gluon. The initial state collinear singularities are absorbed into the parton distribution functions (PDFs). The combination <jsoft + ahard + crvirtual is finite.
281 m,=174 GeV , mH=120 GeV , n=m t , 8C=0.001
Figure 21. NLO QCD corrections to pp —> tth at \ / S = 2 TeV. This figure shows the independence of the total cross section on the soft cutoff, Ss.34
4.8. Collinear
Singularities
and Phase Space
Slicing
To illustrate the effects of collinear singularities, we consider the parton level subprocess, q(p) + Q(Q) - • t(pt) + t(pt<) + h(ph) + g{pg) .
(109)
As in the e+e~ example, we divide the gluon phase space into a soft and a hard region. We then further divide dhard into a region with collinear divergences and a finite region, &hard — &hard \coLl \^hard
\not—coll
(110)
The separation depends on a new cut-off, 5C, although a hard must be independent of Sc. If the emitted gluon is from the initial state quark line, then there is a quark propagator, 1
1
(P ~ PgY
2p • Pg
1 y/sEg{l
— COS t
(111)
282
(1 - z)p
Figure 22.
Diagram contributing to collinear singularity for the process qq —> tthg.
which becomes singular when p and pg are parallel (cos# —> 1). hard/not-collinear region is defined by, 2p-Pg >5C EgV~s
2q-pg EgV~s
>
The
(112)
$c
In this region the quark propagator of Eq. ( I l l ) is finite and the cross section can be computed numerically. In the collinear region, 2p-Pg < Sc EgV~s
Eg^S
(113)
<Sc,
and the matrix element factorizes into a parton i splitting into a hard parton i' plus a collinear gluon with kinematics Pi' - zpi Pg = (1~ z)Pi 2pi • Pg
°ig
These kinematics are illustrated in Fig. 22. In the current example, q —> qg and the matrix element factorizes,34 Aiij -> tth + g) \2-+couinear
Y,i>g2s \ ALO(i'j
-> tth)
2Pu,(z)
(114)
zs 1-9 where PiV is the probability that parton i splits into parton i', with fraction z of the initial momentum. In this example, Pu>(z) = Pqg(z) = CF
l + z2 1-z
s(l-z)
(115)
283
The phase space also factorizes in the collinear limit: 31
(ps)A(ij -+ tth+g)collinear - (psui'j -v m ^ n ' - V ^ l 1 (1 — 2e) 167T2 Z xz dz dsig[(l-z)sig}-ee([ 'p^ -pjSc-Sig) (116) Combining Eqs. (115) and (116), the hard-collinear parton level cross section becomes:45
_as r ( l - e ) (AitfXf Vhard/coll
~ ^
r ( 1
x S^ •( /rl-6.
_
2 e
) \
?
)
\
r{l__z)2pi,.p.
dz
+ (i"j)j.
M
\\s.
zM?
e)
'
Pu'{z)(JLo{i'j
-^tth)
-
(117)
This result has a dependence on both Sc and 5S. The 1 — 6S limit on the z integration excludes soft gluon region. The remaining singularities in Eq. (117) are absorbed into the mass factorization as described in the next section. 4.9. Mass
Factorization
The hadronic cross section depends on the PDFs, fi(x): °H(S)
— Sy / fi{xi)fj(x2)<Tij(xix2S)dxidx2
,
where a is the parton level cross section. At lowest order, there is no dependence on the renormalization/ factorization scale, /i. Since the only physical quantity is the hadronic cross section, <JH(S), we can define scale dependent PDFs to absorb the hard/collinear singularity of Eq. (117), 30,31
\[5S TPlAz)fi' (l)
+CF 21
( °e^) + I)} • (118)
The upper limit on the z integration excludes the soft region. After convoluting the parton cross section with the renormalized quark distribution function of Eq. (118), the infrared singular counterterm of Eq. (118) exactly cancels the remaining infrared poles of &Virt + asoft + & hard/coll-
284
6.5
£
5.5 O _l Z O _i
0 4.5
3.5 150
250
350
450
Figure 23. Leading order and next-to-leading order rate for pp —> tth at the Tevatron for M/, = 120 GeV and '/S = 2 TeV as a function of the unphysical factorization/renormalization scale. 34
The cancellation of the cutoff dependence at the level of the total NLO cross section is a very delicate issue, since it involves both analytical and numerical contributions. It is crucial to study the behavior of <JNLO m a region where the cutoff(s) are small enough to justify the approximations used in the analytical calculation of the infra-red divergent soft and collinear limits, but not so small to give origin to numerical instabilities. This effect is illustrated in Fig. 21. The NLO result for pp —> tth at the Tevatron is shown in Fig. 23 as a function of the unphysical factorization/ renormalization scale. The complete calculation includes not only the qq initial state described here, but also the gg and qg initial states. Note the reduced \i dependence of the NLO rate from the lowest order prediction. The cross section is very small, making this a challenging measurement. At the LHC, the process pp —> tth has a significant rate for Mh < 130 GeV and we can look for h —> 66, giving the final state of W+W~bbbb. This is an important discovery channel for a Higgs with Mh < 130 GeV. The ATLAS experiment, with L = 100 / 6 ~ \ and assuming Mh = 120 GeV can measure ^ ^ ~ 16%. This is illustrated in Fig. 24. 26 The signal and the background have similar shapes at the LHC, increasing the difficulty of this measurement.
285
> O 150
m 100
50
mbb(GeV) Figure 24. Invariant mass for the bb pair from the Higgs decay in the process pp —* tih at the LHC with M^ = 120 GeV (data points). The dashed line is the background. The error bars correspond to the statistical uncertainty with an integrated luminosity of L = 100 fb-1.26
5.
Heavy Top as Inspiration for Model Builders
We turn now to a discussion of the top quark in models where there is new physics involving the top quark beyond that of the Standard Model. The fact that the top quark is much heavier than the other quarks gives it a special role in electroweak symmetry breaking. In this section, we examine the role of the top quark in the minimal supersymmetric model and in models with dynamical symmetry breaking. We pay particular attention to how data from the Tevatron and the LHC can help us to test these ideas. The large value of the top quark mass has important consequences in a supersymmetric (SUSY) model. 46 In the SM, the Higgs boson mass is a free parameter, while in a SUSY model there is an upper limit on the Higgs mass. At tree level in the minimal SUSY model, Mh < Mz, but the large top quark mass, and the subsequent large coupling to the Higgs boson, mt
V2
1,
(119)
lead to large radiative corrections to the prediction for the Higgs boson
286
mass, raising the lower limit on the Higgs mass significantly. In addition, corrections due to the large top quark mass typically imply that the scalar partner of the top, the stop, is the lightest squark. A further difference between the SM and a SUSY model which is due to the large top quark mass is the understanding of the mechanism of electroweak symmetry breaking in a SUSY model. If the supersymmetric model is embedded in a grand unified model, then the simplest version (mSUGRA) has the interesting property that the evolution of the parameters from the GUT scale to the weak scale drives the Higgs mass-squared negative as a consequence of the large value of Mt, and therefore explains electroweak symmetry breaking. In fact, this mechanism requires that the top quark have a mass near its observed value. In models with dynamical symmetry breaking, the large value of the top quark mass can imply that the top quark is different from the lighter quarks, perhaps having different gauge interactions in an extended gauge sector. In Section 5.6, we discuss topcolor as an example of a model of dynamical symmetry breaking where the top quark plays a special role. Many possibilities for dynamical symmetry breaking in electroweak interactions are detailed in the review by Hill and Simmons. 47 5.1. EWSB
and the Top Quark
Mass
We begin by considering the importance of the top quark mass in the SM. In the SM, electroweak symmetry breaking occurs through the interactions of an SU(2)L doublet scalar field, , with a scalar potential, 48 V = A * 2 * ^ + A($i- f ) 2
.
(120)
If y? < 0, the neutral component of the Higgs doublet gets a vacuum expectation value, <*o> - ^=
(121)
and the scalar potential has the shape shown in Fig. 25. Electroweak symmetry breaking (EWSB) occurs and masses are generated for the W and Z gauge bosons. After EWSB, there remains in the spectrum a physical particle, the Higgs boson, h, and at tree level the quartic self- coupling of the Higgs boson, A, is related to the Higgs mass by Ml = 2Xv2.
(122)
A measurement of this relationship would test the mechanism of EWSB.
287
Figure 25.
Higgs potential in the Standard Model with p? < 0.
Equation (120) is the scalar potential at the electroweak scale, A ~ v. All parameters of the electroweak theory change with the energy scale A in a precisely calculable way, and so will be different at higher energies. The most interesting parameters for us are the Higgs quartic self-coupling, A, and the top quark Yukawa coupling, gUh = — . To one loop accuracy,0 A — - —— 24A2 - (A d\
A
dgtth dA
~ 16TT2
1 f dtth 167T 2 V 1 2
Wtth )X + B
54flL
ugtth
17g'2 - 27g'2 - 96g2s
(123)
where, A = B
^g2+g12) 3 2g* + 8
/2\2 (g2+g")
(124)
We have omitted the equations for the running of the gauge coupling constants. They can be found in Ref. 48. In order for electroweak symmetry breaking to occur, the quartic coupling must remain finite at all scales, 1 >0. A(A)
(125)
As A is evolved from the weak scale to higher energy, it grows and eventually becomes infinite at some large scale. When A becomes large, the scaling of c
g and g' are the S[/(2)x, and U(V)Y gauge coupling constants.
288
Eq. (124) can be approximated, A ^ = ^
2TT2
d\
.
(126)
Equation (126) is easily solved,
AM = A(k)"ilog(^:)-
(127)
The position where A(A) —> oo is known as the Landau Pole. Since A is related to the Higgs mass, M% = 2v2X, the requirement that the theory not approach the Landau pole gives an upper bound on the Higgs boson mass, Ml <
4 v2
f x 31og[A/M h
.
(128)
This means that at the scale A there must be some new physics beyond that of the SM which enters. The SM simply makes no sense at energy scales larger than A since in this region the Higgs quartic self-coupling is infinite. Assuming no new physics before the GUT scale, A ~ 10 16 GeV, gives a bound on the Higgs mass of49'50 Mh < 160 GeV .
'
(129)
This bound corresponds to the upper curve in Fig. 26. Non-perturbative studies using lattice gauge theory confirm the validity of this limit. 51 Conversely, we require that the potential be bounded from below, which is equivalent to the restriction that A > 0 at large $. The scaling of the quartic coupling near A = 0 is, dX
1
~dA
16TT2
B - I2gfth
(130)
which can be solved, A(A) = A(M,) + ^ — l ^2 M l o g ( A ) . 167T \Mh
(131)
The requirement that A (A) > 0 gives a lower bound on the Higgs mass,
A complete approach must include the full set of renormalization group equations, including the running of the coupling constants for large values of $.
289 800.0
600.0
>~ CD CD 400.0 x
200.0
""10"
10°
10*
10"
10°
A (GeV) Figure 26. Limits on the Higgs boson mass as a function of the cutoff scale A. The theoretically consistent region lies between the two curves. 4 9
The restrictions on the Higgs mass from consistency of the theory are shown in Fig. 26, where the allowed region for a scale A is the area between the curves, known as the "chimney". At a given scale A, the Higgs mass has a lower limit from bounding the potential from below and an upper limit from the requirement that Higgs quartic coupling, A, be finite. The exact shape of the allowed region depends sensitively on the value of M 4 . 49 ' 50 The SM with a Higgs mass in the 130 GeV region is consistent at energy scales all the way to the Planck scale. The theory is still unsatisfactory however, because it provides no explanation of why /i 2 < 0, as required for EWSB. In addition, the Higgs mass is a free parameter in the SM and there is no understanding of why it should have any particular value. In the next section, we will see that low energy supersymmetry can potentially solve both these problems. 5.2. Supersymmetry
and the Top
Quark
TeV scale supersymmetric models explain several mysteries about the SM: • A heavy top quark and mSUGRA (minimal supergravity) explain why fi1 < 0. • A heavy top quark and the MSSM (minimal supersymmetric Standard Model) explain why the Higgs mass may light, as suggested by precision measurements.
290
In this section, we give a brief review of phenomenological models of supersymmetry and discuss the relationship between the top quark mass and the requirement that supersymmetry exist below the TeV scale. Supersymmetry is a symmetry which relates the masses and couplings of particles of differing spin. To each SM chiral fermion is associated a complex scalar and each of the SU(3) x SU(2)L X U(1)Y gauge bosons similarly acquires a Majorana fermion partner. Supersymmetry connects particles of differing spin, but with all other characteristics the same. 52 In an unbroken supersymmetric model, particles and their superpartners have identical masses. It is clear, then, that supersymmetry must be a broken symmetry if it is to be a theory of low energy interactions. There is no scalar particle, for example, with the mass and quantum numbers of the electron. In fact, there are no candidate supersymmetric scalar partners for any of the fermions in the experimentally observed spectrum. The most general soft breaking of supersymmetry introduces masses and mixing parameters and the theory loses much of its capabilities for specific predictions. The Higgs sector in a SUSY model always contains at least two SU(2)L doublets. In the supersymmetric extension of the Standard Model, the Higgs doublet acquires a SUSY partner which is an SU(2)L doublet of Majorana fermion fields, hi (the Higgsinos), which contribute to the triangle SU(2)L and U(1)Y gauge anomalies as described in Section 2.1. Since the fermions of the Standard Model have exactly the right quantum numbers to cancel these anomalies, it follows that the contribution from the fermionic partner of the Higgs doublet remains uncancelled. 6 ' 7 This contribution must be cancelled by adding a second Higgs doublet (and its Majorana fermion partner) with precisely the opposite U(l)y quantum numbers from the first Higgs doublet. All supersymmetric models thus have at least 2-Higgs doublets and so the minimal supersymmetric model has 5 physical Higgs bosons: h°, H0^0,!!^. The ratio of the vacuum expectation values of the neutral Higgs bosons, Vo
tan/3=—
(133)
Vi
is a fundamental parameter of the theory. At tree level, the neutral Higgs boson masses are predicted in terms of two parameters, which are usually taken to be tan/3 and the mass of the pseudoscalar Higgs boson, MA- The neutral Higgs boson masses are then predicted, KH
= \{M\ + Ml)
T
\y/(MA
+ Mlf-
4M2ZM2A cos2 2/3
(134)
291
which gives an upper bound on the lightest Higgs mass: Mh<Mz
| cos 2/3 | <MZ
(135)
which is excluded experimentally. (In a SUSY model, the LEP and LEPII results require Mh > 89.8 GeV.4) The Higgs mass prediction in a supersymmetric model receives large radiative corrections from top and stop squark loops. At 1-loop, and neglecting possible soft SUSY breaking tri-linear scalar mixing, the lightest Higgs mass becomes: „.r9
•
Ml =
Ml+Ml+e A
z 2
If,
,
9
,9
9 9 9
1 (M 2 + M\ + e)2 - 4M 2 M 2 COS2 2/3 >. 1/2
- 4 e ( M l sin2 /3 + M§ cos2 /3) I
(136)
where, 3GF
Ml
lo
(M?
sh^ h
(137)
and Mt is the average stop squark mass. For unbroken supersymmetry, the top and stop have the same masses, Mt = M t , and the bound of Eq. (135) is recovered. The mass splitting between the top and stop in a broken SUSY theory changes the upper bound on the Higgs mass to, Ml < M § cos2 2/3 + e sin2 /?.
(138)
The large Mf corrections raise the Higgs mass bound above the experimental limit, as shown in Fig. 27. There are many analyses 53 which include a variety of two-loop effects, renormalization group effects, etc. but the important point is that for given values of tan/3, the scalar mixing parameters, and the squark masses, there is an upper bound on the lightest neutral Higgs boson mass. For large values of tan /3 the limit is relatively insensitive to the value of tan (3 and with a squark mass less than about 1 TeV, the upper limit on the Higgs mass is about 110 GeV if mixing in the top squark sector is negligible. For large mixing, this limit is raised to around 130 GeV.
5.3.
mSUGRA
The minimal supersymmetric model (MSSM) is constructed by adding to the Lagrangian all of the soft supersymmetry breaking terms allowed by the gauge symmetries. These include masses for all the new SUSY particles
292
Maximum Higgs Mass (No mixing) 120.0
'
110.0
1
tan (3 =
100.0 • . - - - r ^
tan p=10
> O
90.0
80.0
-
^ a n p =2 70.0
60.0 200.0
I
400.0
.
600.0
.
800.0
msquark (GeV) Figure 27. mixing. 5 3
I
1000.0
Upper bound on Higgs mass in MSSM at one loop and with no soft trilinear
and tri-linear scalar mixing terms. T h e most general Lagrangian of this type has 105 new parameters beyond those of the S t a n d a r d Model. In order to make definite predictions, the simplifying assumption t h a t the soft parameters unify at MQUT is often made. (The resulting model is usually called m S U G R A or the CMSSM). In this framework the model is completely specified by five parameters: • • • • •
Common scalar mass at MQUT, mo Common gaugino mass at MQUT, Afi/2 1 Higgs mass-squared, m\2 1 tri-linear coupling at MQUT, AoXp sign(/t)
T h e parameter ft describes the mixing between the two Higgs doublets. At the G U T scale, the Higgs masses are, MlH(MGUT)
mn
•M
(139)
while the squark and slepton masses are M?(MGUT)
=
MKMGUT)
(140)
293
The theory must explain why the lightest neutral Higgs mass-squared becomes negative at the weak scale and breaks the electroweak symmetry, while the scalar fermion masses remain positive so as to not break color and electromagnetism. In an mSUGRA model, the masses and parameters are evolved using the renormalization group equations (RGE) to scale from MQUT down to the weak scale in order to make predictions for the masses at the weak scale. The large top quark mass plays an important role here. We can understand the basic scaling by considering only the Higgs doublet, i?2, d the scalar partners of the third generation SU(2)L fermion doublet, (£L, &L) = Qs, and the scalar partner of the right-handed top quark, in. The renormalization group scaling is given by: 54 dM2H2 = ^L(3glhXt dlog(A) dM? tR dlog(A)
3g2M2 - g'2M2)
- i {2g^Xt - f^ M °" f9'2M?
dM"k Qs = J L (&hXt dlog(A)
- fg2M2
- 1g2M2 - -\g'2M2
where,
Xt = M2H2+M2Q3+M2R+A2
.
(141)
-^3,2,1 a r e the gaugino masses associated with the SU(3) x SU(2)L X U(1)Y gauge sector, g^ is the coupling of the Higgs to the top quark in the MSSM, and At is the soft SUSY mixing term in the top sector. The QCD term (proportional to g2) makes M2R,MQ3 increase faster than MH2 a s the scale is reduced from MQUT- The Yukawa term (proportional to gtth) drives Mfj < 0, while the other scalar mass-squared terms remain positive. This behavior is a consequence of the large Mt value. Neglecting the gauge couplings, we have:
(142)
dlog(A)
i
H2 is the SU(2)L
doublet with isospin Y -
+\2 '
294 Evolution of sparticle masses
700 600
> o
500 400 300 200 100 n
io 6
io 10
io 17
Q (GeV) Figure 28.
Typical pattern of masses in an mSUGRA model. 5 5
which can be easily solved, Ml{K) = Ml{MGUT) - | j f Xt log ( ^ p )
.
(143)
For heavy Mt, M\ becomes negative and triggers EWSB. It is interesting that this behavior requires Mt ~ 175 GeV.55 The RGE scaling from the GUT scale to the weak scale in mSUGRA type models yields a a typical pattern of masses, as seen in Fig. 28. The gluino is the heaviest gaugino, while the squarks are significantly heavier than the sleptons. 5.4. Charged Higgs
Decays
The pattern of top quark decays is changed in the MSSM from that of the SM, due to the extended Higgs sector. The presence of a charged Higgs boson allows the decay, t —* H+b if the mass of the charged Higgs is less than Mt— m^. This decay looks similar to t —> Wb and can have a significant rate. The coupling of the charged Higgs to the t b system is, 9tH+b ~ mt cot f3P+ + nib tan/3P_
(144)
295
and so this decay is relevant both for very large and for very small tan/?. For small tan/3, H+ —> cs and H+ -> t*b, while for large tan/3, H+ —> ri/. These decays would suppress the rate for top decays to leptons plus jets in the SM top search. In Run I, the Tevatron experiments placed a limit on MH+ of roughly MH+ > 100 GeV for tan/3 < 1 or tan/3 > 50. 56 5.5. The Top
Squark
The mass-squared matrix for the scalar partners of the left and right handed top quarks (ti,, IR) includes mixing between the partners of the left- and right- handed top: M2=( M2Q3 + M? + M\Lt cos 2/3 Mt{At-ii * V Mt{At-(i cot (3) M?R+M? +
cot /3) \ Ml.Rtcos2p)
The mixing is proportional to the top quark mass and so the large value of Mt can cause one of the top squarks to be relatively light. If the scalar mass scale, MQ3 is much larger than Mz, Mt,At, then the stop masses are roughly degenerate ~ MQ3. On the other hand if MQ3 ~ Mz, Mt,At, then the large mixing effects drive the stop mass to a small value and the stop squark becomes the lightest squark. It is possible to search for a light stop in the decays of the top, t —> tx°, where x° is the lightest neutral SUSY particle. The limits then depend on the assumed branching ratio for the top into stop decays. The Run I data was sensitive to stop and chargino masses in the 100 GeV range. 57 5.6. The Top Quark and Dynamical
Symmetry
Breaking
The top quark has a special role in technicolor models where a ti condensate can play the part of the Higgs boson in generating EWSB. This can happen in models where there is a new strong interaction felt by the top quark, but not by the lighter quarks. The proto-type model of this type is called topcolor. A review of this class of models can be found in Ref. 47. Topcolor models are constructed by expanding the strong gauge group from the SU(3) of QCD to, SU{3)tc x SU{3) - •
SU(3)QCD
(145)
SU(3)tc couples only to the 3rd generation with a coupling gtc, while the second SU(3) gauge symmetry couples only to the first two generations. The symmetry is broken to SU(3)QCD at a scale M. Below M, the gauge
296 bosons of the SU(3)tc group, the "top-gluons", are massive and lead to effective 4-Fermi interactions between the top quarks: L ~ | |
( Q
3
L 7 ^ Q
3
L )
( W ^ * * )
,
(146)
where TA is the SU{2>) generator in the adjoint representation and Q^,L is the left-handed (tL,bL) doublet. If the SU(3)tc coupling, gtc, is larger t h a n a critical value, a top condensate forms, (tt), which breaks the electroweak symmetry and generates a mass for the top quark. One must arrange t h a t {tt) ^ 0 and (bb) = 0. This is usually done by adding an additional U(l)h gauge symmetry under which the t and b quarks transform differently. This class of models can be tested at the Tevatron and the LHC by searching for effects of the top gluons and the new Z', and through B decays. 5 9 Since the Z' boson corresponding to the U(l)h and the top gluons of SU(3)tc couple differently to the third generation and to the lighter quarks, they lead to flavor changing neutral currents. Present limits from B meson mixing require t h a t the topgluon be heavier t h a n 3 — 5 TeV and Mz, > 7 - 10 GeV.58
6.
Conclusions
T h e story of the top quark is far from over. Its properties and its role in the SM and in physics beyond the SM will be further elucidated in the emerging R u n II d a t a at the Tevatron. T h e increased statistics of R u n II will enable more precise measurements of the mass, couplings, and decay properties of the top. T h e LHC will provide even more insight into the role of the top quark. It is possible t h a t surprises are just around the corner! Acknowledgements I would like to t h a n k K.T. Mohanthapa, H. Haber, and A. Nelson for organizing such an enjoyable and successful school.
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.JOHN WOMERSLEY
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TEVATRON PHYSICS* JOHN WOMERSLEY Fermi National Accelerator Laboratory Batavia, IL 60510, USA E-mail: [email protected] These lectures form a personal, and not necessarily comprehensive, survey of physics at the Fermilab Tevatron proton-antiproton collider. They cover detectors, analysis issues, and physics prospects for the current Tevatron run.
1.
Introduction
1.1. Hadron
Colliders
The term "hadron colliders" is used to describe accelerators where a beam of protons is collided with a contra-rotating beam of protons or antiprotons. The next decade belongs to these machines: first the Fermilab Tevatron, and (starting around 2008) the LHC at CERN. Historically, there has been a complementarity of capability between hadron colliders and electron-positron colliders. Hadron machines emphasize maximum energy and hence maximum physics reach for new discoveries, while the electron-positron machines tend to have lower center-of-mass energy but cleaner events and better capabilities for precision measurements. Hadron Colliders have the advantage that protons can be easily accelerated to very high energies and stored in circular rings. They have the disadvantage that antiprotons, if used, must be collected from the results of colliding a lower energy, high intensity beam on a target. This can of course be avoided by building a proton-proton collider, at the cost of needing a second ring of magnets. They also have the disadvantage that protons are made of quarks and gluons, so the whole of the beam energy is not concentrated in a single point-like collision. Quarks and gluons are also strongly interacting particles, so the collisions are messy. Despite these problems, hadron colliders are the best way to explore the highest mass scales for new physics.
* Lectures presented at the TASI 2002 Summer School, University of Colorado, Boulder, CO, June 2-28, 2002. 303
304
1.2. The Tevatron The overarching question for the world high energy physics program is "what sets the mass scale of the weak interaction to be -100 GeV?" This question is addressed solely by colliders operating at the energy frontier. In the 1990's, there were four such machines: the Tevatron collider (Run I), LEP at CERN, the SLC at SLAC, and HERA. In contrast, between 2002 and roughly 2008, the Tevatron is the only machine that can address the central problems of the field: LEP and SLC have closed, and HERA's reach is limited. Increased energy and a greatly increased luminosity at the Tevatron make possible a new round of experimentation. The Tevatron physics program involves: • Precise measurements of the known quanta of the standard model, to search for indirect hints (or constraints) on new particles or forces; • Direct searches for new physics i.e. beyond the known standard model particles and forces. The Tevatron program has the potential for a discovery that would change the direction of particle physics. Between 1992 and 1995 (Run I), the Tevatron delivered about 120 pb~' to each experiment at a center-of-mass energy of 1.8 TeV. Run II started in 2001, with an increased center-of-mass energy of 1.96 TeV, using the new Fermilab Main Injector as part of the accelerator chain. We expect 2-4 ftf1 in the first phase of Run II, after which detector upgrades will be needed; the goal is to have delivered 10-15 ftf1 before the LHC becomes competitive. 2.
Detectors
A typical detector surrounds the interaction point with concentric layers of particle measurement devices, as shown in Fig. 1. The innermost parts of the detector measure charged particle trajectories in a magnetic field. Particle direction and momentum can be inferred. The innermost layers need to make the most precise measurements and use silicon wafers; these tend to be expensive, and so the outer layers use less precise but less costly technologies. Surrounding the tracker is a calorimeter. The calorimeter induces particle showers in dense material and measures the particle energies. It also distinguishes electrons and photons from hadrons by the shower topology. Finally, the outermost layers of the detector measure and identify muons. Combining all these measurements, it is also possible to infer the
305
presence of non-interacting particles like neutrinos, since they leave unbalanced momentum in the transverse plane ("missing transverse energy", ET™SS). Interaction point
Magnetized volume Tracking system \
Innermost tracking layers use silicon
Calorimeter induces shower j n dense material J,
EM layers fine sampling
Hadronic layers
Absorber material
Muon detector
Experimental signature of a quark or gluon
"Missing transverse energy" Signature of a non-interacting (or weakly interacting) particle like a neutrino
Figure 1: schematic of an azimuthal section of a tyrpical collider detector, and the particle signatures seen in it. There are two large multi-purpose detectors at the Tevatron: CDF and D 0 (D Zero). The detectors are complementary in that CDF detector places more emphasis on tracking measurements, while D 0 emphasizes calorimetry. The physics reach of the detectors is not dramatically different: it tends to either be driven by cross sections (as in searches for SUSY) or the respective strengths of the detectors tends to balance (as for the top quark in Run I).
2.1. Tracking The ability to identify b-quarks is very important at hadron colliders, as a signal for top, supersymmetry and Higgs. Experimentally, we can exploit the fact that a b-quark forms a B-meson which travels ~ 1mm from the collision point before decaying. To reconstruct this decay, we need to reconstruct tracks with a precision at the 10 urn level. We can tag the B-decay either by asking for two or three tracks having a significant "impact parameter" (distance of closest approach to the fitted primary vertex), or by explicitly reconstructing a
306
secondary decay vertex separated from the primary. Figure 2 shows these two methods schematically. Both CDF and D 0 use silicon detectors close to the beam pipe to achieve the necessary tracking precision. Silicon detectors use wafers of silicon of order 2 cm x 12 cm with -50 Jim pitch between readout strips and on-board amplifier-discriminator chips. These "ladders" are arrayed in cylindrical layers around the beam pipe and supported on structures that provide rigidity and cooling. Both the CDF and D 0 silicon detectors were assembled at Fermilab's Silicon Detector Facility. They are commissioned and working well in the collider. In CDF, the silicon detector feeds an impact parameter trigger which allows B-decay candidates to be selected online: a first at a hadron collider.
Figure 2: Impact parameter and secondary vertex techniques for b-tagging.
The detectors use separate outer tracker systems to cover the tracking region beyond the silicon. The CDF Central Outer Tracker is a large drift chamber with 96 wire planes and 30,000 sense wires. The D 0 Central Fiber Tracker uses 8 layers of 1mm diameter scintillating fibers, read out with cryogenic photodetector devices. Both the COT and CFT have the capability to provide triggers on high-transverse-momentum charged tracks.
2.2. Calorimetry The basic tool for detection of electrons, photons and hadronic jets (the signatures of quarks and gluons) is a segmented calorimeter surrounding the tracking system. The basic idea is to induce a shower of interactions between the incident particles and a dense material, and to measure the ionization energy deposited in the shower. From this, the particle energy is deduced. For electromagnetically interacting particles (e*, y) the situation is straightforward. Above -100 MeV, pair production and bremsstrahlung dominate the energy loss mechanisms. Shower development involves the processes y —> e+e~ and e* —> e*y and it scales with the radiation length Xo- It is easy to show that the total charged track length in the shower, and hence the sum of ionization, is proportional to the incident energy E, and that the RMS is proportional to VE.
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Strongly interacting particles (hadrons) also cascade in matter, but many more processes are involved: as well as inelastic hadronic collisions, energy is lost to ionization, electromagnetic cascades from produced n° mesons that decay to photons, nuclear binding energy, neutrino production, and nuclear excitation. Hadronic showers scale with the nuclear interaction length X: the showers are longer, wider, start later and have more fluctuations than an EM shower of the same energy. Because not all of the above processes are detectable, the response to a hadron is usually lower than to an electron of the same energy, the so-called e/7t ratio. For reasons of cost and compactness, collider calorimeters are "sampling calorimeters" that alternate dense but inert absorber with layers of sensitive medium. A fixed fraction of the shower energy is sampled in the sensitive medium. In the case of CDF, the sampler is scintillator tiles. New forward calorimeters were added for Run II and these exploit wavelength-shifting fibers to give a compact and flexible way to route the light signals to photomultiplier tubes. D 0 uses liquid argon as the sampling medium: the calorimeter consists of an absorber structure with electrodes, all of which is immersed in a vessel containing liquid argon, which serves as the ionization collector. Calorimeter energy resolution is usually dominated by statistical fluctuations in the number of shower particles, which scale as VE. The fractional resolution is therefore often quoted as "x%/VE" (where E is assumed to be measured in GeV): typical real-world values are 15%/VE for electrons, 50%/VE for single hadrons and 80%/VE for jets. Other terms contribute in quadrature: a "noise term", independent of E, which dominates at low E, and a "constant term", a constant fraction of E, which dominates at high energies and can come from calibration uncertainties, nonlinear response, and unequal response to hadrons and electrons. At high energies, jets become very clear in hadron-hadron collisions, and the calorimeter gives a very visual picture of the event topology. It also allows electrons to be clearly distinguished from jets.
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2.3. Muon System The outermost part of the detectors is the muon system. Since all other types of charged particles are absorbed, muons can be identified as charged tracks that traverse absorber outside the calorimeter. The outside of the detector is therefore covered with large-area detector planes to identify and trigger on muons. In the case of D0, an approximate stand-alone muon momentum measurement can be made using magnetized iron, but in both D 0 and CDF the central tracker provides a precise measurement once the muon track has been linked to matching trace in the central tracker. 2.4. Trigger The accelerator luminosity is set by the physics goals. For example, to find the Higgs we need -lOfb"1 of data; to do this in a reasonable time requires a collision rate of 2 x 1032 cm'2 s"1. The total inelastic proton-antiproton cross section is huge compared to the processes we are interested in studying. Even with bunch crossings every 396 ns at the Tevatron, there is more than one inleastic collision each time the bunches cross. The triggering challenge is then the real-time selection of perhaps 50 events per second (the maximum we can store to tape) out of this collision rate of 2.5 million per second. We do this by rapid identification of high energy particles, and of comparatively rare objects (electrons, muons...). Triggering is perhaps the greatest challenge to experiments at hadron colliders. Both CDF and D 0 use a three-level trigger system to reduce the rate progressively. The first level selects 10-40 kHz of collisions based on fast information from specialized detectors. The second level reduces this to a few hundred Hz using microprocessors that perform fast calculations on a subset of the data and reject most of the level 1 candidate events. The third level uses a farm of computers, with access to the full event readout; again most of the candidates from the previous level are rejected and 50 Hz is stored to tape. These events are passed to a computer farm where the reconstruction program is run on them within a few days of data taking, followed by storage in a tape robot system. The experiments operate with a "trigger list" of order 100 different trigger requirements at each trigger level, optimized for particular physics processes or final states. For many relatively high-rate processes, like jet production, we cannot run the trigger without restriction, because the cross section at low energy would exceed our capabilities. Such triggers are "prescaled", and only a known, fixed fraction of the events satisfying the trigger are saved.
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2.5. Simulation Tools A "Monte Carlo" is a Fortran or C++ program that generates events. Events vary from one to the next (random numbers are used), so we can expect to reproduce both the average behavior and the fluctuations of real data. Event generators may be parton level, which include just the parton distribution functions and the hard matrix element, or they may additionally include initial state radiation, final state radiation, hadronization of quarks and gluons, decays of unstable particles, and generation of the underlying event. Separate programs are used for simulation of the detector response to events. These are written by the detector collaborations but based on standard toolkits (GEANT being the best known and most widely used). 2.6. Computing Computing for data processing and analysis is a challenge for modern experiments both because of the quantity of data and because of the size and distributed geography of the collaborations. In the past, this led to the invention of the World-Wide Web as a way to share information. Now the challenge is to share data and computing power. There is a natural synergy between the needs of our experiments and current ideas about "Grid" computing. The Tevatron experiments are already making something like a Grid a reality and are distributing their data for analysis using a Fermilab-developed system called SAM. They are also exploring ways for remote collaborators to assist in monitoring detector operations using web tools over the internet. 3.
Hadron-Hadron Collisions
Hadron-hadron collisions are complicated by the fact that a hadron collider is really a broad-band quark and gluon collider. The incident particles to the parton-level hard scattering are selected from parton distributions at some factorization scale. The hard scattering is calculated at a given order in as, and at some renormalization scale. There may be initial state radiation before the hard scattering, or final state radiation after it. Outgoing quarks and gluons fragment into jets of hadrons, and the remaining quark/gluon content of the interacting beam particles forms a soft "underlying event." Since the incoming parton momentum fractions x, and x2 are a priori unknown, and usually beam particle fragments escape down the beampipe, the longitudinal motion of the parton-parton center of mass cannot be reconstructed. We therefore focus on transverse variables. We use the transverse energy and
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momentum, ET = E sinG and p T = p sinG, where G is the angle between a particle direction and the beam. We also use longitudinally boost-invariant quantities, the primary one being the pseudorapidity T| = -In tan(G/2). This maps on to angles in the detector: r\ = 0, 1, 2 being G = 90°, and roughly 40° and 15° respectively. For massless particles, pseudorapidity and true rapidity coincide. Particle production usually scales per unit of (pseudo)rapidity. 4.
Quantum Chromodynamics
No one doubts that Quantum Chromodynamics (QCD) describes the strong interaction between quarks and gluons. Its effects are all around us: it is the origin of the masses of hadrons, and thus of the mass of stars and planets. This doesn't mean it is an easy theory to work with. As well as using hadron colliders to test QCD itself, we find that it is so central to the calculation of new physics processes, and their backgrounds, that we need to make sure we can have confidence in our ability to make predictions in this framework. QCD is a gauge theory describing fermions (quarks) which carry an SU(3) color charge and interact through the exchange of vector bosons (gluons). It has the interesting features that the gluons are themselves colored, and that (as a consequence) the coupling constant runs rapidly and becomes weak at momentum transfers greater than a few (GeV)2. These features lead to a picture where the quarks and gluons are bound inside hadrons if left to themselves, but behave like free particles if probed at high momentum transfer. This is exactly what was seen in the deep inelastic scattering experiments at SLAC in the late 1960's which led to the genesis of QCD. The short-timescale, hard scattering involves the electron scattering elastically off a single, pointlike constituent inside the nucleon; the other quarks do not participate. Afterwards (over a longer timescale) the scattered quark, knocked out of the proton, radiates lots of gluons and quark-antiquark pairs which combine with each other and the colored remnant of the proton to form colorless hadrons. This is called "fragmentation" or "hadronization." At high momentum transfers these hadrons form a jet of roughly collinear particles whose energy and direction correspond to that of the quark or gluon. This picture becomes very intuitive when compared with highenergy events from LEP. One can see the topologies of two jets corresponding to e+e~ —» qq and three jets from e+e" —> qqg. High energy events at the Tevatron are also simple to recognize (see Fig. 3). Event generators model the hadronization process by dividing in into two phases: a perturbative phase, wherein a shower of quarks and radiated gluons
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develops and Q2 evolves downwards; and a nonperturbative phase, once Q2 ~ 1 GeV2 and the coupling constant becomes strong, where those quarks and gluons are grouped into hadrons (which may then decay). The PYTHIA and HERWIG programs contain two popular Monte Carlo treatments of this process.
Figure 3: High energy dijet event in Run II, as recorded by D0. 4.1. Jets Experimentally, we cannot make the same identification between hadrons and their "parent" parton. We need a jet algorithm to identify jets and define their properties rigorously. We would like an algorithm that can be run both on partons and on final state particles, and on detector measurements, and yield comparable results in all cases: in other words, one that does a good job of integrating over the non-perturbative and hard-to-calculate hadronization process. It is important to define jets properly even at the parton level: a naive one parton = one jet identification makes no sense for soft or collinear gluon radiation, for example.
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The traditional choice at hadron colliders has been the "cone algorithm." Jets are defined as the sum of energy within a cone of radius R2 = Ar)2+A
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balance in two jet events. For two jets, we can define an asymmetry A = (E-n-E-rc )/(ET1+Ex2); then the E T resolution oET/ET = ^2 oA- Since real events have additional soft jets and are not perfectly balanced in ET, it is necessary to progressively tighten cuts and extrapolate to the limit of no soft radiation. Typical jet resolutions are in the range 75-100%/VE(GeV).
4.2. Jet Cross Sections at the Tevatron In Run I, both CDF and D 0 measured jet cross sections at 1.8 TeV using the cone algorithm (R = 0.7). The cross section falls by seven orders of magnitude from 50 to 400 GeV. Agreement with NLO QCD is generally good, except perhaps at the highest energy end of the spectrum. When these results first appeared, there was some excitement that the CDF data, especially, were significantly in excess of QCD for ET > 250 GeV. This sparked a renewed investigation of uncertainies in the prediction arising from parton distributions, and it was found that the gluon distribution is not in fact well determined in this kinematic regime; a small enhancement in the gluon content is allowed and can model the CDF data very well. The latest PDF fits from CTEQ and MRST now make use of the Run I jet cross sections from D 0 to pin down the gluon distribution at high x. Both CDF and D 0 have now started measuring jet energy distributions from Run II. CDF are making use of their new forward calorimetry to cover the whole range of pseudorapidity (see Fig. 4). Jet calibrations are not yet final, but already we see events with transverse energies beyond 400 GeV. With the full Run II dataset this will reach as far as 600 GeV, allowing us to pin down the high-energy behavior of the cross section and thus better determine the gluon content of the proton.
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4.3. Heavy Flavor Production Run I left many unanswered questions about heavy flavour (charm and bottom) production. Resolving these is important because many new particles result in heavy flavour signatures. The inclusive b-jet and B-meson production cross sections lie significantly above the QCD prediction (see Fig. 5), though it can be made to fit better using resummation and retuned fragmentation functions (from LEP data). For charmonium, the measured cross section requires a large colour-
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octet component but that is not consistent with the observed JA|/ polarization. The CDF secondary vertex trigger in Run II is working beautifully, and the resulting huge charm and bottom samples will allow these puzzles to be explored in much more detail. D 0 now has preliminary Run II JA|/ and muon+jet cross sections which are the first steps in measuring the charmonium polarization (and thus production process) and the b-jet cross section.
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4.4. Hard Diffraction Another QCD-related puzzle is hard diffraction. In this kind of event, a highmomentum-transfer collision occurs but one of the incoming beam particles appears to leave the collision intact, instead of being destroyed in the process. This observation is rather surprising and needs to be pinned down better, and related quantitatively with similar phenomena observed at HERA. Both CDF and D 0 have new instrumentation for diffractive physics in Run II. This will allow us to test some of the basic assumptions on how to tag hard diffraction that have been used in earlier studies and will provide a sanity check for ideas of Higgs production through this mechanism at the LHC.
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5.
B-physics
The mixing between the three generations of quarks results in subtle violations of the CP symmetry relating particles and antiparticles. Understanding this symmetry will help explain why the universe is filled with matter, not antimatter. In the decays of B-mesons, these symmetry violations can be large, and so Bhadrons have become an important laboratory to explore the "unitarity triangle," which relates the elements of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. In Run II we want to confront the CKM matrix in ways that are complementary to the electron-positron B-factories. CP violation is now established in the B system through the decay Ba —> JAj/ Ks. The measured mixing angle is consistent with the standard model but, by itself, cannot exclude new physics. The BaBar and BELLE experiments can and will do much more with their data, but the Tevatron can uniquely access the B s meson, which is not produced at the B-factories and has therefore been called the "el Dorado" for hadron collider B-physics. By measuring the mixing rate between B s and B s , we can determine the length of one of the sides of the unitarity-triangle and complement the B-factories' measurements of its angles. CDF expect to be sensitive to Standard Model mixing with a few hundred inverse picobarns. It will also be interesting to see if there is sizeable CP violation in B s —» JA|/ <>| (it is expected to be small); while the decay Bs —> KK at the Tevatron complements Bd —» Tin that is measured at the B-factories. Together they can pin down the unitarity triangle angle Y- There are many other opportunities, such as Ab properties and searches for rare decays. CDF already have most impressive results from Run II, building on their Run I experience together with new detector capabilities (silicon vertex trigger and time of flight detector). Lepton-
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triggered signals for B* -> JA|/ K*, B° -> JA|/ K*°, and B s -> JA|/ 0 are seen; while using the Silicon Vertex Trigger, the purely hadronic modes B* —> D°7t —> Krai and B —> hadron hadron are being recorded (see Fig 6). We can also look forward to CDF exploiting an enormous sample of charm mesons. In D 0 the tools are being put in place for a B-physics program. The inclusive B lifetime has been measured and B mesons are being reconstructed. D 0 does not exploit purely hadronic triggers but benefits from its large muon acceptance, forward tracking coverage, and ability to exploit JA|/ —> e+e". 6.
W and Z Bosons
In Run II, we will complement our direct searches for new phenomena with indirect probes. New particles and forces can be seen indirectly through their effects on electro weak observables. The tightest constraints will come from improved determination of the masses of the W and the top quark. Until the LHC starts operations, the Tevatron is the world's only source of W and Z bosons. Production is dominated by quark-antiquark annihilation. The typical boson p T is a few GeV, but about 10% of the W/Z bosons are produced with one or more jets of significant E T (>25 GeV). Though the dominant decays of W's and Z's are into jets, they are swamped by the enormous QCD jet cross section. This means that experimentally we focus on the leptonic decay modes of the W and Z.
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Center of Mass Energy (TeV) Figure 8: W and Z cross section measurements at proton-antiproton colliders. W bosons are identified in the detector as a high-pT lepton together with a neutrino, identified as unbalanced transverse energy. We cannot reconstruct the longitudinal momentum of the neutrino but we can construct a transverse mass rriT :
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the maximum transverse mass is equal to the the W mass. Transverse mass distributions from Run II are shown in Fig. 7. The W and Z cross sections can be measured extremely well and are in excellent agreement with the QCD prediction (Fig. 8). In fact it is likely we shall start to use the W cross section as an absolute luminosity normalization at some point in Run II, as it is easier both to measure and to predict than the total inelastic proton-antiproton cross section which is used at present. One of the major goals of the Tevatron program is the W mass measurement. The simplest method is to fit the transverse mass distribution; recently we have started to fit the lepton p T and the ETmss distributions as well, and combine the three results. The whole key is to understand the systematics: one must keep beating them down as the data demand more precision. One can use the Z to constrain many effects, such as the lepton energy scale, the boson production process and boson pT. Currently, the W mass is known to be m w = 80 451 ± 33 MeV; the measurement is dominated by LEP data. The Tevatron Run I results fixed the W mass at the 60 MeV level, but it will take a Run II dataset of order 1 ftf before we can significantly improve the world knowledge of m w — not a
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short-term prospect. Given 2 fb"1 we will be able to drive the uncertainty down to the 25 MeV level per experiment, with an ultimate capability of 15 MeV per experiment. Both experiments now have preliminary results from their Run II samples of W and Z candidates. They have measured the cross sections at the Tevatron's new centre of mass energy of 1.96 TeV (Fig. 8) and used the ratio of the W to the Z to indirectly extract the W width. CDF have also taken a first look at the forward-backward asymmetry in e+e~ production in Run II. 6.1. QCD Aspects of W and Z Production To describe the production of bosons with large p T (> 30 GeV), we can use perturbative QCD; NLO calculations exist. For small p T (< 10 GeV), the fixed order calculation fails, and one needs to resum large logarithms of mw2/pT2. At intermediate p T , the two regions must be matched. This approach works well, but one needs to use data to extract a couple of non-perturbative parameters for the resummation calculation. We can use Z data to predict the W p T in this way. W and Z production with jets is an important background to the top quark, to the Higgs, and to many new physics searches. CDF have measured the W+jet cross section as a function of jet E T and find good agreement with NLO QCD. For 2, 3, 4 or more jets, only leading order calculations exist, and the CDF data agree reasonably well as long as the renormalization scale is chosen appropriately. D 0 has used their W+jet sample to study color coherence. By comparing the pattern of energy flow around the (colorless) W to that around the jet, evidence for the QCD-predicted color interference effects in soft gluon emission is seen in the data. Because leptons can be measured well, and the production process is wellunderstood, Drell-Yan lepton pair production at dilepton masses above the Z pole provides a sensitive test for new physics. Examples are searches for compositeness and for indirect effects of extra dimensions.
7.
The Top Quark
The top quark is unique among the elementary constituents of matter because of its high mass (175 times the proton mass, compared to 5 times the proton mass for the next heaviest quark). In the standard model this means that it alone has a strong (non-perturbative) coupling to the Higgs boson. Is nature giving us a hint
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here? Whether the Higgs or something else turns out to be the origin of the electroweak symmetry breaking, the top quark seems to be uniquely connected to the mechanism of mass generation, and the Tevatron collider is the world's only source of top quarks. The search for the top was pursued directly at e+e~ and pp colliders for many years, with mass limits increasing from m, >23 GeV in 1984 to mt > 131 GeV by 1994. In parallel, the increasingly precise electroweak parameter measurements made at LEP and SLC permitted indirect estimates of the top mass to be made, which by the early 1990's favored the mass range around 150-200 GeV. Direct and indirect measurements converged in 1995 when the top quark was discovered by CDF and D 0 with a mass around 175 GeV. Top quarks are produced in tt pairs at the Tevatron (electoweak single-top or tb production has not yet been observed, and will be discussed later). In the Standard Model, the top decays almost 100% to a W and a b quark. The most accessible signatures are those where one or both W's decays to an electron or muon, i.e. lepton + 4 jets + ETmiss, or 2 leptons + 2 jets + ETmiss. The all-hadronic (6 jets) channel has also been investigated but is challenging because of the large QCD backgrounds. Tagging the jets from the b-quarks, either using soft lepton tags or the silicon vertex detector, helps to pull the top signal out of the W+jets background. Both D 0 and CDF are on the road to "rediscovering" top for the spring 2003 conferences, and both experiments have candidate events. We can look forward to significant improvements in the short to medium term because the Run I dataset was so statistically limited—of order 20 clean events per detector. In Run II, we expect roughly 500 such clean events for every inverse femtobarn recorded. 7.1. Top Mass The top mass is a fundamental parameter of the Standard Model and critical input to electroweak fits. It affects the Standard Model prediction of the W and Higgs mass through radiative corrections. In the lepton + jets channel, there is one unknown in reconstructing the event kinematics (the longitudinal momentum of the neutrino). There are three constraints (the lepton+neutrino and two of the jets should reconstruct to the W mass, and the top mass should be equal to the antitop). This allows a twoconstraint kinematic fit, but there are up to 24 combinatoric ambiguities. The
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latter can be greatly reduced if one, or even better, two jets are b-tagged. In addition to the combinatorics, gluon radiation can add extra jets which worsens the mass resolution significantly. The basic fitting procedure is to evaluate a function /which is an estimator of the top mass (such as the result of a kinematic fit) for each event in the sample. The distribution of/is then compared to what is expected, given a hypothesised mt, for a sample of simulated signal and background events. The value of mt giving the best fit t o / i s then the estimated top mass. In Run I, CDF and D 0 obtained: mt = 176.1 ± 5.1 (stat.) + 5.3 (sys.) GeV (CDF) 173.3 ± 5.6 (stat.) ± 5.5 (sys.) GeV (D0) The largest systematic, for both experiments, is the uncertainty on the jet energy scale (~ 4 GeV). CDF also measured the top mass in the 6-jets sample. Here there is a threeconstraint fit, but the signal-to-background ratio is much worse. The result is mt = 186.0 ± 10.0 (stat.) ± 5.7 (sys.) GeV (CDF) Again, the largest systematic is the uncertainty on the jet energy scale (4.4 GeV). In the dilepton sample, things are harder, because two neutrinos are produced. Only four particles (two leptons, two jets) and two components of ETmss are observed. There remain three constraints, so the system is underconstrained. A dynamical likelihood analysis is therefore performed. For each assumed value of the top mass, the kinematics can be reconstructed. An event weight is then calculated which parametrizes how probable it is that this event originated from a top-antitop of this given mass. The event weight distributions, as a function of mt, are combined for all events, and compared with Monte Carlo samples as before. In Run I, CDF and D 0 obtained: mt = 168.4 ± 12.3 (stat.) ± 3.6 (sys.) GeV (D0) 167.4 ± 10.3 (stat.) ± 4.8 (sys.) GeV (CDF) The largest systematic, for both experiments, is once again the uncertainty on the jet energy scale, but the systematics in this method are reduced compared to the lepton+jets channel.
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The overall Tevatron average top mass is mt = 174.3 ± 5.1 GeV (CDF+D0). In Run II, the data samples will be much larger. The systematic errors will need to be reduced to match, and this will require improved knowledge of the jet energy scale using data. This can come from Z+jet events, from photon+jet events, by using the W —> jj decays within the top events, and by using Z —> bb as a calibration sample. Gluon radiation effects will also need to be constrained using data, or reduced with harder cuts given a sufficient number of top events to afford the loss in statistics. Using double b-tagged events, again with a loss of statistics, we can greatly reduce the combinatorics. New mass extraction techniques are also being developed: D 0 has reported a new, preliminary determination of the top mass using existing Run I lepton+jets data. The new technique makes use of more information per event, giving better discrimination between signal and background than the published 1998 analysis, and improves the statistical error equivalently to a factor 2.4 increase in the number of events. With 2 ftf1 of Run II data we will be able to drive the uncertainty on mt down to about 3 GeV/c2 per experiment, and on mw down to about 25 MeV/c per experiment. Measurements at these sensitivities will present a powerful test of the consistency of the standard model. With an ultimate capability of 15 MeV/c2 for m w and 1-2 GeV/c2 for mt per experiment, the indirect evidence for physics beyond the standard model would be compelling.
7.2. Production Cross Section and Kinematics The top cross section is a test of QCD, and any discrepancy could possibly indicate new physics. In Run I, the measurement was performed in the lepton+jets, dilepton and all-jets final states. The results (5.9 ± 1.4 pb for D0, 6.5 +1.7/-1.4 pb for CDF) are well within the range of QCD predictions (5-7 pb). The top quark transverse momentum is another test of the QCD prediction; in Run I, CDF found good agreement between data and expectations. In Run II we will also test for resonances in the top-antitop invariant mass that would signal the production of new particles decaying into top. The Standard Model predicts that the top and antitop spins should be correlated. This is because their production is predominantly through quark-antiquark annihilation with a spin-1 s-channel gluon. Since the top lifetime (4 x 10~25 s) is less than the timescale of QCD hadronization (1/AQCD ~ few x 10-24 s), the top decays before the spin information is lost. The spin correlation can therefore be reconstructed from the decay products. The motivation to do this is to test the top quark's spin (is it really a spin-Vi object as it should be?) and to check these
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assumptions about production and decay. The optimal spin-quantization basis is off-diagonal and has been derived by Mahlon and Parke [1]. Only like-spin combinations are produced in this reference frame. The analysis uses dilepton events, so in Run I it was very statistically limited; nevertheless, D 0 carried the analysis through on the six available events. The result is consistent with expectations; with 2 ftf1, it should be possible to distinguish between uncorrelated spins and the standard model expected correlation at better than the 2o level. The top is expected to decay predominantly (70%) into a longitudinally polarized W, with a 30% admixture of left-handed W's. The lepton p T distrribution can distinguish the various states. CDF find the Run I data are consistent with the standard model expectation, and the fraction of right-handed W's is consistent with zero. 7.3. Single-Top Production Single top (top+bottom) production tests the electroweak properties of the top quark, and allows the Wtb coupling and the CKM element |Vtb|2 to be extracted.
Figure 9: Single top production. The cross section is about 0.7 pb for the s-channel process in Fig. 9 (a) and 1.7 pb for the t-channel process (b). The signature consists of a W (lepton and missing ET) together with two b-jets, one coming from top decay. In the s-channel process both b-jets are high-pT and central, while in the t-channel case the jet from top decay is still hard and central but the other is much softer, there also being a high-pT light quark jet (shown as a d-quark in Fig. 9(b)). It is desirable to separate the two processes based on these kinematic differences since they have different systematic errors and different sensitivities to new physics. The backgrounds for both are significantly higher than they are for top-antitop production (the W + 2 jet cross section is much higher than the W + 4 jet cross section).
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In Run I, both experiments could only set limits (at several times the standard model level) on single top production. With 2 ftf1 in Run II, we expect to see a clear signal. We will use it to measure the cross section to ~ 20%, to indirectly extract the width for t -> Wb to ~ 25%, and thus |Vtb| to ~ 12%. It is important to measure single top production since it is a background to many new physics searches; it is also sensitive to new dynamics like topcolor. 8.
The Higgs Boson
In the standard model, the weak force is weak because the W and Z bosons interact with a scalar field (called the Higgs field) that permeates the universe with a finite vacuum expectation value of 246 GeV. If this same field couples to the fundamental fermions, it can explain their masses too. It should be possible to excite this field and observe its quanta—the long sought Higgs boson. It is the last piece of the standard model, and also a key to understanding any beyondthe-standard-model physics like supersymmetry. Finding it is a very high priority, but will require large datasets because the production cross section is low and the irreducible backgrounds are large. All of the properties of the Higgs are fixed in the standard model with the exception of its own mass: its couplings and decays are all determined. There is no doubt that electroweak symmetry breaking occurs, so we know that something is coupling to the W and Z. We should remember, however, that the Higgs field need not result from a single, elementary scalar boson: there could be more than one particle (as occurs in SUSY), or composite particles could play this role (as in technicolor or topcolor). Whatever the field is, precision electroweak measurements tell us that it looks very much like a Standard Model Higgs (though its fermion couplings are less constrained). We also know that the WW cross section would violate unitarity at Vs ~ 1 TeV without the existence of a Higgs; this is a real experiment accessible to the LHC, so there is no doubt that whatever plays the role of the Higgs will eventually be seen. Over the last decade, the focus was on experiments at LEP. Direct searches excluded masses below 114 GeV, while indirect electroweak measurements combined with Tevatron top mass data in global fits to exclude Higgs masses above about 200 GeV. In mid 2000, there was excitement that hints of a signal for a Higgs with a mass of 115 GeV, right at the limit of sensitivity, had been seen at LEP. In the event, CERN management decided to shut off LEP operations in order to expedite construction of the LHC, and the resolution of the puzzle is now left to the Tevatron.
325
A relatively light Higgs, with a mass in the range between the current lower bound of 114 GeV and about 140 GeV, is produced with a cross section of about 1 pb at the Tevatron and decays mainly to bb quark pairs. This gives a signature of two b-jets, which is almost impossible to extract from the huge background of two b-jets from QCD processes. Instead, we will search for the production of a Higgs together with a W or Z boson. The cross section is a factor of 5 lower, but the W or Z decays a significant fraction of the time into an electron or a muon, and a high energy electron or muon is relatively easy to trigger on and isolate. We then have the simpler, but still very challenging, task of separating the signal of a vector boson plus a Higgs, from the backgrounds: vector bosons plus two bjets, vector bosons plus a Z, with Z —» bb, top pairs and single top. The bb mass resolution directly influences the signal significance; Z —» bb will serve as a calibration for H —> bb. As an example, consider mH = 115 GeV (interesting because of the LEP "hint"). In 15 fb"1, we expect[2] 92 WH -» lv bb events (with a background of 450); 90 ZH —> vv bb events (with a background of 880); and 10 ZH —» 11 bb events (with a background of 44). These event rates would allow a 95% exclusion (if no signal is seen) with 2fb_1 per experiment, and three and five standard deviation signals to be established with 5 and 15fb_1 respectively. If we do see something, we will want to test whether it is really a Standard Model Higgs by measuring the production cross section (which is predicted for any given ma), and searching for other decay modes. The modes H —> xx and H —> WW are expected to have branching ratios around 8-9% for mH = 115 GeV and might just be visible. H —> YY should not be visible for a SM Higgs at the Tevatron. It will also be interesting to look for the process where the Higgs is produced in association with a top-antitop pair. The cross section is very low (a few fb) but the signal to background ratio is good, and one might see a signal at the few event level. This process tests the top quark Yukawa coupling. At higher Higgs masses, above about 140 GeV, the dominant Higgs decay mode becomes W pairs. Here, the challenge is not to reconstruect b-jets, but to separate the H —> WW signal from the other sources of W pairs. Angular cuts can be used to extract the Higgs signal from the WW background (the Higgs is spin-0 while a virtual Z is spin-l). With tight cuts one can obtain a signal to background of order 1 to 3. All of these Higgs strategies and estimated sensitivities are taken from the Higgs and Supersymmetry workshop held at Fermilab in 1999 [2]. Right now, the
326
experiments are developing the foundations needed to do this analysis for real in Run II: good jet resolution, high b-identification and trigger efficiencies, and a good understanding of all the backgrounds. These will also enable us to firm up these earlier estimates of Higgs sensitivity as a function of luminosity. In the most popular extension to the Standard Model, supersymmetry (described below), there is still a Higgs boson: in fact the existence of a light Higgs is a very basic prediction of SUSY. These models contain an extended suite of Higgs particles: two neutral scalars h and H, a pseudoscalar A and a charged pair H*. At tree level, the Higgs sector is decsribed by two parameters (usually taken to be mA and tan(3); on top of this, there are radiative corrections that depend on sparticle masses and the top mass. LEP has excluded mh < 91 GeV, mA < 92 GeV and mH± < 79 GeV, and tan|3 < 2.4. Over much of the remaining allowed parameter space, the h is "light" (115-130 GeV) and the H, A and H* are much more massive. In this "decoupling limit", the h looks very much like the standard model Higgs and decays similarly, while the heavy Higgses tend to have enhanced decays to b's and taus. Tevatron searches for the standard Higgs therefore apply to its supersymmetric cousin h as well; if we exclude the existence of such a Higgs, we will have gone a long way towards excluding minimal supersymmetry at the electroweak scale. One area of Higgs physics that can be attacked with relatively modest luminosities (already feasible in 2003) is to search for the other supersymmetric Higgs bosons. Associated production of a neutral SUSY Higgs (h/H/A) together with a b b quark pair is enhanced for high but plausible values of tanp\ and tighter limits than those from LEP will already be reachable with a few hundred inverse picobarns. The Tevatron can also search for the production of charged Higgses in top decays: the decay to H ^ competes with the standard model Wb decay, and would result in an enhanced production of tau leptons in top decays at large tan(3. As long as we have adequate sensitivity, exclusion of a Higgs would still be a very important discovery for the Tevatron. In the Standard Model, we can only exclude the lower part of the mass range (up to 175 GeV with -10 fb"1), but this is the region favored by electroweak fits. In minimal SUSY, we can potentially exclude all of the allowed parameter space with ~5 fb"1. There is also the possibility of discovering something other than a Higgs. In dynamical models like technicolor and topcolor, the role of the Higgs is played by a composite particle. Such models predict many other new particles in the mass range 1001000 GeV, with strong couplings, large cross sections, and clear signatures at the Tevatron.
327
It is useful to remember what the Higgs search will and will not tell us. It will tell us what is the source of mass of the W and Z, and therefore why the weak force is weak. It will tell us the source of mass of the fundamental fermions. It will tell us whether there are fundamental scalars, and give a strong indication whether there is weak-scale SUSY (since a light Higgs is a basic feature of such models). It could tell us that there is other new physics at the weak scale, should we discover something like technicolor, or should we find a Higgs, but one whose mass, together with the top and W masses, is not consistent with precision electroweak fits. It will tell us the mass scale of new physics and point towards the machine we want to build after the linear collider. The search will not reveal why fermion masses have the values they do, nor what is the origin of flavor (what distinguishes a top quark from an up quark). It will not reveal what is the origin of mass in the universe, since the baryon masses are almost all due to QCD, while that of dark matter, if it turns out to be the lightest supersymmetric neutralino, is due to some unknown SUSY breaking mechanism. We don't even know what two thirds of the universes's mass—dark energy—is made of.
9.
Searches for New Physics
As the world's highest energy collider, the Tevatron is the most likely place to directly discover a new particle or force. We know the standard model is incomplete; theoretically the most popular extension is to make it a part of a larger picture called supersymmetry (which is a basic prediction of superstring models). Here each known particle has a so-far unobserved and more-massive partner, to which it is related through a change of spin. If it exists, the lightest supersymmetric particle would be stable, and vast numbers of them would pervade the universe, explaining the astronomers' observations of dark matter. LEP has excluded sparticle masses below the 80-100 GeV range (and lightest neutralinos below 36 GeV); the Tevatron is now the only place to directly search for supersymmetry. There are a number of search strategies. The most copiously produced SUSY particles at a hadron collider would be the colored squarks and gluinos. These have cascade decays giving final states with missing energy ^miss-j p j u s j e t s ^ an( j 0ften iepton(s) from charginos and neutralinos in the cascade). In Run I, squark and gluino masses below about 200 GeV were exluded (Fig. 10); with 2 fb"1, this mass reach can be roughly doubled.
328
Ms > 300 GeV/c2 Mr, a Mg
Figure 10: Squark and gluino mass limits from Run I (CDF). Another attractive search mode is for charginos and neutralinos through multilepton final states. Associated production of a chargino and a neutralino can give a trilepton signature for which the standard model background is very small. In Run I, this search was not competitive with LEP, but in Run II it will become increasingly important as squark and gluino production reaches its kinematic limits. The chargino mass reach is 150-180 GeV. We will also search for SUSY signatures with heavy flavor. Quite often the lightest squarks are the sbottom and stop, which can decay to b or c-jets. Gauge mediated SUSY models result in signatures with ETmiss + photon(s). Searches for other new phenomena include leptoquarks, dijet resonances, new heavy W and Z' bosons, massive stable particles, and monopoles. The Tevatron allows us to experimentally test the new and exciting idea that gravity may propagate in more than four dimensions of spacetime. If there are extra dimensions that are open to gravity, but not to the other particles and forces of the standard model, then we could not perceive them in our everyday lives. But particle physics experiments at the TeV scale could see signatures such as a quark or gluon jet recoiling against a graviton, or indirect indications like an increase in high energy electron-pair production. These studies use the Tevatron to literally measure the shape and structure of space-time.
329
While it is good to be guided by theory, one should also remain open to the unexpected. Therefore both experiments have .developed quasi-modelindependent (signature-based) searches, which look for significant deviations from the Standard Model. In the Run I dateset, no significant evidence for new physics was found. Perhaps revealing different psychologies, D 0 has quantified its agreement with the Standard Model at the 89% confidence level, while CDF has preferred to highlight some potential anomalies as worth pursuing early in Run II.
Figure lis Run II search for virtual effects of extra dimensions in the pair production of photons and electrons (D0). The experiments have already embarked on a number of searches using Run II data. Work has started on understanding the Bfm$s distribution in multijet events as a prelude to squark and gluino searches; trilepton candidates are also being accumulated. At D0, a gauge-mediated SUSY search has set a limit on the cross section for pp —» Ejmss + YY- Also at D0, virtual effects of extra dimensions are being sought in e+e", p+f4.~ and YY fina^ states, and limits on the scale of new dimensions at the 0.9 TeV level can already be set. A search for leptoquarks decaying to electron+jet has been carried out. None of the cross sections or mass limits is better, yet, than published Run I results, but serves as a demonstration that the pieces are all in place.
330
10. Physics Prospects for Run II At the same time as Fermilab's accelerator experts have been working to improve Tevatron operations, they have been trying to incorporate the lessons learned into a solid plan for the future. The planning for the accelerator complex is in two phases. The first focuses on US Fiscal Year 2003, which ends in September 2003. A full plan and schedule are in place. By the summer of 2003, each experiment should have recorded around 200 pb_1 of Run II data (almost twice the Run I dataset). The centrepiece will be a greatly increased top quark sample, thanks to the higher beam energy and the much improved b-tagging capabilities of the detectors. A first look at B s mixing will be possible, together with lifetimes and branching ratio measurements from the B, B s , Ab and charm samples. Jet distributions at the highest energies will constrain proton structure, and searches will follow up on Run I anomalies and extend the Run I reach for many extensions to the standard model. The second phase covers 2004 and beyond. It is now clear that it will take somewhat longer than had been anticipated to accumulate the large datasets ultimately foreseen for Run II: such is the price of realism. As long as the Tevatron remains the world's highest energy collider, it is a unique facility that must be exploited to the fullest; this will remain true until the LHC experiments start producing competitive physics results. If the LHC delivers first beam to ATLAS and CMS in 2008, we might expect a year to be spent commissioning the detectors and accelerator, with the first physics data by 2009. One year of concurrent running (2009-10) between LHC and Tevatron would not be unreasonable, since the low-mass Higgs searches at the two machines are complementary in several ways. With this in mind, we are preparing to run the Tevatron until the end of the decade in order to fully realize its physics potential. The Run II physics program is a broad and deep one and will answer crucial questions about the universe. As Fig. 12 shows, there is no threshold at which this starts. There is compelling physics to be done each year and with each doubling of luminosity, starting now with a few hundred inverse picobarns and to the end of the decade with multi femtobarn data sets. To explore the 5 ftf1 to 15 ftf1 domain calls for upgrades to the CDF and D 0 detectors. Primarily, these involve new trigger systems to handle more than ten interactions per crossing at the expected luminosity, and new silicon detectors that make use of LHC R&D to sustain the high radiation doses. These upgrades were successfully reviewed by the Department of Energy in September and are now moving towards approval with installation planned for 2005-6.
331
Run I I Physics Program iiiiiii
* 5
* 30 Higgs signal « m H - 115-125, 155-170 GeV * Exclude Higgs over whole range of 115-180 GeV * Possible discovery of supersymmetry in a larger fraction of parameter space
* • • *
3?j Higgs signal @ m^ = 115 GeV Exclude SM Higgs 115-130, 155-170 GeV Exclude much of SUSY Higgs parameter space Possible discovery of supersymmetry in a significant fraction of minimal SUSY parameter space (the source of cosmic dark matter?)
* * * * *
Measure top mass ± 3 GeV and W mass ± 25 MeV Directly exclude m^ =* 115 GeV Significant SUSY and SUSY Higgs searches Probe extra dimensions at the 2 TeV (10- 19 m) sea If B physics: constrain the CKM matrix
* * * * *
Improved top mass measurement High p j jets constrain proton structure Start to explore B§ mixing and B physics SUSY Higgs search <§ large tan p Searches beyond Run I sensitivity
Figure 12; Summary of Run II physics reach for various integrated luminosities.
332
Run II is a marathon and not a sprint. The combination of high accelerator energy, excellent detectors, enthusiastic collaborations and data samples that are doubling every year guarantees interesting new physics results at each step. Each step answers important questions. Each leads on to the next. This is how we will lay the foundations for a successful LHC physics program—and hopefully a linear collider to follow. References 1. G. Mahlon and S. Parke, Phys. Lett. B 411, 173 (1997). 2. M. Carena et ah, Report of the Higgs Working Group of the Tevatron Run II SUSY/Higgs Workshop, hep-ph/0010338.
333
Appendix: Student Exercise for TASI 2002
A Search for New Physics using D 0 Run 1 Data
John Womersley, Fermilab TASI Summer School, June 2002 Goal of this exercise: explore the limits that can be placed on the production of technicolor particles using the data already taken at the Tevatron in 1992-95. Extrapolate to Run 2. We will use the public interface to D 0 Run 1 data, which is called Quaero. 1. First, look at the PRL* article describing Quaero: http://www-dO.fnal.gov/www buffer/pub/pub 228.pdf or http://www-dO.fnal.gov/www buffer/pub/pub 228.ps You'll see (in Table III of the paper) a number of channels where limits were set, including pp —> WH —» evjj. The paper did not, however, set limits on the technicolor process with a similar final state, pp —» W% —> evjj. It is suggested that this process may have a much higher cross section than the Higgs process, so it's interesting to see what can be done with the relatively low luminosity accumulated in Run 1. This process, and a suggested search strategy, are described in: http://arxiv.org/abs/hep-ph/9704455 2. Go to the Quaero website: http://quaero.fnal.gov Look at the examples to see what one can do with this interface. You can ask Quaero to generate the signal events itself, using Pythia, or you can feed it an input file. In this case we'll ask it to use Pythia because Technicolor is implemented in Pythia. The Pythia web page is http://www.thep.lu.se/~torbjorn/Pvthia.html but a more useful short guide from LAPP can be found at http://wwwlapp.in2p3.fr/Pythia/. Page 28 and 29 of the short guide (PDF version) describe the technicolor implementation. 3. Now let's set up our search.
334
Under "signal:" • Select the final state corresponding to evjj (e met 2j (nj)) • Select "smear" (we want the detector resolutions to be modelled) • Select "Pythia input" since we want Quaero to generate the technicolor events itself. • Enter the Pythia commands to generate W% events: The easiest way is MSEL=50 which turns on all the technicolor processes. • Click on "search" (that's what we want to do) • Check the boxes corresponding to the backgrounds to be considered Now under "variables:" • If you like, enter a constraint (e.g. you could constrain the e and the missing E T to be a W). This is optional. • Enter up to 3 variables in which Quaero will search for signal events: the hep-ph paper suggests the p T of the W, the p T of the dijet system, A^Qet], jet2), and the mass of the dijet systemUnder "Requestor," enter your name and e-mail, and a short note describing what you are doing (please mention TASI). 4. Now click "submit"! Stay logged in - if there are errors in the submission, Quaero will bounce an email right back to you. 5. Check your e-mail tomorrow for the results. See the examples on the following pages for typical output. 6. What limit did you get? How do this limit, and the Pythia cross section reported by Quaero, compare with the expected theoretical cross section from the hep-ph paper? Remember that Quaero quotes limits on (cross section x branching ratio), so you'll have to multiply by 9 to remove B(W —> ev). How much more luminosity would it take to observe a signal in Run 2? 7. If you have time, try optimizing the constraints or variables to see if you can get a better limit on the same process. Try constraining the e and Missing E T to a W mass, for example, if you didn't already. Try other variables which might be better than mine.
335
Here is an example Quaero submission: httpv' /^na«m. ttwlgtw / quAvm;
Q».*rev
Quaero
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** £!gafiL£B£i
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336
Example of the e-mail response: i!hrip:A^w«im^toy®tiiM|^^i2J'^
Date; Morv 20 May 2002 1557:54 -ffiPQ To: wcirassteieastosfcgafc' Dearitfiri Vtonarsfciy: Therea]i!ofycuet^£Kraatm^&aKa*iad.. R«asefn£hidfi*eaoai«5{BfffWgre(«patioeti «vpitecaw!aeftatus«s.^nire5*it, Regard^ The DZeta aastxYattan
V.-J«jasKw {\sl at af}., \#241 Man Kay 33 203Z
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MaddOeserts&ori Tsttt fcr sttfimar street e«SMise
P>ora oo»* sectftn xfcrarKJiHgfatio = &3S fife, ujapw ***& afi trie trass, sadg-wt m ttfe process at cmSOensEtewfeof 59%, 93%, and 95%. ar« fcund to &e 8 5 fife I $ pb, and i.a jp^, jespsrttwsy. Madras* sensiStfty (fl,635 f* *-1) fs =K*)feed ai a aegSMi of vatfatfe SJB<E v * * 2.9 sfigpas «**]ES a p « X a ^ 23.4 *- 5,4 SBdapsatid events esfift^cKKi, se\d 14 (wtrts abs«8"w>dfatfe d£&< Piots Plots af$K> va^aHe* Sis*: you used aneavaiMIe f » ^gwtiq ^ t i g ^ j / ^ a r g j ^ a f l ^ ^ u ^ ^ / f ^ ^ i t ^ ^ f e i S i f ; ^ ! p& "TO red «jtvefs»eaKpecj«i »iactapa*«dj efts grseri curve rsym* sscjcaiflxiKpled by a feoaraf 50; ate ESack daisare D&dasa," Pieass &»p tie Mtfti&tg caveats £i mtid vtfietftHKtirasintjtrt"s result: '* Your Sgrtai tiasfeem&m tlraagh a fast eisteaaF sfiTV-iatof. '• Your Sgnai ha* tjeen generated wiSi PyU«a. Any inadequacytoP>»is*ii nxtfattrig cT trie <jg«3l praeefci w^ prepagasa tfto a oifrasgorelsig &i adequacy fet •* a amrrpmQ mim$e ariaysesti*tfte same ssgnat, you shouH qjote asthe inai aaswsf tte resufeeorresptadMg to-tfte greatest sensiSyty.. *No&&^h2£be^in^e&a!*^tesEa^^
337
Example of the plots that are produced:
I
I 'lint)
|a>
l| ' ' | I ' ' I |"|'Enfcfai ' | '
Uif
. j - l . , , | ,.,|,..! I l h.
0
0
1 2 deltaphi(j'1,j2)
3
100 200 300 (e+met)_pt
100 2 0 0 3 0 0 4 0 0 mass(j1 ,j2)
The plots show the three variables I specified in my search. The black points are the data, the red curve is the background, and the green is the signal expected multiplied by 50. You can see that the deltaphi and mass variables give quite good distinction between signal and backgound, but the px variable isn't so great. Try to do better!
Quaero FAQ: 1. Yes, this is real collider data. 2. Yes, if you find a signal you can publish it.
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PART II: TeV-Scale Physics Lecture Series
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JOHN TERNING
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NON-PERTURBATIVE
SUPERSYMMETRY*
JOHN TERNING Theory Division T-8, Los Alamos National Laboratory Los Alamos, NM 87545 E-mail: [email protected]
These lectures contain a pedagogical review of non-perturbative results from holomorphy and Seiberg duality with applications to dynamical SUSY breaking. Background material on anomalies, instantons, unitarity bounds from superconformal symmetry, and gauge mediation are also included.
Contents
1
Holomorphy 1.1 Non-Renormalization Theorems 1.2 Wavefunction Renormalization 1.3 Integrating Out 1.4 The Holomorphic Gauge Coupling
345 345 347 347 349
2
R e v i e w of Anomalies and Instantons 2.1 Anomalies in the Path Integral 2.2 Gauge Anomalies 2.3 Review of Instantons 2.4 Instantons in Broken Gauge Theories
352 353 356 358 359
3
Gaugino Condensation
361
4
The 4.1 4.2 4.3 4.4 4.5 4.6
363 363 366 368 371 373 374
Affleck-Dine-Seiberg Super-potential Symmetry and Holomorphy Consistency of W A D S : Moduli Space Consistency of W A D S : Mass Perturbations Generating W A D S from Instantons Generating W A D S from Gaugino Condensation Vacuum Structure
'Printed with permission from the author. The copyright of these lectures is retained by the author. 343
344
5
't Hooft's A n o m a l y Matching
374
6
Duality for S U S Y Q C D . 6.1 The Classical Moduli Space for F > N 6.2 The Quantum Moduli Space for F > N 6.3 Infrared Fixed Points 6.4 An Aside on Superconformal Symmetry 6.5 Duality 6.6 Integrating out a flavor 6.7 Consistency
375 376 379 380 383 388 392 393
7
Confinement in S U S Y Q C D 7.1 F = N: Confinement with Chiral Symmetry Breaking 7.2 F = N: Consistency Checks
394 394 397
8
S-Confinement in S U S Y Q C D 400 8.1 F = N + 1: Confinement without Chiral Symmetry Breaking . . . 400 8.2 Connection to theories with F > N + 1 403
9
Duality for SO(N) 9.1 The SO{N) Theories and Their Classical Moduli Spaces 9.2 The Dynamical Superpotential for F < N - 2 9.3 Duality 9.4 Some Special Cases
407 407 409 409 410
10 Sp{2N) and Chiral Theories 10.1 Duality for Sp(2N) 10.2 Why Chiral Gauge Theories are Interesting
412 412 414
11 S-Confinement
414
12 Deconfinement
417
13 Gauge Mediation 13.1 Messengers of SUSY breaking 13.2 RG Calculation of Soft Masses 13.3 Gauge Mediation and the /j, Problem
419 419 421 423
14 Dynamical S U S Y Breaking 14.1 A Rule of Thumb for SUSY Breaking 14.2 The 3-2 Model 14.3 The SU(5) Model 14.4 SUSY Breaking and Deformed Moduli Spaces 14.5 SUSY Breaking from Baryon Runaways 14.6 Direct Gauge Mediation 14.7 Single Sector Models
425 425 425 428 430 432 435 437
345
1. Holomorphy Two of the early exciting observations about J\f = 1 SUSY [1] gauge theories were the absence of quadratic loop corrections to scalar masses and the absence of renormalization for many superpotentials [2-5]. While the former property was an immediately obvious consequence of Bose-Fermi symmetry, the latter was only much later realized to follow in a very simple way from the holomorphy [6,7] of the superpotential, that is from the fact that superpotentials are functions of chiral superfields but not of the complex conjugates of these fields. Holomorphy arguments laid the groundwork for the later development of Seiberg duality [8,9]. 1.1. Non-Renormalization
Theorems
Couplings in the superpotential can always be regarded as background fields, and so superpotentials are also holomorphic functions of coupling constants. The fact that superpotentials are holomorphic can be used to prove powerful non-renormalization theorems [6]. If we integrate out physics above a scale [i (i.e. calculate the Wilsonian effective action 1 ) then the effective superpotential must also be a holomorphic function of the couplings. Consider a theory renormalized at some scale A with a superpotential: Wtree = ^
+^ .
(l.l)
Here 0 is a chiral superfield. I will also refer to the scalar component as
(1-2)
so we have R[tp] = R{(j>] — 1, R[8] = 1. Since the Lagrangian in our toy model has Yukawa couplings, £ D ±4>U , 1
(1.3)
As opposed to the 1PI effective action, see ref. [10,11] for a related discussion. For k = 2 there is an SU(2)R R symmetry and for M = 4 there is an S(7(4) H ~ SO(6)R R symmetry. 3 The existence of a discrete symmetry often referred to a -R-parity is completely independent of the continuous R symmetry discussed here [12]. 2
346
which must have zero R charge, we have 3R[<j>] - 2 = 0 ,
(1.4)
and therefore R[W] = 2. More generally we could get the same result by noting: d29W .
/
(1.5)
By convention superfields are labeled by the R charge of the lowest component, which in our example of a chiral superfield is the scalar component. By convention the gaugino assigned R charge 1. In our toy model we can make the following charge assignments: U(l)
U(1)R
m -2 A -3
(L6)
0 -1
where we are treating the mass and coupling as background spurion fields in order to keep track of all the symmetry information. Note that nonzero values for m and A explicitly break both £/(l) symmetries, but these symmetries still lead to selection rules. If we consider the effective superpotential generated by integrating out modes from A down to some scale fi, then the symmetries and holomorphy of the effective superpotential restrict it to be of the form Weff = mcf)2 h, .
/ ^
TO
^a^TO1"".^2 ,
(1.7)
since m 0, and the massless limitTO—> 0 restricts n < 1 so
Weff = jtf
+ ^
= Wtree .
Thus we have shown that the superpotential is not renormalized.
(l.g
347
1.2. Wavefunction
Renormalization
Next consider the kinetic terms: Acin. = Zd^*d^ + iZ^d^
,
(1.9)
where the wavefunction renormalization factor is a non-holomorphic function Z = Z(m, A, m\ \ \ /x, A) .
(1.10)
If we integrate out modes down to /J > ra we have (at one-loop order) Z = l + c\tf\n.(^\ ,
(1.11)
where c is a constant determined by the perturbative calculation. If we integrate out modes down to scales below m we have Z = l + +cAA t ln( -^—r ) (1.12) \mm< ) So there is wavefunction renormalization, and the couplings of canonically normalized fields run. In our example the running mass and running coupling are given by m A Z'Z?' 1.3. Integrating
(1.13)
Out
Consider a model with two different chiral superfields: W = \M
+ -CPH^2
(1-14)
This model has three global (7(1) symmetries: U(1)A U(1)B 0 4>H 1 1 0 -2 0 M -2 A -1
U(1)R
1 l 2
(1.15)
0
where (7(1)A and (7(1)B are spurious symmetries for M, A / 0. If we want to integrate out modes down to \x < M, we must integrate out 4>H- An arbitrary term in the effective superpotential has the form 4PMk\p .
(1.16)
348
To preserve the symmetries we must have j = 4, p = 2, and k = — 1. By comparing with tree-level perturbation theory we can determine the coefficient:
*.-££.
(1-17)
We could also derive this exact result using the algebraic equation of motion ^
= Mfe + ^
2
= 0.
(1.18)
We can simply solve this equation for <J>H and plug the result back into the superpotential (1.14), which yields the perturbative effective superpotential (1.17). Another interesting example is w
=
\MH4>2 + 1^>3H •
. (1.19)
The <J>H equation of motion yields
Note that as y —> 0, the two vacua approach (f>H = —A0/(2M) (as in the previous example) and H = oo. Integrating out CJ>H yields ^
f f
M^ 3Xy£ f Xyf\ \y2 - ^ 2 ( l - ^ r ±2 ( l - ^ - 2) \ / l - i ^ 2 |
3y 1
2M
V
M J V
M
•
(1.21)
The singularities in Weff indicate points in the parameter space and the space of # becomes massless and we shouldn't have integrated it out. The mass of <\>H can be found by calculating d2W -^r=M
+ y^H,
(1.22)
and substituting in the solution of the equation of motion: d2W b2H
A#2 M l ^ \ -^W' '" V * M 2
(1-23)
We could also apply the holomorphy analysis to this problem. First, we can maintain the symmetries of the previous example by assigning charges (-3,0,-1) under U(1)A x U(1)B x U{1)R to the coupling y. Then requiring
349
that the effective superpotential has no dependence on u and maintains the three symmetries we find that it must have the form
for some function / , just as we found from integrating out #. 1.4. The Holomorphic
Gauge 4
Coupling M
Using the superspace notation y = x^ — ido^B (where 9 is an anticommuting Grassmannian variable) we can write a chiral supermultiplet consisting of a scalar (f>, a fermion ijj and an auxiliary field F as a chiral superfield $ = cf,(y) + V26 i/>(y) + 92F(y) .
(1.25)
We can also use this notation to represent an SU(N) gauge super-multiplet as a chiral superfield W% = -iK(v)
+ eaDa{y) - (a^0)aF^(y)
- (06)
,(1.26)
where the index a labels an element of the adjoint representation, running from 1 to N2 — 1, Aa is the gaugino field, F " is the usual gauge field strength,and Da is the auxiliary field. We have used the notation that the a% are the usual Pauli matrices and *" = (/,*') afM = (I,-al)
(1-27) (1.28)
a"" = - ( c r ' V - o - ^ ) .
(1.29)
The field strength can be written as F% = d^Al - dvA% + gthcAlAcv
,
(1.30)
where fabc is the structure constant of the gauge group determined through the adjoint generators Ta by [Ta,Tb]=ifabc
.
Finally, using the notation
*For an excellent review see ref. [12].
6»YM
4?n
2TT
gz
(1.31)
350
we can write the SUSY Yang-Mills action as a superpotential term
— [d*xId'28TWX
16iri >7T« J
(1.33)
+ h.c.
J
= / (fx /
^ Y M j?ap,v
~4g2
327T2
""
+ -Xa^D^Xa g2
1
+
pa
""
-^DaDa 2
2g
where 1 Aivu0
pa
pa ap •
(1.34)
Note that the gauge coupling g only appears only in r which is a holomorphic parameter, but the gauge fields are not canonically normalized. To go to a canonically normalized basis 5 we would rescale the fields by (A«,\aa,Da)^g(Al,\aa,Da)
(1.35)
Recall that the one-loop running of the gauge coupling g is given by the renormalization group equation: dg where for an SU(N) gauge theory with F flavors and N = 1 supersymmetry, b = 3N - F .
(1.37)
The solution for the running coupling is 1
•Xf
2
9M
(1.38)
where |A| is the intrinsic scale of the non-Abelian gauge theory that enters through dimensional transmutation. We can then write the one-loop running version of our holomorphic parameter r as 4iri Ti-loop — —
2TT
h
_1_ In 2ni
(1.39)
A* J
(1.40)
Due to subtleties with the measure in the path integral there is a non-trivial relation between the running of the holomorphic gauge coupling and the running of the canonical gauge coupling [3,4,13,14].
351
We can then define a holomorphic intrinsic scale A = \k\ei9™'b = fie27riT/b
(1.41) ,
(1.42)
or equivalently r1_loop = ^ l n ( ^ ) .
(1.43)
In order to take account of non-perturbative effects we need to understand the term in the action proportional to #YM- This FF term violates a discrete symmetry: CP (the product of charge conjugation and parity). The CP violating term can be rewritten as Fa^F^
= 4e^ C T a M Tr [AudpAa
+ -A„ApAa
) .
(1.44)
Thus the CP violating term is a total derivative and can have no effect in perturbation theory since it integrates to terms at the boundary of spacetime. Nevertheless it is well known that it can have non-perturbative effects. To see this consider the following semi-classical instanton configuration of the gauge field:
where ft" describes how the instanton is oriented in the gauge space and spacetime. Equation (1.45) represents an instanton configuration of size p centered about the point XQ. Such instantons have a non-trivial, topological winding number, n, which takes integer values. The CP violating term measures the winding number:
Jd4xF"""f£,
^
=n9YU.
(1.46)
eiS ,
(1.47)
Since the path integral has the form /
VAaVXaVDa
and the action S depends on #YM only through a term which is an integer times #YM it follows that #YM -> 0YM + 2TT ,
(1.48)
is a symmetry of the theory since it has no effect on the path integral.
352
The Euclidean action of a instanton configuration can be bounded, since 0 < I dAxTr {F^ ± F^Y
. 2
= fd4xTr
(2F2 ± 2 F F ) ,
(1.49)
so we have f d4xTrF2
> I Jd4xTrFF\
= 32ir2\n\ .
(1.50)
Thus one instanton effects are suppressed by e -&ot = e P & + t f ™ =
^ Y
(1.51)
If we integrate down to the scale JJ we have the effective superpotential WeS = TJ^WZWS
.
(1.52)
Since the physics must be periodic in #YM, A _> e27ri/bA
,
(1.53)
is a symmetry. To allow for non-perturbative corrections we can write the most general form of r as:
T(A;M) = l n
i (j) + / ( A ; A t ) '
(L54)
where / is a holomorphic function of A. Since A —> 0 corresponds to weak coupling where we must recover the perturbative result (1.43), / must have Taylor series representation in positive powers of A. Since plugging A -> e27ri/feA
(1.55)
into the perturbative term already shifts #YM by 2TT, f must be invariant under this transformation, so the Taylor series must be in positive powers of A6. Thus in general we can write: h
/ A\
.°°.
T(As )
/ A\
" -2s ta u) + S""W
bn
(L56)
So the holomorphic gauge coupling only receives one-loop corrections and non-perturbative n-instanton corrections, or in other words in perturbation theory there is no additional running beyond one-loop. 2. Review of Anomalies and Instantons The reader should feel free to skip over this section if they already have a basic familiarity with anomalies and instantons.
353
2.1. Anomalies
in the Path
Integral
Consider some fermions V coupled to a gauge field Aa: Sferndon = / dtxitfa"^
+ iA»T0)^
(2.1)
Under a position dependent chiral rotation i> - • eia{x)ip
(2.2)
Sfermion - • SWmion ~ / d^Xa^O^i^a**^)
(2.3)
Thus, since a = constant is a symmetry of the action, classically we find
c^W)
= dfifA =
o
(2.4)
Quantum mechanically this is not true: the global current has an anomaly. One way to see this is to calculate the diagram with a fermion triangle and the global current and and two gauge currents at the three vertices, as in Figure 1.
Figure 1. The fermion triangle which contributes to the anomaly. One also adds the crossed graph where the gauge bosons are interchanged.
One finds directly from this calculation that the global current is not conserved: d^fA oc Fa^F^
.
(2.5)
Another way to see this is from Fujikawa's path integral derivation [15] of the anomaly. First define p^i^B^d^ + iA^)
(2-6)
0 = i o ^ - iAJ
(2.7)
354
where a% are the usual Pauli matrices and a% = {U,<Ti),
(2.8)
(2.9)
^ =- ( ^ / - 4 a / ) .
(2.10)
f d4xtp'!ptjj= Jdtxipjp^
(2.11)
Then
and we can define orthonormal eigenfunctions fn and gn D2fn=flPfn 2
D gn = 00gn
= -\2nfn,
(2.12)
2
(2.13)
= -X ngn ,
V>fn = K9n ,
(2.14)
P9n = -Kfn ,
(2.15)
with the usual completeness properties
Y.ti{*)Uy)=5{x-y),
(2.16)
n
Y/9*n(x)gn(y)
= 5(x-y),
(2.17)
Tr Jd4xf:(x)fm(x)
= Snm ,
(2.18)
Tr / dixg*n{x)gm(x)
= 5nm .
(2.19)
n
In general D2 and £> have different numbers of zero eigenvalues. We can expand the fermion fields in this basis: ^(x)=J^affn(x) ,
(2.20)
n
^{x)=^bn9n(x)
.
(2.21)
n
The partition function in the background gauge field A^ is given by a path integral over the fermion fields which in this basis reduces to ipV^ e
S
=
Rnmdandbme
s
.
(2.22)
Under a chiral rotation (Xn &n
> (Xn = {^nmQ"m ?
(2.23)
>
(2.24)
On — Onm^m ?
355
where the transformation matrices are defined by Cnm = TIJd4xeia^f*(x)fm(x)
,
Cnm = Tvjdixe-ia^g*m(x)gn(x)
(2.25) .
(2.26)
So the path integral measure changes by —> ( d e t C d e t C ) - 1 ] ! ^ ^ ^ ™ , .
Unmdandbm
(2.27)
where (detCCy1
= exp (-i
[ d4xa{x)A(x)\
A(x) =TrJ2(fn(x)fn(x)
- g*n(x)gn(x))
,
(2.28)
.
(2.29)
n
Since the partition function, Z, is independent of the chiral rotation parameter a, if we take a functional derivative with respect to it we find: 0 = ^ | | a = 0 = i(d^(x)
- A(x)) .
(2.30)
To evaluate A, we use a smooth regulator function R(z) to suppress the effects of the highest eigenmodes. R(z) is chosen such that R(0) = 1, R(oo) — 0, R'(oo) = 0, R"(oo) = 0, . . . For example, e~z satisfies these conditions. Inserting the regulator we have: A(x) = lim Tr J2^l/M2)
(f*n{x)fn{x)
A
M—»oo
- g*n{x)gn{x)) ,
(2.31)
—'
= lim T r £ (f*(x)R(-D2/M2)fn(x)-g*n(x)R(-D2/M2)gn(x))
,
n
= lim
lim Tr (R(-D2/M2)
y—*x M—>oo
- R{-T>2/M2))
\
5{y - x) .
(2.32)
I
To simplify this first note that D2 = d2 - A2 - ci^{F^
- 2A{lldv]) = d2 - A2 - F+ + 0(d • A),
D2 = d2-A2
- 2A[lidv]) = d2-A2+F'+
+ u^iF^
0(d • A). (2.33)
For simplicity we will chose a gauge where d • A = 0. Using the Fourier transform of 5(y — x), one finds by Taylor expanding R(z) around (p2 + A2)/M2 that (2?r)4 ^
n\
356
The n = 0 in the series vanishes trivially, the Dirac trace of the n = 1 term vanishes, and terms with n > 2 vanish in the large M limit. Defining 2 2 z p /M we are left with 1 f°° A{x) = — ^ /
zdzRW^e^TrF^F^
3271-* J 0 1
=
^
2
^
/
^
(2-35)
•
So the anomaly is given by d»f{xE)
= ^Fa^F;u
.
(2.36)
Now consider integrating this equation over spacetime. The left-hand side is given by integrating Eq. (2.31) which yields [d4xA
y/R(X2jM2)rIt[dix(f:(x)fn(x)-g*n(x)gn(x)),
= lim - n^ - n^
,
(2.37)
which is just the number of fermion zero-modes minus the number of antifermion zero-modes, since all other modes occur in pairs and cancel in the difference. So we have 1 n n ip i/)t — QQ_22 / " X F jiV?\IV 32^ n , (2.38) where n is the winding number (Pontryagin number) of the gauge field configuration. This result is known as the Atiyah-Singer index theorem. 2.2. Gauge
Anomalies
There are also potentially anomalies for three-point functions of gauge bosons which are proportional to Aabc
= Tr[T°{T 6 ,T c }] .
(2.39)
These anomalies are potentially non-vanishing for example in the case with one U(l) gauge boson and and two SU(N) gauge bosons, or with three SU(N) gauge bosons for N > 3. The anomaly for two fermions in two different representations R i and R-2 simply add A a 6 c (R! © R 2 ) = A a6c (Rx) + Aabc(R2)
,
(2.40)
357
while for a fermion that is in two different representations R x and R 2 of two different groups one finds: A a 6 c (Ri ® R 2 ) = dim(R 1 )A a b c (R 2 ) + dim(R 2 )A a b c (R 1 ) .
(2.41)
For the fundamental (defining) representation we define dabc
_ ^ [ ^ { 7 ^ ^ j « j] _
(2.42)
It is then convenient to define an anomaly factor A(R), which measures the anomaly relative to the to the fundamental representation: Aabc(K)
= A(R)d ibc
(2.43)
So the gauge anomaly for a theory vanishes if
X>(R0=0
(2.44)
The dimension, index, and anomaly coefficient of the smallest representations are listed below. Irrep
dim(r) N N2-l
2T(r) 1 2iV
A(v) 1 0
N(N-l) 2 N(N+1) 2 N(N-l)(N-2)
7V-2 N +2
iV-4 iV + 4
(JV-3)(iV-2)
(JV-3)(JV-6)
N{N+l)(N+2)
(N+2)(AT+3)
(JV+3)(iV+6)
iV2-3
N2 - 9
•
Ad
y m 1 U
:n
JV(JV-1)(JV+1)
SU(N)
(2.45)
2
]V (JV+1)(JV-1) AT(iV-2)(iV+2) iV(JV-4)(iV+4) 12 (W+2)(JV+3)(JV+4) (JV+3)(N+4)(iV+8) 1 | 1 1 1W(JV+l)(W+2)(W+3) 6 6 24 i(Af-2)(JV 2 -iV-4) (JV-4)(W 2 -JV-8) N(N+l)(N-l)(N-2) 2 8 2 =
:n
If the gauge anomaly does not vanish, then the theory can only make sense as a spontaneously broken theory, since two triangle graphs back to back generate a mass for the gauge bosons. Alternatively if we start with an anomaly free gauge theory and give masses to some subset of the fields so that the anomaly no longer cancels, then the low energy effective theory has an anomaly, but we can only produce such masses if the gauge symmetry is spontaneously broken.
358
2.3. Review
of
Instantons
Recall that instantons 6 are Euclidean solutions of D»F% = 0 ,
(2.46)
characterized by a size p, which, as |x| —* oo, approach pure gauge: - • iU{x)dllU(x)i
A^x)
.
(2.47)
In general, instantons break one linear combination of axial U(l) symmetries. Consider the axial symmetry that has charge + 1 for all (left-handed) fermions. We have oc Fa^F«„
«
(2.48)
Integrating this current in the instanton background with winding number n, one finds: /
d^xd^
=n
Y2 nr 2T(r)
(2.49)
where nr is the number of fermions in the representation r. Thus instantons can create or annihilate fermions. Also an axial rotation of the fermions ip -> eiai\) ,
(2.50)
eYM^0YM-aJ2^r2T(r) .
(2.51)
is equivalent to a shift of #YM r
In the instanton background the gauge covariant derivative can be diagonalized: WD^i
= Xiipi .
(2.52)
For a fermion in representation r one finds 2T(r) zero eigenvalues corresponding to 2T(r) "zero-modes". (In the anti-instanton background ip^ has 2T(r) zero eigenvalues.) Consider a fundamental of SU(N) with T(D) = \. We can write tpi in terms of a Grassmann variable and a complex eigenfunction ipi = £,ifi(x). We then have
For an excellent review see ref. [16].
359
where /o corresponds to the zero eigenvalue. The path integration over this particular fermion is then
(2-55)
J V^V^ = J d£o J I I <&#] • So
= / ^° / n d ^ exp (- E x^n j = / #° / n <&#* IK1 - A^^«) /< n
= 0,
(2.56)
since integrating a constant over a Grassmann variable vanishes: d£0 = 0 .
(2.57)
/ •
However if we insert the fermion field into the path integral we find f TtyThfi exp (-
j ^a»D^\
^{x) = f0(x) J\
\n .
(2.58)
Thus for an instanton amplitude to be non-vanishing it must be accompanied by one external fermion leg for each zero-mode. At distances much larger than the instanton size 't Hooft [17] showed that instantons produce effective interactions for a quark Qnj with color index n and flavor index j : £inst = cdet H?nQnj + h.c.
(2.59)
This interaction respects the non-Abelian SU(F) x SU(F) flavor symmetry but breaks the U(1)A symmetry. 2.4. Instantons
in Broken
Gauge
Theories
In a theory with scalars that carry gauge quantum numbers, VEVs of the scalars prevent us from finding solutions of the classical Euclidean equation
360
of motion. However we can find approximate solutions [18] when we drop the scalar contribution to the gauge current: D»F% = 0 D»D^
+ ^ ^
= 0 ,
(2.60) (2.61)
As \x\ —> oo we want the gauge field to go to pure gauge and the scalar field to go to its VEV (up to a gauge transformation): A„{x) - iU{x)dlxU{x)^ <jp -> U(x){<jt) ,
(2.62) (2.63)
where {(j>>) is a vacuum solution. For small (p < l/(gv)) instantons with a completely broken gauge symmetry we find Euclidean actions: Sinst = S S^ = 8ir2p2v2 .
(2.64) (2.65)
Integrating over instanton locations and sizes we find d^xa I
-^e~s''nst~s*
\b = Jdix0J%(pA)°e-^-^-,
(2.66)
which is dominated at p2
= 1 6 ^ •
^
Thus the integration is convergent: breaking the gauge symmetry provides an exponential infrared cutoff. Note that since A^ is related to an element of the gauge group at \x\ —> oo, the topological character of the instanton relies on U : S3 - • SU{2) c G
(2.68)
If the scalar fields break the gauge group G down to H, then there will still be pure instanton in the H gauge theory if SU(2) C H. If the instantons in G/H can be gauge rotated into SU(2) C H, then all G instanton effects can be accounted for by the effective theory through H instantons. If not, we must add new interactions in the effective theory in order to match the
361
physics properly. Examples of when this is necessary [18] include SU(N) breaks completely SU(N) - • £7(1) SU(N) x SU(N) ->
SU(N)diag
SU(N) -> SO(N) This obviously happens whenever there are a different number of zero modes for G and H instantons. In general new interactions must be included in the effective theory when n^G/H) is non-trivial. 3. Gaugino Condensation We now turn to the chiral symmetry transformations and condensation of gauginos. Note that in pure SU(N) SUSY Yang-Mills (that is with F = 0 flavors, so the only fermion is a gaugino) the U(1)R symmetry is broken by instantons. We can see this by calculating the mixed triangle anomaly between one (7(1).R current and two gluons (see Figure 1). The anomaly is proportional to a group theoretical factor (the anomaly index) which arises by summing (tracing) over all possible fermions in the loop taking in to account the Abelian and non-Abelian charges. In this example the anomaly index is given by the i?-charge of the gaugino (which is 1) times the index of the adjoint representation (T(Ad) = N), so the anomaly is non-vanishing. Because of the anomaly, the chiral rotation Xa
_> eia>a
j
(3 ^
is equivalent to a shift in the coefficient of the CP violating term in the the action, Fa^vF^v, OYM
- • OYM - 2Na .
(3.2)
The factor 27V arises because the gaugino Aa is in the adjoint representation of the gauge group and thus has 2N zero-modes in a one-instanton background. Thus the chiral rotation is only a symmetry when
where k is an integer, so the U(1)R symmetry is explicitly broken down to a discrete Z2N subgroup. Treating the holomorphic gauge coupling T as a
362
background (spurion) chiral superfield we can define a spurious symmetry given by Xa
_^ eiaxa
j
T-+T+
,
(3.4)
7T
which leaves the p a t h integral invariant. Assuming t h a t SUSY Yang-Mills has no massless particles, just massive color-singlet composites, t h e n holomorphy a n d symmetries determine t h e effective superpotential t o be: T T r
Q
2-KJT
Weff = a/Te = aA3 .
N
(3.5)
This is the unique form because under the spurious U(1)R rotation (3.4) the superpotential (which has J?-charge 2) transforms as WeS
- • e2iaWeS
.
(3.6)
Since the holomorphic gauge coupling parameter r transforms linearly under the spurious U(1)R rotation, it must appear an exponential with the coefficient given in (3.5). Again treating r as a background chiral superfield, t h e n in the SUSY Yang Mills action (1.34) the F component of T (which we will refer to as FT) acts as a source for A a A a . W i t h our assumption t h a t there are no massless degrees of freedom, the effective action at low energies is given by just the effective superpotential (3.5). T h u s t h e gaugino condensate is given by (AaAa> = 1671-1^-1112 oFT
'wj
167TJ—- / =
d26WeS
d lQni-pr-Wes OT
= 167TI-—a/i 3 e w .
(3.7)
Dropping the non-perturbative corrections 7 to the running of r and plugging b = 3AT into (1.56) we find
Which only contribute a phase.
3
,
(3.8)
363
thus there is a non-zero gaugino condensate 8 . The presence of this condensate means that the vacuum does not respect the discrete ZN symmetry since (A a A a )-*e 2 i a (A a A a ) ,
(3.9)
is not invariant for all possible values of a = kir/N. In fact it is only invariant for k = 0 or k = N. So we see that Z^JV symmetry is spontaneously broken to Z2, and that there should be TV degenerate but distinct vacua. It is easy to check that the symmetry transformation OYM —• $YM + 2-7T sweeps out N different values for (AaA°). It should be kept in mind that at this point we have not justified the assumption of no massless degrees of freedom which is crucial to the calculation. 4. The Affleck-Dine-Seiberg Superpotential 4.1. Symmetry
and
Holomorphy
Consider SU(N) SUSY QCD with F flavors (that is there are 2NF chiral supermultiplets) where F < N. We will denote the quarks and their superpartner squarks that transform in the SU(N) fundamental (defining) representation by Q and $ respectively, and use Q and $ for the quarks and squarks in the anti-fundamental representation. The theory has an SU(F) x SU(F) x U{1) x U(1)R global symmetry. The quantum numbers of the chiral supermultiplets are summarized in the following table 9 where • denotes the fundamental representation of the group. SU{N)
SU(F)
SU(F)
17(1)
U{1)R
•
•
1
1
1
D
-1
F-N F F-N F
(4.1)
The SU{F) x SU{F) global symmetry is the analog of the SU(3)L x SU{3)R chiral symmetry of non-supersymmetric QCD with 3 flavors, while the U(l) is the analog 10 of baryon number since quarks (fermions in the fundamental representation of the gauge group) and anti-quarks (fermions in the anti-fundamental representation of the gauge group) have opposite charges. There is an additional ^ ( 1 ) ^ relative to non-supersymmetric QCD since in the supersymmetric theory there is also a gaugino. 8
For related work using instantons, see ref. [19-21]. As usual only the .R-charge of the squark is given, and R[Q] = R[&] — 110 U p to a factor of N.
9
364
Recall that the auxiliary Da fields for this theory are given by (4.2)
where j is a flavor index that runs from 1 to F, m and n are color indices that run from 1 to iV, the index a labels an element of the adjoint representation, running from 1 to N2 — 1, and Ta is a gauge group generator. The D-term potential is given by: V
1 a a -D D 2g2
(4.3)
Classically there is a (D-flat) moduli space (space of VEVs where the potential V vanishes) which is given by \
vi
Vp
($ ) = ($)
0
(4.4)
\0 ... 0 J where ($) is a matrix with iV rows and F columns and we have used global and gauge symmetries to rotate (<&) to a simple form. At a generic point in the moduli space the SU(N) gauge symmetry is broken to SU(N — F). There are N2 -1-({N-
F)2 - 1) = 2NF - F2
(4.5)
broken generators, so of the original 2NF chiral supermultiplets only F2 singlets are left massless. This is because in the supersymmetric Higgs mechanism a massless vector supermultiplet "eats" an entire chiral supermultiplet to form a massive vector supermultiplet. We can describe the remaining F2 light degrees of freedom in a gauge invariant way by an F x F matrix field M/
Wr%r,
(4.6)
where we sum over the color index n. Since M can be written as a chiral superfield which is a product of of chiral superfields11, then, because of 11
That is there is a fermionic superpartner of M (the "mesino") given by M^, = Q
<£„
365
holomorphy, the only renormalization of M is the product of wavefunction renormalizations for $ and $. The axial U(1)A symmetry which assigns each quark a charge 1 is explicitly broken by instantons, while the U(1)R symmetry remains unbroken. To check this we can calculate the corresponding mixed anomalies between the global current and two gluons. For U(1)R we multiply the i?-charge by the index of the representation, and sum over fermions. The gaugino contributes 1 • N while each of the 2F quarks contributes ^(F~^N — 1). Adding these together we find that the mixed anomaly for (7(1) .R vanishes: ARgg
\ 1
(F-N
= N+{—___ij-=o.
(4.7)
For the (7(1)^ anomaly, the gauginos do not contribute since they have no (7(1)A charge and we find the anomaly coefficient is non-vanishing: AAgg
= l-2F-1-.
(4.8)
To keep track of selection rules arising from the broken U(1)A we can define a spurious symmetry in the usual way. The transformations Q -* eiaQ , Q - • eiaQ , GYM
-+ GYM + 2Fa ,
(4.9)
leave the path integral invariant. Under this transformation the holomorphic intrinsic scale (1-42) transforms as Ab
_^
ei2FaAb
_
(410)
To construct the effective superpotential we can make use of the following chiral superfields: Wa, A, and M. Their charges under the U(1)R and spurious U{1)A symmetries are given in the following table. U(1)A U(1)R 0 2 ww b 2F 0 A 2F 2{F - N) detM a
a
Note that detM is the only SU(F) x SU(F) invariant we can make out of M. To be invariant, a general non-perturbative term in the Wilsonian superpotential must have the form Abn(WaWa)m(detM)p
.
(4.12)
366
As usual to preserve the periodicity of 8YM we can only have powers of A6 (for m = 1 we can still have the perturbative term b log AW a W a because the change in the path integral measure due to the anomaly). Since the superpotential is neutral under U(1)A and has charge 2 under U(1)R, the two symmetries require: 0 = n2F + p2F 2 = 2m + p2(F-
.
N) .
(4.13) (4.14)
The solution of these equations is n
(4 15)
= -P=W^F-
'
Since b = 3N — F > 0 we can only have a sensible weak-coupling limit (A —• 0) if n > 0, which implies p < 0 and (because N > F) m < 1. Since WaWa contains derivative terms, locality requires m > 0 and that m is integer valued. In other words, since we trying to find a Wilsonian effective action (which corresponds to performing the path integral over field modes with momenta larger than a scale /i) which is valid at lowenergies (momenta below n) it must have a sensible derivative expansion. So there are only two possible terms in the effective superpotential: m = 0 and m = 1. The m = 1 term is just the tree-level field strength term. The coefficient of this term is restricted by the periodicity of #YM to be proportional to b In A. So we see that the gauge coupling receives no nonperturbative renormalizations. The other term (m = 0) is the Affleck-DineSeiberg superpotential 12 : / \3N-F\
W
=C
Trh?
F
™ »' {li*M)
'
(4 16)
-
where CN,F is in general renormalization scheme dependent. 4.2. Consistency
of
WADS-'
Moduli
Space
We can check whether the Affleck-Dine-Seiberg superpotential is consistent by constructing effective theories with fewer colors or flavors by going out in the classical moduli space or by adding mass terms for some of the flavors. Consider giving a large vacuum expectation value (VEV), v, to one flavor. This breaks the gauge symmetry to SU(N - 1) and one flavor is partially 12
First discussed by Davis, Dine, and Seiberg [22] and explored in more detail by Affleck, Dine, and Seiberg in [23]
367
"eaten" by the Higgs mechanism (since there are 2N — 1 broken generators) so the effective theory has F—l flavors. There are 2F—1 additional gauge singlet chiral supermultiplets left over as well since 2NF - (2JV - 1) - (2F - 1) = 2(N - 1)(F - 1) .
(4.17)
We can write an effective theory for the SU(N — 1) gauge theory with F — l flavors (since the gauge singlets only interact with the fields in the effective gauge theory by the exchange of heavy gauge bosons they must decouple from the gauge theory at low energies, i.e. they interact only through irrelevant operators with dimension greater than 4). The running holomorphic gauge coupling, g^, in the low-energy theory is given by 871-2
u
( A*
i
where bi is the standard /3-mnction coefficient of the low-energy theory bL = 3(N-l)-(F-l)
= 3N-F-2
,
(4.19)
and AL is the holomorphic intrinsic scale of the low-energy effective theory \AL\eie™/bL
AL =
= iie2viTL/bL
,
(4.20)
This coupling should match onto the running coupling of the high-energy theory 8TT2
(5) '
•6 In
52(M)
at the scale of the heavy gauge boson threshold 2
8TT
gi(v)-
_
^ 13
v.
2
8TT
gl(v)+C>
(4.22)
where c represents scheme dependent corrections, which vanish in the DR renormalization scheme 14 . So we have A\ f e
(KL^H
13 The fact that the heavy gauge boson mass is v rather than gv is a result of having an unconventional normalization for the quark and squark fields, this is necessary to maintain gL and A^ as holomorphic parameters. For related discussions on this point see [3,4,13]. 14 DR uses dimensional regularization through dimensional reduction with modified minimal subtraction [24,25].
368 A3iV-F
li
^2
-
A3N-F-2 iV-l,F-l '
— A
/ 4 2 ON ^.Z«3J
where we have started labeling (for later convenience) the intrinsic scale of the low-energy effective theory with a subscript showing the number of colors and flavors in the gauge theory that it corresponds to: A J V - I , F - I = A^. If we represent the light (F — l ) 2 degrees of freedom (corresponding to the gauge invariant combinations of the chiral superfields that are fundamentals under the remaining gauge symmetry) as an (F — 1) x (F — 1) matrix M then we have detM = v 2 detM + . . . ,
(4.24)
where . . . represents terms involving the decoupled gauge singlet fields. Plugging these results into the Affieck-Dine-Seiberg superpotential for N colors and F flavors (which we denote by WADS(N, F)) and using
(which follows from equation (4.23)) we reproduce WADS(N — 1,F — 1) provided that CJV,F is only a function oi N — F. Giving equal VEVs to all flavors we break the gauge symmetry from SU(N) down to SU(N - F), and all the flavors are "eaten". Through the same method of matching running couplings,
we then have
ir-w A3JV-F V2F
l(N-F) '~F)
(4.26)
,3(JV-F) iV-F,0 '
A
(4.27)
So the effective superpotential is given by Weff — CN,F-^-N-F,0
i
(4.28)
which agrees with the result (3.5) derived from holomorphy arguments for gaugino condensation in pure SUSY Yang-Mills, as described in section 3. 4.3.
Consistency
O / W A D S - ' Mass
Perturbations
Now consider giving a mass, m, to one flavor. Below this mass threshold the low-energy effective theory is an SU(N) gauge theory with F-1 flavors.
369
Matching the holomorphic gauge coupling of the effective theory to that of the underlying theory at the scale m gives: (^ ) b \m. )
V J
(AL
\m
F+l
A 3 7 V " -F _ A3N-
(4.29)
-1
Using holomorphy the superpotential must have the form / A3N~F\ Wexact = ( - ^ )
wzr¥
/(*) ,
(4.30)
where
and /(£) is an as yet undetermined function. Note that since mMf is actually a mass term in the underlying superpotential, it has U(1)A charge 0, and i?-charge 2, so t has i?-charge 0. Taking the weak coupling, small mass limit A —• 0, m —> 0, we must recover our previous results with the addition of a small mass term, hence f(t) = CN,F+t.
(4.32)
However in this double limit t is still arbitrary so this is the exact form of f(t). Thus we find / \3N-F\
W^F
Wexact = CN,F (-^f)
+ mMf
.
(4.33)
The equations of motion for Mf and MF -8Mr dWexa, dMF (where coi(Mf)
=
CN F
{-N^F)+m
' {-detM)
, -1
s^-ry^
}
C
°
= °>
(4 34)
W )
(4 351
is the cofactor of the matrix element Mf)
- 0
'
imply that
F
and that the cofactor of Mf is zero. Thus M has the block diagonal form M=(™
°F)
,
I 0 MfJ '
(4.37) K
!
370
where M is an (F — 1) x (F — 1) matrix. Plugging the solution (4.36) into the exact superpotential (4.33) we find We^N,F-l) = (N-F+l)^)
( ^ ^ ^ j
(4.38)
which is just WADS up to an overall constant. Thus,for consistency, we have a recursion relation: C W . F - 1 = (N-F
+ 1) ( j ^ j
""'+1
•
(4-39)
An instanton calculation is reliable for F = N — 1 since the non-Abelian gauge group is completely broken, and such a calculation [20,25] determines CN,N-I = 1 in the DR scheme, and hence CNtF = N -F
.
(4.40)
We can also consider adding masses for all the flavors. Holomorphy determines the superpotential to be Wexact = CN,F
( -
^
detM
)
+ T^M?
,
(4.41)
where mj is the quark (and squark) mass matrix. The equation of motion for M gives / \3N-F\
Mt = (m-1n
W=F
-r-TT) V detM
•
(4-42)
Taking the determinant and plugging the result back in to (4.42) gives &$i = M{ = (m-l){
(Aetmh3N-FY
.
(4.43)
Note that since the result involves the Ar-th root there are N distinct vacua that differ by the phase of M. This is in precise accord with the Witten index argument [26]. Matching the holomorphic gauge coupling at the mass thresholds gives A 3 i V - i ? detm = A ^ r 0 .
(4.44)
So Weff = NA3Nfi
,
(4.45)
371
which agrees with our holomorphy result 15 , equation (3.5), for pure SUSY Yang-Mills and determines the parameter a = N up to a phase. So the gaugino condensate (3.8) is given by (AaA°) = -32n2e2*ik'N
A%fi ,
(4.46)
where k — 1...N. Thus starting with F = N — 1 flavors we can consistently derive the correct Affleck-Dine-Seiberg effective superpotential for 0 < F < N — 1, and gaugino condensation for F = 0. This retroactively justifies the assumption that there was a mass gap in SUSY Yang Mills. 4.4. Generating
W A D S from
Instantons
Recall that the Affleck-Dine-Seiberg superpotential WADS OC AT^F ,
(4.47)
while instanton effects are suppressed by 16 e-sinst
^ Ab _
(448)
Sc for F = N — 1 is is possible that instantons can generate WADS- Since SU{N) can be completely broken in this case, we can do a reliable instanton calculation. When all VEVs are equal the ADS superpotential predicts quark masses of order d2WADS
A2N+l
(4.49)
v2N
d^idtf and a vacuum energy density of order 8WADs d$i
2
A2^1 „2JV-1
2
(4.50)
Looking at a single instanton vertex we find 2N gaugino legs (corresponding to 2N zero-modes) and 2F = 2N - 2 quark legs, as shown in Figure 2. All the quark legs can be connected to gaugino legs by the insertion of a scalar VEV. The remaining two gaugino legs can be converted to two quark legs by the insertion of two more VEVs. Thus a fermion mass is generated.
15 16
Which assumed a mass gap. See Eq. (1.51).
372
2N-2
Figure 2. Instanton with 2N-2 quark legs (solid, straight lines) and 2N gaugino legs (wavy lines), connected by 2N squark VEVs (dashed lines with crosses).
Prom the instanton calculation we find the quark mass is given by m ~ e
_s^ 2 /„^„i /S'(P)
V
v2N
p2JV-l
b
V2N p2N-l
2^V+1
-)
V2N n2iV-l
(4.51)
PJ The dimensional analysis works because the only other scale in the problem is the instanton size p, and the integration over p is dominated by the region around b Forcing the quark legs to end at the same space time point generates the F component of M, and hence a vacuum energy of the right size. From our previous arguments we recall that we can derive the ADS superpotential for smaller values of F from the case F = N — 1, so in particular we can derive gaugino condensation for zero flavors from this instanton calculation with N — 1 flavors.
373
4.5. Generating
W A D S from
Gaugino
Condensation
For F < N — 1 instantons cannot generate WADS since at a generic point in the classical moduli space with detM ^ 0 the SU(N) gauge group breaks to SU(N — F) D SU(2). Matching the gauge coupling in the effective theory at a generic point in the classical moduli space gives A3N-F
= AHN-F)detM
(453)
In the far infrared the effective theory splits into and SU(N — F) gauge theory and F2 gauge singlets described by M. However these sectors can be coupled by irrelevant operators. Indeed they must be, since by themselves the SU(N-F) gauginos have an anomalous R symmetry. The R symmetry of the underlying theory was spontaneously broken by squark VEVs but it should not be anomalous. An analogous situation occurs in QCD where the SU(2)L x SU(2)R chiral symmetry is spontaneously broken and the axial anomaly of the quarks is reproduced in the low-energy theory by an irrelevant operator (the Wess-Zumino term [27]) which manifests itself in the anomalous decay ir° —> 77. In SUSY QCD the correct term is in fact present since the effective holomorphic coupling in the low-energy effective theory,
depends on In detM through the matching condition (4.53). The relevant term in the low-energy Lagrangian is (factoring out 3(iV — F)/(32ir2))
J d26 In detMWaWa + h.c. = [ r r ( F M M - 1 ) A a A a + A r g ( d e t M ) F a ^ F ; i / + . . . ] + h.c.
(4.55)
where FM is the auxiliary field (i.e. the coefficient of 92 in superspace notation) for M. The second term can be seen to arise through triangle diagrams involving the fermions in the massive gauge supermultiplets. Note Arg(detM) transforms under a chiral rotation in the correct manner to be the Goldstone boson of the spontaneously broken R symmetry: Arg(detM) -> Arg(detM) + 2Fa . The equation of motion for FM gives dW FM
~dM =
M-l(XaXa)
(4.56)
374
M-1A%_Ffi
oc
/ \3N-F\
7T=F
This gives a vacuum energy density that agrees with the Affleck-DineSeiberg calculation. This potential energy implies that a non-trivial superpotential was generated for M, and since the only superpotential consistent with holomorphy and symmetry is WADS we can conclude that for F < N — 1 flavors gaugino condensation generates WADS • 4.6. Vacuum
Structure
Now that we believe WADS is correct what does it tells us about the vacuum structure of the theory? It is easy to see that
y = Y,
dW dQi
dW
Wi = J2\Fi\ + \Fi\2, 2
2
(4.58)
i
is minimized as detM —> oo, so there is a "run-away vacuum", or more strictly speaking no vacuum. A loop-hole in this argument would seem to be that we have not included wavefunction renormalization effects, which could produce wiggles or even local minima in the potential, but it could not produce new vacua unless the renormalization factors were singular. It is usually assumed that this cannot happen unless there are particles that become massless at some point in the field space, which would also produce a singularity in the superpotential. This is just what happens at detM = 0, where the massive gauge supermultiplets become massless. So we do not yet understand the theory without VEVs. 5. 't Hooft's Anomaly Matching 't Hooft made one of the most important advances in understanding strongly coupled theories with composite degrees of freedom by pointing out that the anomalies of the constituents and the composites must match [28]. Consider an asymptotically free gauge theory, with a global symmetry group G. We can easily compute the anomaly for three global G currents in the ultraviolet by looking at triangle diagrams of the fermions. We will call the result Auv. Now imagine that we weakly gauge G with a
375
gauge coupling g < 1. If Auv ^ 0, then we can add some spectators that only have G gauge couplings, they can be chosen such that their G anomaly is As = -Auv, so the total G anomaly vanishes. Now construct the effective theory at a scale less than the strong interaction scale. If we compute the G anomaly at this scale we add up the triangle diagrams of light fermions, which consist of the spectators and strongly interacting or composite fermions. If G is not spontaneously broken by the strong interactions its anomaly must still vanish 17 , so 0 = AIR + As .
(5.1)
Thus we have AIR
= Auv
.
(5.2)
Taking g —> 0 decouples the weakly coupled gauge bosons but does not change the three point functions of currents. 6. Duality for S U S Y QCD Theoretical effort in the mid 1990s (mainly due to Seiberg [8,29]) led to a dramatic break-through in the understanding of strongly coupled Af = 1 SUSY gauge theories 18 . After this work we now have a detailed understanding of the infrared (IR) behavior of many strongly-coupled theories, including the phase structure such theories. The phase of a gauge theory can be understood by considering the potential V(R) between two static test charges a distance R apart 1 9 . Up to an additive constant we expect the functional form of the potential will fall into one of the following categories: Coulomb :
V(R) ~ ^
free electric :
V(R) ~
flln(RA)
free magnetic : V(R) ~ ]^)Higgs :
V(R) ~ constant
confining :
V(R) ~ crR .
(6.1)
17 If G is spontaneously broken the anomaly will still be reproduced by the interactions of the Goldstone bosons, this is the origin of the Wess-Zumino term [27]. 18 For other review of these developments see [9,30,31]. 19 Holding the charges fixed for a time T corresponds to a Wilson loop of area T • R.
376
The explanation of these functional forms is as follows. In a gauge theory where the coupling doesn't run (e.g. at an IR fixed point or in QED at energies below the electron mass) then we expect to simply have a Coulomb potential. In a gauge theory where the coupling runs to zero in the IR (e.g.. QED with massless electrons) there is an inverse logarithmic correction to squared gauge coupling and hence to the potential. Since electric and magnetic charges are inversely related by the Dirac quantization condition, the squared charge of a static magnetic monopole grows logarithmically in the IR due to the renormalization by loops of massless electrons. Using electricmagnetic duality to exchange electrons with monopoles one finds that the logarithmic correction to the potential for static electrons renormalized by massless monopole loops appears in the numerator since the coupling grows in the IR. In a Higgs phase the gauge bosons are massive so there are no long range forces. In a confining phase 20 we expect a tube of confined gauge flux between the charges, which, at large distances, acts like a string with constant mass per unit length, and thus gives rise to a linear potential. The familiar electric-magnetic duality exchanges electrons and magnetic monopoles and hence the free electric phase with the free magnetic phase. Mandelstam and 't Hooft [32] conjectured that duality could also exchange the Higgs and confining phases and that confinement can be thought of as a dual Meissner effect arising from a monopole condensate 21 . Electricmagnetic duality also exchanges an Abelian Coulomb phase with another Abelian Coulomb phase. Seiberg [9,29] conjectured that analogs of the first and last of these dualities actually occur in the IR of non-Abelian J\f = 1 SUSY gauge theories and demonstrated that his conjecture satisfies many non-trivial consistency checks.
6.1. The Classical Moduli Space for F > N Consider SU(N) SUSY QCD with F flavors and F>N. This theory has a global SU(F) x SU(F) x t/(l) x U(1)R symmetry. The quantum numbers 22
20 More precisely a confining phase with area law confinement. Note however that with dynamical quarks in the fundamental representation of the gauge group we can produce quark anti-quark pairs which screen the test charge, so there is no long range force, that is the flux tube breaks. 21 Seiberg and Witten later showed that this is actually the case in certain Af = 2 SUSY gauge theories [33]. 22 As usual only the .R-charge of the squark is given, and R[Q] = R[&] — 1.
377
of the squarks and quarks are summarized in table 6.2. SU{N)
SU(F)
SU(F)
C/(l)
U(1)R
*, Q
•
D
1
1
$, Q
3
1
D
-1
F-N F F-N F
(6.2)
The SU{F) x SU{F) global symmetry is the analog of the SI7(3) L x SU(3)R chiral symmetry of non-supersymmetric QCD with 3 flavors, while the U(l) is the analog 23 of baryon number since quarks (fermions in the fundamental representation of the gauge group) and anti-quarks (fermions in the anti-fundamental representation of the gauge group) have opposite charges. There is an additional U(1)R relative to non-supersymmetric QCD since in the supersymmetric theory there is also a gaugino. Recall that the .D-terms for this theory are given in terms of the squarks by Da = g(*jn{Ta)™<5>ri
•* 3 "n;i),
(6.3)
where j is a flavor index that runs from 1 to F, m and n are color indices that run from 1 to AT, the index a labels an element of the adjoint representation, running from 1 to N2 — 1, and Ta is a gauge group generator. The D-term potential is: V
1 a a -D D 2g2
(6.4)
where we sum over the index a. Define (6.5) (6.6) Dm and Dm are N x N positive semi-definite Hermitian matrices. In a SUSY vacuum state the vacuum energy vanishes and we must have: Da = gTZm(Dm
-Dm)=0
(6.7)
.Since Ta is a complete basis for traceless matrices, we must have that the second matrix is proportional to the identity: Dn Up to a factor of N.
-D
Pi
(6.8)
378
D^ can be diagonalized by an SU(N) gauge transformation (6.9)
D' = U^DU , so we can take -D" to have the form:
/hi
\ «2
D =
(6.10)
KIV
V
In this basis, because of Eq. (6.8), Dm must also be diagonal, with eigenvalues \vi\2. This tells us that
k l 2 = \vi\2 + p
(6.11)
Since D^ and Dm are invariant under flavor transformations, we can use SU(F) x SU(F) flavor transformations to put ($) and ($) in the form 0...0\
Vl
(6.12)
<*> VJV
0 ... 0 /
«i
($)
0 ... 0
3.13)
\ ° - °/ Thus we have a space of degenerate vacua, which is referred to as a moduli space of vacua. The vacua are physically distinct since, for example, different values of the VEVs correspond to different masses for the gauge bosons. With a VEV for a single flavor turned on we break the gauge symmetry down to SU(N — 1). At a generic point in the moduli space the SU(N) gauge symmetry is broken completely and there are 2NF— (N2 — 1) massless* chiral supermultiplets left over. We can describe these light degrees of freedom in a gauge invariant way by scalar "meson" and "baryon" fields and their superpartners: M? = Wn<$>ni ,
(6.14)
379
(6.15) = $
(6.16)
The fermion partners of these fields are the corresponding products of scalars and one fermion. There are constraints relating the fields M and B, since the M has F2 components, B and B each have
components, N and all three are constructed out of the same 2NF underlying squark fields. For example, at the classical level, there is a relationship between the product of the B and B eigenvalues and the product of the non-zero eigenvalues of M: Mj71 . . . M3"
,Bn'
Biu
(6.17)
where [] denotes antisymmetrization. Up to flavor transformations the moduli can be written as: \
fvivi
vNvN
{M) =
0/
\ (BI,...,N)
(B
(6.18)
=VI...VN
)=vi
VN
(6.19) (6.20)
with all other components set to zero. We also see that the rank of M is at most N. If it is less than N, then B or B (or both) vanish. If the rank of M is k, then SU(N) is broken to SU(N - k) with F -k massless flavors. 6.2. The Quantum
Moduli
Space for F > N
Recall that the ADS superpotential made no sense for F > N however the vacuum solution
Ml = {m~l)i (detmA3Ar-F)
3.21)
is still sensible. Giving large masses, m # , to flavors N through F and matching the gauge coupling at the mass thresholds gives
A3JV-^detmff = A 2 / + V
(6.22)
380
The low-energy effective theory has N — 1 flavors and an ADS superpotential. If we give small masses, mi, to the light flavors we have M( = {nq})\
(detmLA2NN^1)Jr
= (m^(detmLdetmHA3iv-F)* .
(6.23)
Since the masses are holomorphic parameters of the theory, this relationship can only break down at isolated singular points, so equation (6.21) is true for generic masses and VEVs. For F > N we can take m*- —> 0 with components of M finite or zero. So the vacuum degeneracy is not lifted and there is a quantum moduli space [34] for F > N, however the classical constraints between M, B and B may be modified. Thus we can parameterize the quantum moduli space 24 by M, B and B. When these fields have large values (with maximal rank) then the squark VEVs are large (compared to A) and the gauge theory is broken in the perturbative regime. As the meson and baryon fields approach zero (the origin of moduli space) then the gauge couplings become stronger, and at the origin we would naively expect a singularity since the gluons are becoming massless at this point. We shall see that this expectation is too naive.
6.3. Infrared Fixed
Points
For F > 3N we lose asymptotic freedom, so the theory can be understood as a weakly coupled low-energy effective theory. For F just below 3iV we have an infrared fixed point. This was pointed out by Banks and Zaks [36] as a general property of gauge theories. By considering the large N limit of SU(N) with F/N infmitesimally below the point where asymptotic freedom is lost they showed that the /? function has a perturbative fixed point. Here we will apply their argument to SUSY QCD. Novikov, Shifman, Vainshtein and Zakharov [37] have shown that the exact (3 function for the running canonical 25 gauge coupling is given by
24
There is a theorem which shows that in general a moduli space is parameterized by the gauge invariant operators, see [35]. 25 For discussions of how the holomorphic gauge coupling is related to the canonical gauge coupling see [3,4,13,14].
381
where 7 is the anomalous dimension of the quark mass term. In perturbation theory one finds
-< = -h—+0^a2
<6-25>
N2-l
So 2
16vr /3(g) = - 5
3
o5 / (3iV - F) - -^ (w2
+0(g7)
-FN-
N2 — 1 F ^ -
.
(6.26)
Now take the number of flavors to be infinitesimally close to the point where asymptotic freedom is lost. For F = 2>N — eN we have
L
167r2/3(ff) = -g\N
- f j (3(iV2 - 1) + 0(e)) 8TT 2 7
+0(g ) . (6.27) So there is an approximate solution of the condition (3 = 0 where there first two terms cancel. This solution corresponds to a perturbative infrared (IR) fixed point at
£ = *rw=i*>
(6 28)
-
7
and we can safely neglect the 0(g ) terms since they are higher order in e. Without any masses this fixed point gauge theory theory is scale invariant when the coupling is set to the fixed point value g = g*- A general result of field theory is that a scale invariant theory of fields with spin < 1 is actually conformally invariant [38]. In a conformal SUSY theory the SUSY algebra is extended to a superconformal algebra. A particular .R-charge enters the superconformal algebra in an important way, we will refer to this superconformal i?-charge as Rsc- Mack found [39] that in the case of conformal symmetry there are lower bounds on the dimensions of gauge invariant fields. In the superconformal case 26 it was shown for A/" = 1 by Flato and Fronsdal [40] (and for general M by Dobrev and Petkova [41]) that the dimensions of the scalar component of gauge invariant chiral and anti-chiral superfields are given by 3 d = -Rsc, for chiral super fields , 3 d = — R s c , foranti — chiral superfields . 'A brief review is given in the next section.
(6.29) (6.30)
382
Since the charge of a product of fields is the sum of the individual charges, RSc[0102]
= R^ld]
+ RSC[02] ,
(6.31)
we have the result that for chiral superfields dimensions simply add: Z?[0i<9 2 ] = D[Oi] + D[02] .
(6.32)
This is a highly non-trivial statement since in general the dimension of a product of fields is affected by renormalizations that are independent of the renormalizations of the individual fields. In general the R symmetry of a SUSY gauge theory seems ambiguous 27 , since we can always form linear combinations of any U(1)R with other U(l)'s, but for the fixed point of SUSY QCD i? sc is unique since we must have RSC[Q] = RSC[Q] .
(6.33)
Thus we can identify the R charge we have been using (as given in table 6.2) with Rsc. If we denote the anomalous dimension of the squarks at the fixed point by 7* then the dimension of the meson field at the IR fixed point is D[M] - D[*«] = 2 + T , =
\2{F~FN)
- 3 - ^ .
(e-34)
and the anomalous dimension of the mass operator at the fixed point is 37V 7* = 1 - — . (6.35) We can also check that the exact (3 function (6.24) vanishes: /3oc3A^-F(l-7»)=0 .
(6.36)
For a scalar field in a conformal theory we also have [39] D{4>) > 1 ,
(6.37)
with equality holding for a free field. Requiring that D[M] > 1 implies F > ^N .
(6.38)
Thus the IR fixed point is an interacting conformal theory for |iV < F < 3N. Such conformal theories have no particle interpretation, but anomalous dimensions are physical quantities. 27
A general procedure for determining the superconformal R symmetry was given by Intriligator and Wecht [42]
383
6.4. An Aside
on Swperconformal
Symmetry
The reader may skip this section on her first time through, and return to it later if she remains curious about how the results quoted above relating scaling dimensions to i?-charges were obtained. I will loosely follow the discussion of ref. [43]. The generators of the conformal group can be represented by Mpv = -iix^dy
-
xvd^)
Pp. = -id^, K^ - -i{x2dfj, D = ixada
2Xfj,xada)
,
(6.39)
where M^u are the ordinary Lorentz rotations/boosts, PM are the translation generators, K^ are referred to as the "special" conformal generators, and D is the dilation generator. (D is also know as the "dilatation" generator by those who like extra syllables.) In 4 spacetime dimensions, the conformal group is isomorphic to SO(A, 2) We would like to find the restrictions that can be placed on conformal field theory operators by constructing the unitary irreducible representations of the conformal group. The simplest method to perform this construction is not with the generators defined above but by a related set of generators that correspond to "radial quantization" in Euclidean space. These generators are defined with indices that runs from 1 to 4: % = Mjk
*u =\{Pj-Ki) D' = -\{P* + KQ) J =••^(Pj+K^
P
+ iM'jo
l Pi = -D- -(P0-K0)
.^ - > «'i = iPj+Kj) iM 0 l
K = :-D+ -(P0-K0),
(6.40)
where j = 1,2,3 and D' acts as the "Hamiltonian" of the "radial quantization" . The eigenstates of the "radial quantization" are in one to one correspondence with the operators of the conformal field theory. From these
384
definitions we see that D't =
-D'
P? = K'j .
(6.41)
An SO (4, 2) representation can be specified by its decomposition into irreducible representations of its maximal compact subgroup which is 50(4) x 50(2). The 50(4) « 5(7(2) x SU(2) representations are just the usual representations of the Lorentz group which can be specified by two half-integers (J,j) (Lorentz spins): (scalar) = (0,0) (spinor)= (vector) =
(l,o)
or (o,±)
(6.42)
1 1 2'2
The 50(2) representation is labeled by the eigenvalue of D' acting on the state (operator). I will denote this eigenvalue by —id where d is the scaling dimension of the operator. To complete the construction we need raising and lowering operators for the 50(2) group. We can choose a basis such that P' is a raising operator and therefore K' is a lowering operator. Then we can classify multiplets by the scaling dimension of the lowest weight state (the state annihilated by K'). The operator corresponding to the lowest weight state is also known as the primary operator. One can check (using the commutator [D',P'] = —iPL) that acting on the lowest weight operator with the raising operator P', gives a new operator (sometimes called a descendant operator) with scaling dimension d + 1. Unitarity requires that any linear combination of states have positive norm, so in particular the state aP'm\d,(ji,ji))+bP'n\d,U2,J2))
,
(6.43)
(with m ^ n) must have positive norm. Using the commutator [P'm, K'n] = -i(25mnD'
+ 2M'mn) ,
(6.44)
we find (\a\2 + \b\2)d + 2Re(a*b{d,(j1,j1)\iM^nn\d,(j2,j2))
>0 .
(6.45)
Given that iM'mn has real eigenvalues and restricting to \a\ = \b\ this condition can be written as d > ±(d,(ju~jl)\iM'mn\d,(J2,h))
•
(6.46)
385
To find the eigenvalues of iM'mn write (for fixed m, n) iM
Ln
= ^(Smadn0
~ 6mp5na)M'a0 ,
(6-47)
which can be re-written as M'mn = (V.M')mn
,
(6.48)
where V is the generator of 50(4) rotations in the vector representation: Va0mn = i(5ma5n/3 — Smf35na) ,
(6.49)
and A.B = ^AaPBa0
.
(6.50)
By a similarity transformation we can go to a Clebsch-Gordan basis where (V + M'), V, and M' are "simultaneous (commuting) quantum numbers" and use the relation 28 V.M = 1 [(V + M'f
- V2 - M'2]
(6.51)
From this one can show that that the most negative eigenvalue of V.M has a larger magnitude than the most positive eigenvalue, so the most restrictive bound from the inequality (6.46) is the case with the — sign. If our state |d, j'2, J2) corresponds to the representation r of 50(4), and r' is the representation with the smallest quadratic Casimir in the product rxV then we have d > \ [C2 (r) + C2 (V) - C2 (r')]
(6.52)
C 2 (V) = V.V
(6.53)
where
and so on. Using the fact that the 50(4) Casimir is twice the sum of the SU(2) Casimirs (i.e. C2(jJ) = 2(J 2 + J2) = 2(j(j + l)+](j + 1))), one can check that C2(scalar) = 0 3 C2{spinor) = -
(6.54)
C2 {vector) = 3 , 28 Readers of a certain age will recognize this technique from the calculation of the mostattractive channel in single gauge boson exchange [44], while all readers should recognize the calculation of the spin-orbit term.
386
and thus: d > 0 , (scalar) 3 d > — , (spinor)
(6.55)
d > 3 , (vector) . The first two bounds are perfectly reasonable since the identity operator is a scalar and has dimension 0, and a free (massless) spinor has dimension 3/2. The third bound may be a little surprising since a free massless vector (gauge) boson field has dimension 1, however such a field is not gauge invariant, and thus unitarity cannot be applied. A conserved current is a gauge-invariant vector field and does have dimension 3. Applying similar arguments to states with P[P'k acting on them, one finds for scalars d(d-l)>0,
(6.56)
which means that for scalar operators with d > 0 (i.e. operators other than the identity) d>1.
(6.57)
Turning to superconformal symmetry, first note that in addition to the usual (Euclidian) supersymmetry generators Q'ia (where a is a spinor index and i runs from 1 to M) there is also a superconformal generator S'^ such that Q ,f = S' .
(6.58)
Q' and S' can be chosen to be real Majorana (four component) spinors. There is also an iZ-symmetry which is 1/(1) for M = 1, U(2) for A/" = 2, and SU(4) for A/" = 4. In general we can take the matrix (Tij)pq = Sip5jq ,
(6.59)
as a generator of the full i?-symmetry, and R as the generator of the Abelian .R-symmetry (i.e. for M = 1 we have R = T n ; for A/" = 2, R = J^ Ty; while for M = 4 set R = 0). The /^-symmetry does not commute with the supersymmetry generators, to see this explicitly we need a few more definitions. Define the Euclidian T matrices in terms of the Lorentzian 7 matrices by r< = H ,
(6.60)
T 4 = -«7o ,
(6.61)
387
and choose a basis where Ta is real and hermitian Then we can define left-handed and right-handed projection operators by P- = i ( l - 7 5 ) ,
(6-62)
P+ = i ( l + 7 5 ) .
(6.63)
The convention is that (j,j) = (1/2, 0) corresponds to the fermion component of chiral supermultiplet which is projected to zero by P _ . Finally we can then write [Ta, Q'm\ = P+ Q[Sjm - P Q'3^m
(6.64)
[Ttj, S'm] = P+ S<8jm - P- S'jSjm
(6.65)
Most importantly for the unitarity bounds, the anticommutator of Q' and S"is: {Q'ia, S'jP} = i5-f[{M'mnTmTn)a0 -2{P+)aPTij
+ 2Sa0D'}
+ 2{P-)a0Tji
+-^(rfB)apR.
(6.66)
Now apply the requirement of unitarity to the state aQja|d,iJ)(jiJ1)>+6Q^|d,JR,(j2j2)> ,
(6-67)
(with a ^ /?, and for simplicity I will only consider TV = 1). The result is d>±(d,RJ(jl,jl)\l-(M^nTmTn)a0
- l(
-(6.68)
The operator inside the matrix element can be split into left and righthanded parts (i.e. proportional to P+ and P_): 4J.S - ^R\
+ P_ Uj.S
+ ^R\
,
(6.69)
where S is the rotation generator in the spin-half representation of the first SU(2) embedded in SO(4), and S is the corresponding generator of the second SU(2). The eigenvalues of 2J.S are —j — 1 and j for j > 0 and 0 for j = 0, so, as before, the most negative eigenvalues provide the strongest bounds. d > P+ Uj + 2 - 25j0
+ ^R\+P-(2]
+ 2 - 2S-J0
-
^R\
. (6.70)
388
Although P+ and P_ were denned in the superconformal algebra to act on spinors, they have been implicitly extended to act as projection operators on superpartners of spinors in the same way they act on the spinors themselves. The bound we have found above is not the whole story. The bound (6.70) is a necessary condition [43] but if either j or j are zero there are more restrictive conditions [40,41] that require: d > dmax = max (2j + 2 + ^R, 2j + 2 - ^Rj > 2 + j + j or d = | | i ? | .
(6.71)
Here dmax is the boundary of the continuous range, while the isolated point (which happen only in the supersymmetric case [40,41]) is achieved for chiral and anti-chiral supermultiplets. We can check the result (6.70) by considering a chiral supermultiplet with i?-charge Rsc. The primary field is the lowest component of the supermultiplet, which is just the scalar with (j,j) = (0,0). Equality is achieved in the necessary condition (6.70) on the scaling dimension of the scalar component at: d = \Rsc ,
(6.72)
in agreement with the results of [40,41]. It is this special case which is of interest for the chiral supermultiplet. The reason is that superconformal multiplets are generally much larger than supermultiplets (due to the existence of the S' generator), but the superconformal multiplets can degenerate ("shorten" in the language of ref. [45]) precisely at the point where some of the states have zero norm, which is the point where the bound is saturated. In other words, in a superconformal theory, chiral supermultiplets are "short" multiplets.
6.5.
Duality
In a conformal theory (even if it is strongly coupled) we don't expect any global symmetries to break, so 't Hooft anomaly matching should apply to any description of the low-energy degrees of freedom. The anomalies of the mesons and baryons described above do not match to those of the quarks and gaugino. However Seiberg [29] found a non-trivial solution to the anomaly matching using a "dual" SU(F — N) gauge theory with a "dual" gaugino, "dual" quarks and a gauge singlet "dual mesino" with the
389
following quantum numbers 29 : SU(F-
N)
SU{F)
SU{F)
C/(l)
U(1)R
N F-N N F-N
N F N F r> F-N Z F
n
1
q
n n
1
D
mesino
1
•
•
q
0
(6.73)
In the language of duality the dual quarks can be thought of as "magnetic" quarks, in analogy with the duality between electrons and magnetic monopoles. The anomalies of the two dual theories match as follows: SU{Ff
:-{F-N)
U(1)RSU(F)2 U{1)
N
N ^ < N-F/ri F -2N ArN l „ (F-N)+ — F F~ 2
U(1)SU{F)2
3
+F
1
AT 2
2F
:0
C/(l) : 0 U(1)U(1)2R : 0 N -F 2{F-N)FJ7(l) F = ~7V 2 N -F 2{F-N)FU(l) F 2N4 + N2 - 1 2
F
-2N F
F2 + (F-
F-2N
N)2 - 1
F2 + (F - N)2 - 1
F
U{l)2U{l)j
N ^F-NJ
N
-F F
2F(F - N) =
-2N2
(6.74)
This theory admits a unique superpotential: (6.75) where represents the "dual" squark (that is the scalar superpartner of the "dual" quark q) and M is the dual meson (the scalar superpartner of the "dual" mesino). This ensures that the two theories have the same number 29
As usual only the R-charge of the scalar component is given, and i?[fermion] R [scalar] — 1.
390
of degrees of freedom since the M equation of motion removes the color singlet (/>(/> degrees of freedom. The counting works because both theories have the same number of massless chiral superfields at a generic point in moduli space: 2FN - 2(N2 - 1) = 2FN - 2(N2 - 1) ,
(6.76)
where N = F — N is the number of colors in the dual theory. The dual theory also has baryon operators: b
i l , .•,IF-N
bn,.
• •AF
_
(
JJIF-NIF-N
Arllil
r n ^ i •
—N
,
.,TIF-N • •
ni
d> • ~rnF-NtF-N
e '-
(6.77) (6.78)
Thus the two moduli spaces have a simple mapping
Bilt. Bh>-
..,iN
•<-> C i i , . .•;iN,jl,-
...xN
^
e i u
,3FN
..JF-N'S
. •;iN,jl,-
•JF-NT.
•
JFN
(6.79)
The one-loop (3 function in the dual theory is (3(g) oc -53(37V - F) = -g3(2F
- 3JV) .
(6.80)
So the dual theory loses asymptotic freedom when F < 3N/2. In other words, the dual theory leaves the conformal regime to become infrared free at exactly the point where the meson of the original theory becomes a free field which implies very strong coupling for the underlying quarks and squarks. We can also apply the Banks-Zaks [36] argument to the dual theory [46]. When F = 37V - eiV
=
3
2 V
-(l+e-)N ) 6/
(6.81)
there is a perturbative! fixed point at 8TT2
=
A2
=
TV
3 ^ _ ! ( 16TT2 ~—f
3N
F
/
1
+
N
(6.82) (6.83)
At this fixed point D(M<j>4>) = 3, so the superpotential term is marginal. If the superpotential coupling A = 0, then M has no interactions (it is a free field) and therefore its dimension is 1. If the dual gauge coupling is set
391
close to the Banks-Zaks fixed point and A « 0 then we can calculate the dimension of (jxf) from the i? sc charge for F > 3N/2: D(H>) =
3(F - N) 3N K >= — < 2 . p
, (6.84)
So the superpotential is a relevant operator (not marginal) and thus there is an unstable fixed point at _a yq
8TT2
~2
N
= yqt = ———2 e * 3 N - 1 A2 = 0 .
(6.85) (6.86)
In other words the pure Banks-Zaks fixed point in the dual theory is unstable and the superpotential coupling flows toward the non-zero value given in (6.83). So we have found that not only does SUSY QCD have an interacting IR fixed point for 3N/2 < F < 3N there is a dual description that also has an interacting fixed point in the same region. The original theory is weakly coupled near F = 3N and moves to stronger coupling as F is reduced, while the dual theory is weakly coupled near F = 3N/2 and moves to stronger coupling as F is increased. For F < 3N/2 the IR fixed point of the dual theory is trivial (asymptotic freedom is lost in the dual): Qt = 0 2
A = 0.
(6.87) (6.88)
Since M has no interactions it has scaling dimension 1, and there is an accidental U(l) symmetry in the infrared. For this range of F, Rsc is a linear combination of R and this accidental U{1). This is consistent with the relation D{M) = (3/2)i? sc (M). Surprisingly in this range we find that the the IR is a theory of free massless composite gauge bosons, quarks, mesons, and their superpartners. We can lower the number of flavors to F = N + 2, but to go below this requires new considerations since there is no dual gauge group SU(F — N) when F = N + 1. We will examine what happens in this case in detail later on. To summarize, for 3./V > F > 3N/2 what we have found is two different theories that have IR fixed points that describe the same physics. For 3N/2 > F > N + 1 we have found that a strongly coupled theory and an IR free theory describe the same physics. Two theories having the same
392
IR physics are referred to as "being in the same universality class" by condensed matter physicists. This phenomenon also occurs in particle theory, a well known example of this is QCD and the chiral Lagrangian. Having two different description can be very useful if one theory is strongly coupled and the other is weakly coupled. (Two different theories could not both be weakly coupled and describe the same physics.) Then we can calculate non-perturbative effects in one theory by simple doing perturbative calculations in the other theory. Here we see that even when the dual theory cannot be thought of as being composites of the original degrees of freedom (since there is no particle interpretation of conformal theories it doesn't make sense to talk about composite particles) it still provides a weakly coupled description in the region where the original theory is strongly coupled. The name duality has been introduced because both theories are gauge theories and thus there is some resemblance to electric-magnetic duality and the Olive-Montonen duality [47] of Af = 4 SUSY gauge theories. However. Olive-Montonen duality is valid at all energy scales, while for these Af = 1 theories, as we go up in energy the infrared correspondence of Seiberg duality is lost.
6.6. Integrating
out a flavor
If we give a mass to one flavor in the original theory we have added a superpotential term Wmass = m ¥ F $ f .
(6.89)
In the dual theory we then have the superpotential Wd = XMffifa + mM£ ,
(6.90)
where we have made use of the mapping (6.79). Because of this mapping it is common (though somewhat confusing) to write ~ M XM = — ,
(6.91)
which trades the coupling A for a scale // and uses the same symbol, M, for fields in the two different theories that have the same quantum numbers. With this notation the dual superpotential is Wd = -Mffifa
+ mMf
.
(6.92)
393
The equation of motion for Mf is: dWd
dMf
1-rF
M
>F
+m =0
(6.93)
So the dual squarks have VEVs: -rF ,
(6.94)
-fj,m .
We saw earlier that along such a D-flat direction we have a theory with one less color, one less flavor, and some singlets. The spectrum of light degrees of freedom is: q' q' M' q" q"
s
Mf MF
Mf
SU(F -ND D 1 1 1 1 1 1 1
-1)
SU{FD 1 D D 1 1 D 1 1
1)
SU(F - 1) 1
•
•1 •
(6.95)
1 1
•1
The low-energy effective superpotential is:
WeS = - (tf)MF'; + (fcjMfJ* + Mfs) + -M'q
(6.96)
So we can integrate out M3F, ", Mf, 4> , Mf, and S since they all have mass terms in the superpotential. This leaves just the dual of SU(N) with F — 1 flavors which has a superpotential W = -M'4>'4>' .
(6.97)
Similarly one can check that there is a consistent mapping between the two dual theories when one flavor of the original squarks and anti-squarks have £>-flat VEVs, and the corresponding meson VEV gives a mass to the dual quarks and dual squarks, which can then be integrated out. The analysis of baryonic D-flat directions was done in ref. [48]. 6.7.
Consistency
We have seen that Seiberg's conjectured duality satisfies three non-trivial consistency checks:
394
• The global anomalies of the original quarks and gauginos match those of the dual quarks, dual gauginos, and "mesons". • The moduli spaces have the same dimensions and the gauge invariant operators match: M
^M t
tl,---,iN,3l,---3F-NLr
-Dll,...,8JV "
^ii,...,*,, ^
il €
--'iNdl-JF~Nbju...jFN
.
(6.98)
• Integrating out a flavor in the original theory results in an SU(N) theory with F — 1 flavors, which should have a dual with SU(F — N — 1) and F — 1 flavors. Starting with the dual of the original theory, the mapping of the mass term is a linear term for the "meson" which forces the dual squarks to have a VEV and Higgses the theory down to SU(F - N - 1) with F - 1 flavors. The duality exchanges weak and strong coupling and also classical and quantum effects. For example in the original theory M satisfies a classical constraint 30 rank(M) < N. In the dual theory there are F — rank(M) light dual quarks. If rank(M) > N then the number of light dual quarks is less than N = F — N, and an ADS superpotential is generated, so there is no vacuum with rank(M) > N. Thus in the dual, rank(M) < iV is enforced by quantum effects. 7. Confinement in SUSY QCD For certain special cases (particular values of the number of flavors) the description in terms of a dual gauge theory breaks down since there is no possible dual gauge group. Remarkably one finds that the low-energy effective theory simply contains mesons and baryons and that their properties can actually be derived from the case where there is a dual gauge group. 7.1. JF = N:
Confinement
with Chiral Symmetry
Breaking
For SUSY QCD with F = N flavors, Seiberg [34] found that all the 't Hooft anomaly matching conditions can be solved with just the color singlet meson and baryon fields, that is no dual quarks are required. Thus this theory is confining in the sense that all of the massless degrees of freedom are color singlet particles. Discussed at the end of Section 6.1.
395
Recall that the classical moduli space of SUSY QCD is parameterized by Ml = Wn
(7.1) nN
Bh,...,iN=^niil...^nNiNe '--' B
= $
...$
(7.2) £„!,..,„„ .
(7.3)
For F = N flavors the baryons are flavor singlets: B = ^-'ip
Bix_iF
(7.4)
B = eiu...tiFBiu-'iF
.
(7.5)
We also saw that classically these fields satisfy a constraint: detM = BB~ .
(7.6)
With quark masses turned on we have: (M/) = ( m - 1 ) ^ ( d e t m A 3 i V - F ) ^ .
(7.7)
Taking a determinant of this equation (and using F = N) we have det(M) = det (m" 1 ) det mA2N
= A2N ,
(7.8)
independent of the masses. However det m / 0 sets (B) = (B) = 0, since we can integrate out all the fields that have baryon number. Thus the classical constraint is violated! To understand what is going on it is helpful to use holomorphy and the symmetries of the theory. The flavor invariants are: detM B B A2JV
U(l)A 2N N N 2N
£7(1) 0 N -N 0
U(1)R_ 0 0 0 0
(7.9)
vanishes since F = N. Thus a o f „MV t h f > oHu^iivo, snnarlc: ,,«»v u„™ uiiv ii,-vu Ui.rP'e , 6 >, ^J. F -, vanishes generalized form of the constraint that has the correct A —> 0 and B,B —> 0 limits is
detM -BB = A™ (l + Y: Cpq ^ J j ^
V9) ,
(7-10)
with p, q > 0. For (BB) ;§> A2N the theory is perturbative, but with Cpq / O w e find solutions of the form d e t M « (BB)*^
,
(7.11)
396
which do not reproduce the weak coupling A —> 0 limit, thus we can conclude Cpq = 0. Therefore the quantum constraint is: detM - ~BB = A2N .
(7.12)
First note that this equation has the correct form to be an instanton effect since e -s l n .t
oc Ab = A2N .
(7.13)
Also note that we cannot take M = B = B = 0, that is we cannot go to the origin of moduli space (this situation is referred to as a "deformed" moduli space). This means that the global symmetries are at least partially broken everywhere in the quantum moduli space. For example consider the following special points with enhanced symmetry: Ml = A25{, B = ~B = 0 and M = 0, BB~ = -A2N. In the first case, with M = A2, the global SU(F) x SU{F) x U(l) x U(1)R symmetry is broken to SU(F)d x [7(1) x U(1)R, that is the theory undergoes chiral symmetry breaking and, as in non-supersymmetric QCD, the chiral symmetry is broken to the diagonal subgroup. For BB = —A2N the global symmetry is broken to SU(F) x SU(F) x U(1)R, that is baryon number is spontaneously broken. For large VEVs we can understand this symmetry breaking as a perturbative Higgs phase with squark VEVs giving masses to quarks and gauginos (See Figure 3). There is no point in the moduli space where gluons become light, so there are no singular points, and the moduli space is smooth. This is an example of a theory that exhibits "complementarity" [49,50] since we can go smoothly from a Higgs phase (large VEVs) to a confining phase (VEVs of 0(A)) without going through a phase transition. Complementarity holds in any theory where there are scalars (in our case they are squarks) in the fundamental representation 31 of the gauge group. In such a theory we can take any colored field and, by multiplying by enough scalar fields, we can form a color singlet operator 32 . In other words any color charge can be screened. In a theory where screening does not happen a Wilson loop 33 can obey an area law (which indicates confinement) or a •"•Also known as defining or faithful representations of the gauge group. I n the perturbative regime this color singlet operator is just the original colored field multiplied by VEVs, while in the regime of small VEVs it can be a complicated nonperturbative object. 33 A Wilson loop is a closed quark loop. If the expectation value of the loop goes like e —C1 , where A is the area enclosed by the Wilson loop it is said to obey an area law. If the expectation value of the loop goes like e~C2L, where L is the length of the perimeter of the Wilson loop it is said to obey a perimeter law. 32
397
perimeter law (which happens in a Higgs phase) and it can therefore act as a gauge invariant order parameter which can distinguish between the area law confinement phase and the Higgs phase. In a screening theory a Wilson loop will always obey a perimeter law (even if confinement occurs, since all the dynamics takes place along the perimeter of the loop where the screening occurs) as it does in a Higgs phase. Thus in screening theories there is no gauge invariant order parameter that can distinguish between a confining phase and a Higgs phase, so there can be no phase transition between the two regimes and the theory exhibits complementarity.
Figure 3. Squark VEV (dashed line with cross) gives a mass to a quark (solid lines) and a gaugino (wavy line).
7.2. F = N: Consistency
Checks
The constraint (7.12) can be put in the form of an equation of motion arising from a superpotential by introducing a Lagrange multiplier field, which we can refer to as X: Wconstraint = X(detM-BB-A2N)
.
(7.14)
We can now check the consistency of the confined picture by adding a mass for the Nth flavor. It is convenient to re- write the meson field as
M=
r * ;
,
a-is)
where M is an (N — 1) x (N — 1) matrix. We can then write the mass term for the iVth flavor in the confined description as Wmass = rnY, so that the full superpotential is W = X (detM - ~BB - A2N) + mY .
(7.16)
398
We then have the following equations of motion: dW — — = -IB = 0 dW ^L = -XB = 0 dB
(7.17) (7.18) '
K
dW —i=Xcoi{N>)=0 (7.19) dW — =Xcof(PJ)=0 (7.20) dW ~ —-=XdetM + m = 0 , (7.21) where cof(Mj) is the cofactor of the matrix element Mj. The solution of these equations is: X = -m (detM)
(7.22)
B = B~ = Nj = Pi = 0 .
(7.23)
Plugging this solution into the equation of motion for X (the constraint equation) gives ^ = YdetM - A2N = 0 . (7.24) oX So, putting this result into the full superpotential (7.16), we find the lowenergy effective superpotential is
WeS = "^Z
.
(7.25)
detM Using the usual matching relation for the holomorphic gauge coupling we find mA2" = A™+I1} (7.26) so A2N+1
WeS = ^ ^ . (7.27) detM This is just the Afneck-Dine-Seiberg superpotential for SU(N) with N — 1 flavors As a further consistency check let's consider in more detail the points in the moduli space with enhanced symmetry. When M/ = A25j, B = B = 0 the global symmetry is broken to SU(F)d x U{1) x U(1)R. In terms of the elementary fields, the $ and $ VEVs break SU(N) x SU(F) x
399
SU(F) to SU(F)d. The quarks transform as • x D = 1 + Ad under this diagonal group, while the gluino transforms as Ad. The composites have the following quantum numbers 34 :
M-TrM TrM B B
SU(F)d Ad 1 1 1
f/(l) 0 0 N -N
U(1)R _ 0 0 0 0
The field TrM gets a mass with the Lagrange multiplier field X. non-trivial anomalies match as follows (recalling that F = N):
U(1)2U(1)R : U(1)R: U(1)R: U(1)RSU(F)2:
elem. comp. -2FN -2N2 2 2 -2FN + N -1-(F -1)-1~1 -2FN + N2-1~{F2-1)-1~1 -2N + N -N .
(7.28)
The
(7.29)
At M = 0, B~B = -A 2 J V only the U{\) is broken. The composites transform as:
M B B
SU(F) D 1 1
SU(F) D 1 1
U(1)R 0 0 0
(7.30)
The linear combination B + B gets a mass with the Lagrange multiplier field X. The anomalies match as follows: SU(F)3 : U(1)RSU(F)2 U(1)R: U(1)3R:
elem. N : -N\ -2FN + N2-1 -2FN + N2-1
-(F2
comp. F -F\ -F2-l - 1) - 1 - 1 .
(7.31)
which, again, agree because F = N. 34
As usual only the R-charge of the scalar component is given, and it[fermion] R [scalar] — 1.
400
8. S-Conflnement in S U S Y QCD 8.1. F = N + 1; Confinement Breaking
without
Chiral
Symmetry
For SUSY QCD with F = N+l flavors Seiberg [34] again found that all the 't Hooft anomaly matching conditions can be satisfied with just the color singlet meson and baryon fields; thus this theory is also confining. This theory also exhibits complementarity [49,50] since screening can occur (as discussed at the end of section 7.1). However, the theory with F = N + 1 flavors does not require chiral symmetry breaking, that is we can go to the origin of moduli space. Furthermore the theory develops a dynamical superpotential. Confining theories that screen, do not spontaneously break global symmetries, and have dynamical superpotential have been dubbed "s-confming" [51]. To see that spontaneous chiral symmetry breaking does not occur we begin by recalling the classical constraints on the meson and baryon fields. For F = N + 1 flavors the baryons are flavor anti-fundamentals (and the anti-baryons are flavor fundamentals) since they are anti-symmetrized in N = F -1 colors: Bi
=
e^-^Bi,,...^ ,
Bi = eilt...,iN,iB
' '
.
(8.1) (8.2)
With this notation the classical constraints are: (Af-^jdetM = B % , M3iBi = M^Bj = 0 .
(8.3) (8.4)
With non-zero quark masses we have: (M/) = (m- 1 )^(det™A 2 J V - 1 )* , (B*) = (B3) = 0 .
(8.5) (8.6)
So taking a determinant of equation (8.5) gives
(Af-^jdetM = mJA 2 ^" 1 .
(8.7)
Thus we see that the classical constraint is satisfied as m*- —> 0. Taking this limit in different ways we can cover the classical moduli space, so the classical and quantum moduli spaces are the same. In particular chiral symmetry remains unbroken at M = B = B = 0.
401
The most general superpotential allowed for the mesons and baryons is: W •
1 A2JV-1
aB^lBj
+ f3detM + detM /
detM
&MiB<
where / is an as yet unknown function. Actually only / = 0 reproduces the classical constraints: dW _ ~ BM{ ~ A2N dW 1
[aB'Bj + ^(Af-^jdetM] - 0 ;_
(8.9)
n
(8.10)
^Z- = - — 2 A-r - a B ' M / = 0
(8.11)
dW dBj
1
A
-!
provided that (3 = — a. To determine a, consider adding a mass for one flavor so that we can compare with the results for F = N flavors.
a^ W = — 2W-1
BlM\Bj -- detM + mX , B^MlBj
A
(8.12)
where
-
(
^
)
B={U\B')
I
.
5.13)
,
U4)
'-(H
5.15)
For this theory with one massive flavor and F = N light flavors we have the following equations of motion: dW _ a - (B'U - cof (Y)) = 0 ~8Y ~ A™~~ dW dW dU dW dU
a A2N-1
a a
ZB'
(8.16) (8.17)
0
(8.18)
B'Y = 0
(8.19) (8.20) (8.21)
402
The solution of these equations is: Y =Z =U=U=0, detM' - B'B' =
(8.22)
mA2N 1
~ = 1A™ . (8.23) a a Equation (8.23) gives the correct quantum constraint (7.12) for F = N flavors if and only if a = 1. Plugging the solutions of the equations of motion into the superpotential (8.12) we find the effective superpotential is w
<* = X27TT (B'Z'
~
detM
' + mA2""1) •
(8-24)
With the usual matching relation for the holomorphic gauge coupling we find m A 2 " " 1 = A2NNN ,
(8.25)
so ^eff = ^ ^ n (B'B' - detM' + A2NNN)
.
(8.26)
Holding AJV,JV fixed as m —> oo implies that A —> 0, so X becomes a Lagrange multiplier field in this limit. Thus we can completely reproduce the superpotential (7.14) used to discuss F — N flavors in section 7.2. Thus to summarize we have determined that the correct superpotential for the confined description of SUSY QCD with F = N + 1 flavors is: 1 W = A ,2ni >Vr- , IB^fBj - detM 3.27) Since the point M = B = -B = 0 i n o n the quantum moduli space, we should worry about what singular behavior occurs there. Naively gluons and gluinos should become massless. What actually happens is that only the components of M, B, B become massless. That is we simply have confinement without chiral symmetry breaking. This is the type of theory that 't Hooft was searching for when he proposed his anomaly matching conditions [28]. The color singlet composites transform as 35 : M B B 35
SU(F)
SU(F)
•
•
•1
1
•
U{1) 0 N -N
U(1)R 2 F N F N F
3.28)
As usual only the .R-charge of the scalar component is given, and _R[fermion] = it[scalar] — 1.
403
Some of the anomaly matchings go as follows: elem. N
SU(F)3 : U(1)SU{F)2 : U{1)RSU{F)2 U(1)R : U(l)R :
comp. F-l
l y 2 __JV JV 2F 2
JV 2 2-F F _|_ F 2 ^
2
-^F2^
-N -l J
2
F
2(N - F) 2
- (f) 2NF + N -1 (^Y
3.29)
N-F
F + (^Y
2F
which agree because F = N + 1. 8.2. Connection
to theories
with F > iV + 1
We can also check that the confined descriptions of the theories with F = N and F = N + 1 flavors are consistent with dual descriptions of the theories with more flavors. Consider the dual theory for F = N + 2: SU(N + 2)
C/(l)
U(1)R
•
1
JV 2
D
1
•
JV JV+2 JV JV+2
1
•
D
SU(2)
SU{N + 2)
q
•
q M
JV 2
3.30)
4 JV+2
0
The dual theory has a superpotential .31)
W = -M~4>
As we have seen in section 6.6, giving a mass to one flavor in the corresponding SUSY QCD theory produces a dual squark vev —F
3.32)
{
SU(N + 1)
C/(l)
q'
•
1
A^
V
1
•
-N
M'
D
D
0
U{1)R JV JV+l JV JV+l 2 JV+l
That is after integrating out the gauge singlet fields which are massive.
,33)
404
Comparing with the confined spectrum (8.28) we see that we should identify qH = cBi ,
(8.34)
Wj = cBj ,
(8.35)
where c and c are some constant re-scalings. After integrating out the massive gauge singlet fields the remnant of the tree-level dual superpotential is Wtree = -BiM'iB3 . (8.36) A* Since we have completely broken the dual gauge group we expect that instantons will generate extra terms in the superpotential. Indeed one finds:
WW. = ^ ^ d e t (ML) ,
(ftp)
^
(8.37)
/
where A is the intrinsic holomorphic scale of the dual gauge theory. A simple way to verify that this superpotential is generated by instantons is to check that the corresponding interaction between two fermion components of M (the mesinos) and N — 1 mesons is actually generated. The instanton contribution to this interaction can be seen in Figure 4. So the effective superpotential agrees with the result for F — N + 1 flavors (8.27): W
<* = x i
[B'MfBj - detM'] ,
(8.39)
if and only if c
c=T^rT, \2N-1 '
Iy
NJV,iV+2 N4-2
(8.40)
1
fJiN+2m
A2N-1
3.41)
•
The second relation (8.41) actually follows from a more general relation 13N-FA3N-F
=
{_1}F-iVMF
_
(g
42)
To see why this relation is true consider generic values of (M) in the dual of SUSY QCD. All the dual quarks are massive, so we have a pure SU(F-N)
405
N-1 Figure 4. Instanton contribution to the interaction between two mesinos (external straight lines) and N — 1 mesons (dash-dot lines). The instanton has 4 gaugino legs (internal wavy lines) and N + 2 quark and anti-quark legs (internal straight lines). A squark VEV and an anti-squark VEV are shown (dashed line with cross) gives a mass to a quark and a gaugino or a anti-quark and gaugino, however they can be arbitrarily many insertions of these VEVs which must be re-summed.
gauge theory. The intrinsic scale of the dual low-energy effective theory is given by matching:
Af = A3^-Fdet
.43)
This theory undergoes gaugino condensation, and as we have seen the effective superpotential is: WL = NA3L
(8.44)
406
= (F - N) i
=
<
*
-
"
(
j
•
*
*
"
)
(8.45)
(
•
8
-
4
6
)
where we have used equation (8.42). Adding a mass term m^Mj gives: Mi = {m-1yi(detmh3N-F)j}
,
(8.47)
which we have already seen is the correct result. If we consider the dual of the dual 37 of SUSY QCD then (with the assumption that (A = A) equation (8.42) implies A3N-F13N-F
=
^f-N-F
(8>48)
So, since F — N = N, we must have for consistency ji=-li
.
(8.49)
If we write the composite meson of the dual quarks as: N] = tfa
,
(8.50)
and the dual-dual squarks as d, then the superpotential of the dual of the dual is Wdd = -4-1 ij + -^Nl M M The equations of motion give dW 1 ^L = -Ni=0 dW
l-i
.
(8.51)
(8.52) 1
•
— L = -7dj + -M\ = 0 (8.53) y dNl n 3 n J •So, since Jl = —fi, we can identify the original squarks with the dual-dual squarks: *i = dj •
(8-54)
Plugging the solution (8.52) back into the dual-dual superpotential (8.51) we find that it vanishes and we conclude that the dual of the dual of SUSY QCD is just SUSY QCD. 37
Which has F - N = N colors.
407
9. Duality for
SO(N)
Dualities for SO(N) theories have also been found [52], and they exhibit some interesting new features since they can be non-screening if there are no spinor representations. Also there are massless composites made of squarks and gluinos. 9.1. The SO(N) Spaces
Theories
and Their Classical
Moduli
An SO(N) gauge theory with F quarks in the vector representation has a global SU(F) x U(1)R symmetry as follows: SO{N)
SU(F)
•
i-|
U(1)R F+2-JV
LJ Q F Since there are no dynamical spinors in this theory, static spinor sources cannot be screened, so there is a distinction between area-law confining and Higgs phases. Recall that the adjoint of SO(N) is the two-index antisymmetric tensor. For odd N, there is one spinor representation, while for even N there are two in-equivalent spinors. For N = Ak the spinors are self-conjugate, while for TV = 4fc + 2 the two spinors are complex conjugates. The simplest irreducible representations are summarized in the following two tables.
SO(2N +1) d(r) 2N + 1
Irrep
•
2T(r) 2 2 JV-2
S
B m
N{2N+1) AN -2 (N + 1)(2N + 1) - 1 4/V + 6
Irrep
• S S,(S>)
B m
SO(2N) d(r) 2/V •
2T(r) 2
2N-1
2JV-3
2N-1
2 JV-3
N{2N - 1) N(2N+1)-1
AN -A AN + A
The one-loop /3 function coefficient for A^ > 4 is b = 3(A - 2) - F .
(9.3)
408
Solving the D-flatness conditions one finds that up to flavor transformations, the classical vacua for F < N are given by
hi
\ VF
<$)
(9.4)
0 \0
... 0 )
At a generic point in the classical moduli space the SO(N) gauge symmetry is broken broken to SO(N-F) and there are NF-N(N-1) + (N-F)(NF — 1) massless chiral supermultiplets. For F > N the vacua are: 0 ... (T
Vi
($)
(9.5) vN 0 . . . 0,
At a generic point in the moduli space the SO(N) gauge symmetry is broken completely and there are NF — N(N — 1) massless chiral supermultiplets. We can describe these light degrees of freedom in a gauge invariant way by scalar "meson" and (for F > N) "baryon" fields and their superpartners: Mji = Q^i
,
(9.6)
B[iu..,iN] = % • • • * « ] ,
(9-7)
where [] denotes antisymmetrization. Up to flavor transformations the moduli space is described by:
J
N
(M)
\ (BI,...,N)
Vl
• ••VN
(9.
°/
(9.9) (9.10)
with all other components set to zero. The rank of M is at most N. If the rank of M is N, then B = i v d e t ' M , where det' is the product of non-zero eigenvalues.
409
9.2. The Dynamical
Superpotential
for F < N — 2
To construct the effective superpotential we should look at how the chiral superfields transform under the anomalous axial U(1)AU(1)A a
W A6 detM
U(1)R
0 2F 2F
1 0 2(F + 2-
(9.11) N)
So we see it is possible to generate a dynamical superpotential (the analogue of the ADS superpotential for SU(N)): Wdy
CN,F
(9.12)
\detMJ
for F < N - 2. 9.3.
(—Y
Duality
For F > 3(N — 2) we lose asymptotic freedom, so the theory can be understood as a weakly coupled low-energy effective theory. For F just below 3iV we have an infrared fixed point. A solution to the anomaly matching for F > N — 2 is given by: SO(F - N + 4)
SU(F)
q
•
M
1
• m
U(1)R N-2 F 2(F+2-N) F
For N > N — 3 this theory admits a unique superpotential: (9.13) The dual theory also has baryon operators: _§[ii,.-,iff] =Ah
...^1 .
(9.14)
There are additional hybrid "baryon" operators in both theories since the adjoint is an antisymmetric tensor. In the original SO(N) theory we have:
Hu
W $r- . . $ ;*JV-lN_4]
(9.15)
410
While in the dual theory we have: £[
=
w ^
... 4,^-tl .
(9.16)
The two theories thus have a mapping of mesons, baryons, and hybrids: M
^M
h • •Hl,...,lN-4
< - > F-
• tl,---,iFJD
Ftii'--->iN
t
H[n,...,iN-2]
„
i?i 2]
eil,...,iFHt"-
-
•
(9.17)
- 3(N - 2)) .
(9.18)
The dual /3 function is 0(g) ex -g3(3(N
- 2) - F) = -g3(2F
So the dual theory loses asymptotic freedom when F < 3(N — 2)/2. When F = 3N -eN
,
(9.19)
there is a perturbative IR fixed point in the dual theory. One can check that the exact /? function vanishes in this range using the relation between dimensions and R charges in a superconformal theory. So we have found that SO(N) with F vectors has an interacting IR fixed point for 3(N — 2)/2
Cases
For F < N - 5, SO(N) breaks to SO(N - F) D SO(5), which undergoes gaugino condensation and produces the dynamical superpotential: /16A3(AT-2)-F
W d y n oc (AA) oc (^
- ^
j
.
(9.20)
For F = N - 4, SO(N) breaks to SO(4) ~ SU(2)L x SU(2)R, so there are two gaugino condensates /16A2JV_1V
1 ^condensate = 2 ( A A ) L + 2(XX)R
= - ( e L + eR)
(
^
^
j
, (9.21)
411
where es = ±1 .
(9.22)
So there are four vacua corresponding to two physically distinct branches: one with (ex, + eR) = ±2 and the other with (ex + eR) = 0. The first branch has runaway vacua, while the second has a quantum moduli space. At M = 0, the composite M satisfies the 't Hooft anomaly matching conditions. One can check that this only happens for F = N — 4. So we have another example of confinement without chiral symmetry breaking, this time without any baryons. Integrating out a flavor on the first branch gives the correct runaway vacua of the previous case (F + N — 5), while on the second branch we find no supersymmetric vacua after integrating out a flavor, which is a consistency check. For F = 7V-3, SO(N) breaks to SO(4) ~ SU{2)L x SU(2)R, which then breaks to SU(2)d ~ 50(3) so there are instanton effects (since H^G/H) = H^(SU(2)) = Z) and gaugino condensation y^2JV-3
W conde „ sat e = 4(1 + e) — — , detM
(9.23)
e = ±1 ,
(9.24)
where corresponding to the two phases of the gaugino condensate. So there are two physically distinct branches: one with e = 1 and the other with e = — 1. The first has runaway vacua, while the second has a quantum moduli space. Integrating out a flavor, we would need to find two branches again, so W ^ 0 even on the second branch. For this to be true we must have some other fields that interact with M. We also know that M does not match the anomalies by itself. The solution of the anomaly matching is given by: SU{F) q M
• m
U(1)R AT-2 F
2^+2-iV)
The most general superpotential is
where f(t) is an unspecified function. Adding a mass term gives qF = ±iv ,
(9.26)
412
which gives us the correct number of ground states. Note that the operator mapping must be: -.JV-
Q1
(9.27)
w„r
which is a hybrid operator. For N = 4 this is a gluino-ball. This is an example of confinement without chiral symmetry breaking with hybrids. Starting with the F = N dual which has an 50(4) gauge group, and integrating out a flavor there will be instanton effects when we break to 50(3) so the dual superpotential is modified in the case F = N — 1: W
M,
Vrh1 -
2/j, '
6
rdetM
4A2N-5
(9.28)
For F AT — 2 we see in the original theory that we can generically break to SO(2), while in the dual we also break so 5*0(2) ~ U(l). This case needs a separate treatment [52]. 10. Sp(2N) 10.1. Duality
and Chiral Theories for
Sp(2N)
An Sp(2N) gauge theory 38 with 2F quarks (F flavors) in the fundamental representation has a global SU(2F) x U(1)R symmetry as follows:
Q
Sp(2N)
SU(2F)
•
•
U(1)R F-l-N F
(10.1)
Recall that the adjoint of Sp(2N) is the two-index symmetric tensor. The simplest irreducible representations are summarized in the following table. Irrep
•
B
d(r) 2N
T(r)
N(2N-1)-1 N(2N+1)
2N-2 2N + 2
N(2N-l)(2N-2)
cm 3
_
2 N
N(2N+l)(2N+2) 2N(2N-1)(2N+1)
We'll use the notation that Sp(2) ~
_
SU(2).
2N
(2iV-3)(2JV-2) _ 2 (2JV+2)(2JV+3) 2
(2N)2 - 4
(10.2)
413
The one-loop /3 function coefficient for N > 4 is b = 3(2iV + 2) - 2F .
(10.3)
The moduli space is parameterized by a "meson" Mji = QjQi ,
(10.4)
which is antisymmetric in the flavor indices i,j. The holomorphic intrinsic scale (spurion) and the flavor singlet chiral superfield transform under the global symmetry as: U(1)A U{l)R 2F 0 PfM 2F 2(F - 1 N)
(10.5)
where the Pfaffian of a 2F x 2F matrix M is given by PfM
'M,„
(10.6)
• Mi,
So we see it is possible to generate a dynamical superpotential W d y n oc
At
(10.7)
PfM
for F < N + 1. For F = N + 1 one finds confinement with chiral symmetry breaking PfM = A2**"1"1) .
(10.8)
For F = N + 2 one finds confinement without chiral symmetry breaking with a superpotential: W = PfM .
(10.9)
A solution to the anomaly matching for F > N — 2 is given by: Sp(2(F - N -- 2 ) ) q
•
M
1
SU(2F)
B
U(1)R N+l F 2(F+2-N) F
This dual theory admits a unique superpotential: (10.10) For 3(AT + l ) / 2 < F < 3(N + 1) we have an IR point. For N + 3 < F < 3(N + l ) / 2 the dual is IR free and the low energy theory is made up of quarks, gluons, mesons, and their superpartners. A complete discussion of Sp(N) theories with fundamentals is given in [53].
414
10.2. Why Chiral Gauge Theories
are
Interesting
We would eventually like to use non-perturbative methods like duality for understanding SUSY gauge theories that dynamical break SUSY. Usually vector-like gauge theories don't break SUSY, while chiral gauge theories can. If a theory is vector-like we can give masses to all the matter fields. If these masses are large we have a pure gauge theory that has gaugino condensation but no SUSY breaking. By Witten's index argument [26] we can vary the mass but the number of bosonic minus fermionic vacua doesn't change. If taking the mass to zero does not move some vacua in from or out to infinity, then the massless theory has the same number of vacua, and SUSY is not broken. As our first example of a chiral gauge theory consider
Q T
SU(N)
SU(N + 4)
D
•
m
1
I will not write down the charges under the two global U(l)'s. This theory is dual to [54] SO(8)
SU(N + 4)
q
•
D
p
S
1
U ~ detT
1
1
M-QTQ
1
m
with a superpotential W = Mqq + Upp .
(10.11)
This theory is vector-like! The dual /3 function coefficient is: b = 3(8-2)-(N
+ 4)-l
= 13-N
.
(10.12)
So the dual is IR free for N > 13. 11. S- Confinement We need some semi-systematic way to survey chiral gauge theories. One way to do this is to generalize well understood dual descriptions. The simplest of these is confinement without chiral symmetry breaking in SU(N)
415
with N + 1 flavors. Recall (from section 8.1) the confined description had a superpotential W
= A ^ I (detM -
BM
B)
•
(11-1)
The crucial features of this description were that since there was no chiral symmetry breaking and that the meson-baryon description was valid over the whole moduli space. That is, there was a smooth description with no phase-transitions. This is because the theory obeyed complementarity [49,50], since every static source could be screened by squarks. To generalize this we will need to have fields that are fundamentals of SU or Sp and spinors of SO. We will only consider theories that have a superpotential in the confined description. This requirement gives us an index constraint. Theories that satisfy these conditions are called s-confining [51]. Consider a gauge theory with one gauge group and arbitrary matter fields. Choose an anomaly free U(1)R such that 4>i that transforms in the representation ri of the gauge group and has i?-charge q while all other fields have zero charge. The charge q is determined by anomaly cancellation: 0=(q-l)T(ri)+T(Ad)-Y/T(*i) = qpr(rl)+T(Ad)-YtT(ri).
(H-2) (11.3)
3
Since we can do this for any field, and for each choice the superpotential has jR-charge 2, we have 6
Woe A
*(t)
TW
2/(E J T(r J )-T(Ad))
(11.4)
There may in general be a sum of terms corresponding to different contractions of gauge indices. Requiring that this superpotential be holomorphic at the origin means there should be integer powers of the composite fields, which implies integer powers of the fundamental fields. Unless all the T(ri) have a common divisor we must have ^2T{TJ)
- T(Ad) = 1 or 2 for SO or Sp
3
2 ( ^ T ( r - j ) - T(Ad)) = 1 or 2 for SU .
(11.5)
3
The differing cases come from the different conventions for normalizing generators, for SO and Sp we have T(D) = 1, while for SU we have T(D) = 1/2.
416
Anomaly cancellation for SU and Sp require that the left hand side be even. This condition is necessary for s-confinement, but not sufficient. One has to check explicitly (by exploring the moduli space, as discussed below) that for SO none of the candidate theories where the sum is 2 turn out to be s-confining. Thus we have
Y^T{r0)-T{Ad) = {\2 for for
SU or SO Sp
(11.6)
This condition gives a finite list of candidate s-confining theories. We can check whether candidate theories that satisfy the index constraint really are s-confining by going out in moduli space. Generically we break to theories with smaller gauge groups and singlet fields that decouple in the infrared. If the smaller gauge theory is not s-confining the the original theory was not s-confining. Alternatively if we have an s-confining theory and we go out in moduli space we must end up with another s-confining theory. Using these checks one can go through the list of candidates. For SU one finds that the following theories are the only ones that satisfy the conditions for s-confinement. SU(N) (JV + 1)P + D) SU(N) SU(N)
L] + B + 3(n + n)
SU(5) (11.7)
f2D+4D
517(5) SU(6)
U+5D+D
SU(6)
-4(D + D)
SU{7)
B + 3D
Let's consider the special case: SU(2N + l)|5I/(4) SU(2N + 1)
¥D •
1 1 •
1 • 1
[/(!)!
U{1)2
0 2N + 5 4 -2JV + 1 -2iV - 1-2AT + 1
U(1)R 0 0 i
(11.
417
This theory has a confined description in terms of the following composite fields: SU(4:)SU(2N + 1) •
QQ
C7(l)! 3-27V
•
AQ2 ANQ AN-'Q3
1
U 1 1 1
• •
Q2N+1
1
U(l)2 -AN+ 2
U{l)R \
8 -2N + 7 - 2 7 V - 1 27V2 + 37V+1 - 6 7 V - 3 27V2-37V-2 4(27V+1) -47V 2 + 1
0 \ § 0
(11.9)
with a superpotential
W
1 A2N
(A»Q)(QQy(AQT-'
+
.TT2S
N~ir,3\ (A»-'Q*)(QQ){AQ')
~(Qz"+L)(ANQ)(AN-lQ3)
N
(11.10)
One can check that the equations of motion reproduce the classical constraints, and that integrating out a flavor gives confinement with chiral symmetry breaking.
12. Deconfinement Given that we can find confined descriptions of certain SUSY gauge theories, we can also reverse engineer a dual description of a given theory where some of the fields correspond to mesons or baryons of the dual description. Such a procedure is referred to as "deconfinement" [55]. Consider the SU(N) gauge theory with an antisymmetric tensor [56,57] for odd TV theory with F > 5 flavors: SU{N) SU(F) SU(N + F - 4) f/(l)i 1
•
• 1
1 1
•
U(l)2U(l)R -2F TV
-12 N
F
•£•
r
N
- F 2-
N+F-i
—
We can find a dual description of this theory by taking A to be a composite
418
meson of a s-confining Sp theory SU(N) Sp{N --3) SU(F) SU(N + F --4) E/(l)! c/(i)2 u{\)R 1 1 0 -F =$ • • 1 0 1 1 FN 8 • ~P 1 1 1 0 F(l-N)6-% 1 1 • 1 N-F 2-f Q -F 1 1 F -§• Q AT+F-4
DID Dl
Y Z
r
N
(12.1) with a superpotential (12.2)
W = yzP The SU(N) group has iV + F duality: to find another dual: SU(Fy p q q M L B
• • • •
1 1 1
3 flavors so we can using our standard
3) Sp(N - 3) SU(F) SU(N + 1 1 • 1 1 1 1 1 • 1 1 • 1 • • 1 • • 1 1 •
F-4)
(12.3)
with (12.4)
W = Mqq + Bqp + Lyq . But Sp(N - 3) with N + 2F -7 fundamentals has an Sp(2F • SU(Fy p q Q
M I B
• • • •
1 1 1
a
B
H
1
(Ly)
•
3) Sp(2F - 8) SU(F) SU(N + 1 1 • 1 1 1 1 1 • 1 1 • 1 • • 1 • • 1 1 • 1
1
1
1 1
1 1
B •
dual:
F-4)
(12.5)
419
with W = ayy + Hll + (Ly)ly + Mqq + Bqp + (Ly)q ,
(12.6)
which, after integrating out (Ly) and q becomes W = ayy + Hll + Mqly + Bqp .
(12.7)
With F = 5 we have a gauge group 5(7(2) x SU(2) and one can show (using the fact that gauge invariant scalar operators have dimensions larger than or equal to one) that for TV > 11 this theory has an IR fixed point [57]. One can also show that some of the fields are IR free. Integrating out one flavor completely breaks the gauge group and the light degrees of freedom are just the composites of the s-confining description discussed at the end of section 11. With the other dual descriptions we would have to discuss strong interaction effects to see that we get the correct confined description. 13. Gauge Mediation The basic idea of gauge mediation 39 as proposed 40 by Dine, Nelson, Nir, and Shirman [60] is that there are three sectors in the theory, a dynamical SUSY breaking sector, a messenger sector, and the Minimal Supersymmetric Standard Model (MSSM). SUSY breaking is communicated to the messenger sector so that the messengers have a SUSY breaking spectrum. They also have standard model gauge interactions, which then communicate SUSY breaking to the ordinary superpartners. This mechanism has the great advantage that since the gauge interactions are flavor blind it does not introduce flavor changing neutral currents which are an enormous problem for supergravity mediation models. 13.1. Messengers
of SUSY
breaking
We will first consider a model with Nf messengers i, 4>i and a Goldstino multiplet X with an expectation value: (X)=M
+ 02F ,
(13.1)
so the scale of SUSY breaking is set by \/F. The Goldstino is coupled to the messenger fields via a superpotential W = Xfafa . 'For an excellent review see [58]. 'For early work along these lines see ref. [59].
(13.2)
420
In order to preserve gauge unification, fa and fa should form complete Grand Unified Theory (GUT) multiplets. The existence of the messengers shifts the coupling at the GUT scale, /LiGUT, relative to that in the MSSM by ^GUT = - ^ l n ( ^ )
,
(13-3)
where N,
Nm = J22T(ri)-
( 13 -4)
i-l
For the unification to remain perturbative we need Nm < ln(
*50 x •
(13.5)
M^r)
^
>
The VEV of X gives each messenger fermion a mass M, and the scalars squared masses equal to M2 ± F. We will be interested in the case that F
At two-loop order one finds squared masses for squarks and sleptons of order
the sum indicates that there are contributions from all the gauge interactions that the scalar couples to. This is very nice since the squark and slepton masses turn out to be of the same order as the gaugino masses, however doing two-loop calculations is quite tedious. Fortunately since we are only interested in the finite parts of two-loop graphs, there is a simple method for doing these calculations using the renormalization group (RG) and holomorphy [58,61]. The super-partners of the the standard model gauge bosons.
421
13.2. RG Calculation
of Soft
Masses
Below the mass scale of the messengers we can integrate them out and write the pure gauge part of the Lagrangian as: CG = ~^fd20T(X,fi)WaWa,
(13.E
where the effects of the messengers are included through their effects on the one-loop running of the holomorphic gauge coupling. Taylor expanding in the F component of X we find a gaugino mass given by i 9T , MX = - — \X=MF j 91nr , F VI \x=M2dlnX^-' M
(13.9)
The holomorphic coupling is given by
^^-w+4kO+^ln(i)'
(13 10)
-
where, as usual, b' is the (3 function coefficient of the theory including the messenger fields and b is the f3 function coefficient in the effective theory (i.e. the MSSM) below the mass scale of the messengers. These two coefficients are related by: b' = b-Nm.
(13.11)
So the gaugino mass is simply given by
Note that in the MSSM, where we have three gauginos, the ratio of the gaugino mass to the gauge coupling is universal: M± a\
=
M ^ «2
=
M ^
c*3
=
F -iVl
This type of relation was originally found in supergravity mediation models and, at the time, was thought to be a signature of such models. Next consider the wavefunction renormalization for the matter fields of the MSSM: C=
I' dAeZ{X,X])Q^Q'
,
(13.14)
422
where Z must be real and the superscript ' is meant to indicate that we have not yet canonically normalized the field. Taylor expanding in the superspace coordinate 6 we have
c
z+
2+
n
d2z
= /*» ( If™ §T^ + a*§oFeW2
QrtQ' . X=M
(13.15) Canonically normalizing we have:
Zl 2 { l +
MF°2
'
\X=M
(13.16)
Q',
so £ " /
t2 J_IJ5L^Wi^ ZdXdXy
dlnZdln dX 9Xt
QfQ. X=M
(13.17) Thus we have a sfermion mass term: _ d2h\Z FFt « " "51nXainXtU=MMMt ' 2
m
(13.18)
Rescaling the matter fields also introduces an A term in the effective potential from Taylor expanding the superpotential: Z~1'2
dlnZ 8X
dW
(13.19)
FQ
which is suppressed by a Yukawa coupling. To calculate Z, we do a supersymmetric calculation and replace M by vXJO. At I loops an RG analysis gives In Z = a(fi0)
f(a(iJ,0)L0,
a(fi0)Lx)
(13.20)
where In
U Lx d2lnZ SlnXSlnXt
In
a{ii)l+1h(a{n)Lx)
(13.21) (13.22)
(13.23)
Thus the two-loop scalar masses are determined by a one-loop RG equation.
423
At one-loop we have dlnZ
_ C2(r)
dhi/j,
TT
<*(/*) ,
(13.24)
2Cj(r)
2C2(r)
where
a' 1 (X)=a- 1 (// 0 ) + ^ l n f — g - J ,
a-1(M)=^1W + A
l n
(13.26)
^j .
(13.27)
So we finally obtain -Q = 2 C
2
(r)^
m
(e
2 +
^ ( l - a ) ( ^ )
,
(13.28)
where {
-i
+ *°M-*-
(13 29)
-
What is particularly impressive about this calculation [58,61] is that a twoloop result is obtained from a one-loop calculation. 13.3.
Gauge Mediation
and the \i
Problem
Recall that in order to obtain a viable mass spectrum for the electroweak sector in the MSSM, the two Higgs doublets, Hu and Hd, needed two types of masses terms: a supersymmetric \x term: W = fiHuHd ,
(13.30)
and a soft SUSY-breaking b term: V = bHuHd ,
(13.31)
with a peculiar relation between these two seemingly unrelated parameters b ~ fj2 .
(13.32)
In gauge mediated models we need to be of the same order as the squark and slepton masses:
"~1^£-
< 13 - 33 >
424
If we introduce a coupling of the Higgses to the SUSY breaking field X, W = \XHuHd
,
(13.34)
we get (j, = XM ,
(13.35)
b = XF ~ 1 6 T T V ,
(13.36)
so b is much too large for this type of model to be viable. A more indirect coupling W = X ( A i 0 i ^ + A2^2^2)Afl-u0i^2 + XHafafa ,
(13.37)
yields a one-loop correction to the effective Lagrangian: d4e^^f(X1/X2)HuHd^J
.
(13.38)
This unfortunately still gives the same, non-viable, ratio for bjJJ?. The correct ratio can be arranged with the introduction of two additional singlet fields S and N: W = S(X1HuHd + X2N2 + XH> - M2N) + X(f4> ,
(13.39)
then /i = A i ( S ) ,
(13.40)
b = AjFs .
(13.41)
A VEV for S is generated at one loop 1
F\
<^> ~ i ^ J ^ 'WN • L
(13 42)
-
but Fs is only generated at two loops: Fs
~ ( 1 6 ^ W ~ 16^17" •
(13 43)
'
Thus we can obtain b ~ fj,2 provided that we arrange for M2N ~ Fx- So it seems that somewhat elaborate models are needed to solve the fj, problem in the gauge mediated scenario.
425
14. Dynamical SUSY Breaking Non-perturbative SUSY techniques can be used to understand how to dynamically break gauge symmetries (like the the GUT symmetry, of the electroweak gauge symmetry of the standard model), however the most important application is to break SUSY itself. We would like to have a theory of dynamical breaking so that the SUSY breaking scale can be naturally much smaller than the Planck or string scale without fine-tuning the ratio of these scales by hand. 42 This is what naturally happens in asymptotically free gauge theories since the coupling at high energies can be perturbative and slowly running and then get strong at some much lower scale. This is just what happens in QCD: the coupling is perturbative near the (putative) unification scale, but gets strong in the IR, producing a large but natural ratio between the GUT scale and the proton mass.
14.1. A Rule of Thumb for SUSY
Breaking
A theory that has no flat directions and spontaneously breaks a continuous global symmetry generally breaks SUSY [63,65]. This is because there must be a Goldstone boson (which has no interactions in the potential), and by SUSY it must have a scalar partner (a modulus), but if there are no flat directions this is impossible. (Unless the modulus is also a Goldstone boson.) In the early days people looked for theories that had no classical flat directions (assuming that quantum corrections would not cancel the classical potential) and tried to make them break global symmetries in the perturbative regime. This method produced a handful of dynamical SUSY breaking theories. With duality we can find many examples of dynamical SUSY breaking. An important twist is that we will find that non-perturbative quantum effects can lift flat directions both at the origin of moduli space as well as for large VEVs.
14.2. The 3-2
Model
In the mid 1980s, Affleck, Dine, and Seiberg [65] found the simplest known model of dynamical SUSY breaking. Their model has a gauge group SU(3) x SU(2) and two global £/(!) symmetries with the following
For an excellent review see [62].
426
chiral supermultiplets: Q L U D
SU(3) SU(2) U(l) U(1)R 1 D • 1/3 -3 1 • -1 • 1 -4/3 -8 • 1 2/3 4
(14.1)
I will write Q = (U,D), and denote the intrinsic scales of the two gauge groups by A3 and A2 respectively. For A 3 » A2 (that is when the SU(3) interactions are much stronger than the SU(2) interactions), instantons give the standard Affleck-Dine-Seiberg superpotential: Wdyn
A5
(14.2)
det(QQ)
which has a runaway vacuum. Adding a tree-level trilinear term to the superpotential (14.3)
— ^ L — + XQDL det(QQ)
W
removes the classical flat directions and produces a stable minimum for the potential. Since the vacuum is driven away from the point where the VEVs vanish by the dynamical ADS potential (14.2), the global C/(l) symmetries are broken and we expect (by the rule of thumb described above) that SUSY is broken. The L equation of motion dW 0Lo
\ea»QmaD
(14.4)
= 0,
tries to set detQQ to zero since detQQ
=
det
{DQiDQ2
= U QmaD Qnp€ a@
(14.5)
Thus the potential can't have a zero-energy minimum since the dynamical term blows up at detQQ=0. Therefore SUSY is indeed broken. We can crudely estimate the vacuum energy for by taking all the VEVs to be of order 2> A3 and A < 1 we are in a perturbative regime. The potential is then given by V
dW dQ
2
+
dW dU
2
+
dW 3D
2
+
dW dL
(14.6)
427
A3"
, .\ ^Al 3 UO "•" AA-f A3
+, y\2 ^ ,
(14.7)
where in the last line we have dropped the numerical factors since we are only interested in the scaling behavior of the solution. This potential has a minimum near
<> « H ,
(14.8)
A' so we see that this solution is self-consistent: the vacuum is weakly coupled for small A since this ensures that 3> A3. Plugging the solution back into the potential (14.7) we find the vacuum energy is of order V«A^A£,
(14.9)
which vanishes as A or A go to zero, as it must. Using duality Intriligator and Thomas [66] showed that we can also understand the case where A2 ^> A3 and supersymmetry is broken nonperturbatively. The SU(2) gauge group has 4 doublets which is equivalent to 2 flavors, so we have confinement with chiral symmetry breaking. The SU(3) gauge group has two flavors and is completely broken for generic VEVs. It is simpler to consider SU{2) as an SU group rather than an Sp group, so we write the gauge invariant composites as mesons and baryons:
M~(LQ'
LQ
* ci4in N i ' '
VQ3Q1Q3Q2/ B ~ Q1Q2 ,
{
B ~Q3L . In this notation the effective superpotential is
W = X(detM-BB-A*)+\(j2MliDi
+ BD3\
,
(14.11)
where X is a Lagrange multiplier field that imposes the constraint for confinement with chiral symmetry breaking. The D equations of motion try to force M\i and B to zero while the constraint means that at least one of M11, M12, or B is non-zero, so we see that SUSY is broken at tree-level in the dual (confined) description. We can estimate the vacuum energy as V « A2A! .
(14.12)
Comparing the vacuum energies in the two cases we see that the SU(3) interactions dominate when A3 3> A? A2.
428
Without making the approximation that one gauge group is much stronger than the other we should consider the full superpotential
W = A (detM - BB - K\) +
A7 J=— + XQDL , det(QQ)
(14.13)
which still breaks SUSY, although the analysis is more complicated.
14.3. The SU(5)
Model
Another simple model analyzed by Affleck, Dine, and Seiberg [63] as well as Meurice and Veneziano [64] is SU(5) with matter content • + Q, that is an antifundamental and an antisymmetric tensor representation. This chiral gauge theory has no classical flat directions, since there are no gauge invariant operators that we can write down. Affleck, Dine, and Seiberg tried to match the anomalies in order to find a confined description but found only "bizarre", "implausible" solutions. This lead to the belief that the at least one of the global U(l) symmetries was broken and that therefore SUSY was broken (using the rule of thumb described earlier). Adding extra flavors ( • + • ) with masses Murayama [67] showed that SUSY is broken, but taking the masses to oo takes the theory to a strongly coupled regime, so the possibility remained that non-perturbative effects induced some phase transition as the mass was varied. With duality arguments Pouliot [68] showed that SUSY is indeed broken at strong coupling. The simplest way to see SUSY breaking is to consider the case that where there are enough flavors that the theory s-confines. Csaki, Schmaltz, and Skiba [51] showed that with four flavors SU(5) 5J7(4) SU(5) tf(l)i U(l)2 A Q Q
D
•
1 1 •
1
0 • 1
4 -5
9 -3 -3
U(l)R 0 0 \
(14.14)
the theory s-confines. To simplify the notation, rather than introducing new symbols for each color-singlet composite I will simply label it by it's constituents between parenthesis. Thus the meson is denoted by (QQ).
429
The spectrum of massless composites is: SU(4) 577(5) C/(l)i t/(l) 2 (QQ) (AQ2) (A2Q) (AQ3)
•
D
1
B 1 1 1
•
D 1
(Q5)
U(1)R
-1
-6
I 2
8 -5 -15 20
3 15 0 -15
0
(14.15)
1 2 3 2
0
with a superpotential l r
Wdy
(A2Q)(QQ)3(AQZ)
+ (AQ3)(QQ)(AQ
-(Q*)(A2Q)(AQ3)
f
(14.16) (14.17)
Note that by examining the global symmetry index structure one can see that the first term in Wdyn is anti-symmetrized in both the SU(5) and SU(4:) indices, while the second term is anti-symmetrized in just the SU(5) indices. We can add mass terms and Yukawa couplings for the extra flavors: 4
AW = ] T mQiQi + ] T XijAQiQj ,
(14.18)
which lift all the flat directions. The equations of motion give dW
(A2Q)(AQ3)
5
= Q,
(14.19)
d(Q ) d(QQ)
=
3(A2Q)(QQ)2(AQ2)
(AQ3)(AQy
0 . (14.20)
Assuming that (A2Q) ^ 0 then the first equation of motion (14.19) requires (AQ3) = 0 and multiplying the second equation of motion (14.20) by (A2Q) we see that because of the antisymmetrizations the first term vanishes and therefore (AQ3)(AQZ)2
(14.21)
but this contradicts (AQ3) = 0, so there is no solution with our assumption
(A2Q) + 0.
Assuming that (AQ3) ^ 0 then the first equation of motion requires (A Q) = 0, and plugging this into the second equation of motion (14.20) 2
430
we find equation (14.21) directly. Multiplying equation (14.21) by (AQ3) we find that the left hand side vanishes again due to antisymmetrizations, so (AQ3) = 0 but this contradicts equation (14.21) and our assumption. Therefore the equations of motion cannot be satisfied, and SUSY is broken at tree level in the dual description. 14.4. SUSY
Breaking
and Deformed
Moduli
Spaces
The Intriligator-Thomas-Izawa-Yanagida [69,70] model is a vector-like theory which consists of an SU(2) SUSY gauge theory with two flavors43 and a gauge singlet:
Q S
SU(2) SU(4) D
•
1
(14.22)
B
with a superpotential W = XSijQiQj
.
(14.23)
The strong SU(2) dynamics enforces a constraint Pf (QQ) = A4 ,
(14.24)
where the Pfafnan44 of a 2F x 2F matrix M is given by Pf(M) = e*i- i2FMili2 ... Ml2F_li2F .
(14.25)
The equation of motion for for the gauge singlet S is dW — = \QiQj
=0.
(14.26)
Since this equation is incompatible with the constraint (14.24) we see that SUSY is broken. Another way to see this is that at least for large values of XS we can integrate out the quarks, leaving an SU(2) gauge theory with no flavors 43
Since doublets and anti-doublets of SU(2) are equivalent, an SU(2) theory with F flavors has a global SU(2F) symmetry rather than an SU(F) X SU(F) as one finds for a larger number of colors. If we artificially divided Q up into q and q then we could follow our previous notation M = qq, B = e^qtqj, B = e^q^ and write the constraint as Pf(QQ) = d e t M — BB = A4.
431
which has gaugino condensation: 3JV
A:•eff
2 A'3N-2 (XS) 2
Weff = 2A^ff = 2A XS , dWeS 2AA^
as*
(14.27) (14.28) (14.29)
so again we see that the vacuum energy is non-zero. Since this theory is vector-like (it admits mass terms for all the quarks and for S) one would naively expect that this model could not break SUSY. This is because the Witten index Tr(—1)F is non-zero with mass terms turned on so there is at least one supersymmetric vacuum. Since the index is topological, it does not change under variations of the mass. However Witten noted that there is a loop-hole in the index argument since the potential for large field values are very different with AW = msS2 from the theory with ms —• 0, since in this limit vacua can come in from or go out to co and thus change the index. At the level of analysis described above S appears to be a flat direction but a general feature of SUSY breaking theories is that flat directions become pseudo-flat. With a non-zero vacuum energy, flat directions can be modified by corrections from the Kahler potential. For large values of XS in this model there is a wavefunction renormalization for S [71,72]
2s =
H- c A A . l n ( ; » 2 ) .
(14.30)
So the vacuum energy is corrected to be
v = 4 ' A 'V \Zs\ w |A|2A4 1 + cAA1 In — 5 V Mo J.
(14.31)
So we see that the potential slopes towards the origin. This can be stabilized by gauging a subgroup of SU(4). Otherwise there is a calculable low-energy effective theory [73] near ASwO (essentially an O'Raifeartaigh model [74]) which has a local minimum at S = 0. The effective theory becomes noncalculable near XS « A. The behavior near this region is unknown.
432
14.5. SUSY
Breaking from Baryon
Runaways
Consider a generalization [75] of the 3-2 model: Sp(2N) SU(2N - 1) SU(2N-1) 1 1 D B B 1 1 l B
U(l)
U(l)i l
•
• •
-l
2JV-1 2AT+2 2JV-1
o 6
(14.32)
-4JV
with a tree-level superpotential (14.33)
W = XQLU.
If we turn off the SU(2N — 1) gauge coupling and the superpotential, Sp(2N) is in a non-Abelian Coulomb phase for N > 6, it has a weaklycoupled dual description for N = 4, 5, it s-confines for N = 3, and confines with a quantum-deformed moduli space for N = 2. If we turn off the Sp(2N) gauge coupling and the superpotential, SU(2N — 1) s-confines for any N > 2. Here we will consider the case that Asu ^> AspIncluding the effects of the tree-level superpotential, this theory has a classical moduli space that can be parameterized by the gauge-invariants: SU(2N -- l ) t f ( l ) M = (LL) B =
2
•
b = (TJ2N-1)
fi
°
4(JV -iV+l) 2N-1
(14.34)
1
subject to the constraints MjkBte
6
2
(U2N-2D)
klmi
17(1) R
0 ""12JV-3
2A^ + 2
_
(14.35)
These constraints split the moduli space into two = 0, Mjkbranches: b = 0. on one of them M = 0 and B, b ^ 0, and on the other M ^ 0 and B, b = 0. First consider the branch where M = 0. We will see that later that the true vacuum actually lies on this branch. In terms of the elementary fields, this corresponds to the VEVs (up to gauge and flavor transformations)
(U) =
/ v sin I \ 0
v cos 9
(14.36)
<£>> =
vl
2N-2
V oJ
433
For large VEVs, v > ASu, SU(2N — 1) is generically broken and the superpotential gives masses to Q and L or order Xv. The low-energy effective theory is pure Sp(2N), "which has gaugino condensation. The intrinsic scale of the low-energy theory is A 3(2iV+2) A
eff
__ .3(2iV+2)-2(2JV-l) — ASp
(xuf2N-1]
(14.37)
The effective superpotential is given by A3
Weff OC A 3 f f
(
XU
(14.38)
For N > 2 this forces (U) to zero. Now consider the case of small VEVs, v < Asu, then SU(2N — 1) s-confines so we have the following effective theory Sp(2N) SU(2N - 1)
• •
L (QU) (QD) (Q2"-1) B b
• 5
• • 1
1 1 •
1
1
(14.39)
with a superpotential W = X(QU)L 1 A4JV-3 SU
[(Q2N~l)(QU)B + (Q2N-x)(QD)b - det
(14.40)
The first term in the superpotential is a mass term, so (QU) and L can be integrated out with (QU) — 0, and we have a low-energy superpotential W
1 A4AT-3
(14.41)
(Q^-')(QD)b
On this branch of the moduli space (b) = (U ) ^ 0, and this VEV gives a mass to (Q 2 i v _ 1 ) and (QD) which leaves an pure Sp(2N) as the low energy effective theory. So we again find gaugino condensation with a scale given by A3(2N+2) =
A3(27V+2)-2(2N-l)
^ ^ 2 ( 2 ^ - 1 )
(_b_^
A SU
^
^
434
and a effective superpotential Wee oc Ae3ff ~ b ^
(2JV-2) ( A ^ A ^ ASU )
N
^ ,
(14.43)
which forces b —> oo, (this is a baryon runaway vacuum) but the effective theory is only valid for scales below Asu, since we assumed v < Asu- We have already seen that beyond this point the potential starts to rise again, so the vacuum is around 72JV-1,
(b) = (U'"~x)~AlT1.
(14-44)
With some more work [75] one can also see that SUSY is also broken when ASp » AsuAn especially interesting case is when N = 3, where the Sp(2N) sconfines and we have the following effective theory 5(7(5) S77(5)
(QQ)
B
(LL) (QL)
1 D
(Q2^-1)
•
u
n
D
1
B
(14.45)
D 1 D 1
D
with W = \{QL)U + Q21*-^2"-1
.
(14.46)
The non-Abelian global symmetry group in this case is 5(7(5) which is just large enough to embed the standard model gauge groups, and hence this is a candidate model for gauge mediation [60]. After integrating out (QL) and U we find 5C/(5) with an antisymmetric tensor, an antifundamental, and some gauge singlets, which we have already seen breaks SUSY in Section 14.3. Returning to general N and considering the other branch of the moduli space where M = (LL) ^ 0, one can see that the D-flat directions for L break Sp(2N) to 5/7(2), and the spectrum of the effective theory is 5(7(2) SU(2N-1) Q' V
V D
• •
1 1
•
1
• •
(14.47)
435
and some singlets with a superpotential W = XQ'V'L'
.
(14.48)
This is a generalized 3-2 model (see Section 14.2) with a dynamical superpotential. For (L) 3> Asu the vacuum energy is independent of the SU(2) scale and proportional to ^su(2N-i) which itself is proportional to a positive power of (L), thus the effective potential in this region drives (L) smaller. For (L) -C A$u w e can use the s-confmed description above, and find again that the baryon b runs away. For (L) w Asu, t n e vacuum energy is V ~ Asu
.
(14.49)
which is larger than the vacuum energy on the other branch, so we see that the global minimum is on the baryon branch with b = (U ) ^ 0. Further examples of dynamical SUSY breaking can be found in references [62,76-78]. 14.6.
Direct
Gauge
Mediation
A more elegant (and more difficult from the model building view point) approach to gauge mediation is direct gauge mediation where the fields that break SUSY have SM gauge couplings. These types of models only have two sectors (a SUSY breaking sector and an MSSM sector) rather than three. Consider the model [79] SI/(5)i SU{5)2 SU(5) Y
4>
1 •
•
• 1
•
• i
with a superpotential
W = \Y0i .
(14.50)
We can weakly gauge the global SU(5) with the SM gauge groups. For large Y S> Ai, A2, (p and <j> get a mass and, by the usual matching argument, the scale of the effective gauge theory is Ae3ff5 = A?" 5 - 5 (AX) 5 ,
(14.51)
1 5
where X = (dety) / . The effective gauge theory undergoes gaugino condensation, so the superpotential is given by W eff = A3eS - XXAj
,
(14.52)
436
so SUSY is broken a la the Intriligator-Thomas-Izawa-Yanagida model discussed in Section 14.4. The vacuum energy is given by |AAf|2 (14.53) ZX where Zx is the wavefunction renormalization for X. For large X the vacuum energy grows monotonically. A local minimum occurs at the point where the anomalous dimension 7 = 0. For (X) > 10 14 GeV, the Landau pole for A is above the Planck scale. The theoretical problem with this model is that for small values of X there is a supersymmetric minimum along a baryonic direction [80]. For this regime it is appropriate to look at the constrained mesons and baryons of SU(5)\. The superpotential is V:
W = A(detM -BB-
A}0) + XYM .
(14.54)
0
There is a supersymmetric minimum at BB = —A} , Y — 0, M = 0. This supersymmetric minimum would have to be removed, or the nonsupersymmetric minimum made sufficiently metastable (with a lifetime of the order of the age of the Universe) by adding appropriate terms to the superpotential that force BB = 0 The phenomenological problem with this model is that heavy gauge boson messengers can give negative contributions to squark and slepton squared masses. Consider the general case where a Goldstino VEV (X) = M + 62F,
(14.55)
breaks SUSY and breaks two gauge groups down to the SM gauge groups GxH
-f SU(3)C x SU(2)L x U(1)Y ,
(14.56)
with 1 a{M)
1 aG(M)
+
1 aH(M)
(14.57)
Analytic continuation in superspace gives MA
<*(M) 47T
(b-bH-
bG)
(14.58)
M
and m
Q
2 2C 2 (r) a(M)2 2
(16TT )
(b
(R2 ~ 2)bH
2bG)e +
b-bH
~bG
(i-e
(14.59)
437
where
R
=
a
- ^ (14-61) y a(M) ' This typically gives a negative mass squared for right-handed sleptons. Another danger for direct mediation models arises if not all the messengers are heavy. Then two-loop RG evolution gives: d 2 2 2 4 2F (14.62) ex - 5 M A + c S T r ( ( - l ) m- ) the one-loop term proportional to the gaugino mass squared drives the scalar mass positive as the renormalization scale is run down, while the twoloop term can drive the mass squared negative. This effect is maximized when the gaugino is light, so in the standard case where the gluino is heavy, then again it is the sleptons that receive dangerous negative contributions. This effect is also dangerous [81] in models where the squarks and sleptons of the first two generations are much heavier that 1 TeV. 14.7. Single Sector
Models
Another appealing approach to gauge mediation is to have the strong dynamics that break SUSY also produce composite MSSM particles [82]. Thus rather that having three sectors (one for the standard model, one hidden sector that dynamically breaks SUSY, and one "messenger" sector that couples to the SUSY breaking and has standard model gauge interactions) there is really just one sector. Consider a SUSY model with gauge group SU(k) x SO(10) with the following matter content 45 :
SU{k) 50(10) SU(W) SU(2) 1 1 Q • • L • 1 • 1 U 1 • • 1 1 16 1 • s
(14.63)
and a superpotential W = XQLU Note that the spinor representation of 5 0 ( 1 0 ) has dimension 16.
(14.64)
438
The 5(7(10) global symmetry is large enough to embed the standard model gauge group, or a complete Grand Unified Theory gauge group (SO(W) or SU(5)) can be embedded. This is a baryon runaway model (similar to the models described in Section 14.5). For large detU » Ai 0 WeS~UW/k,
(14.65)
while for small detU
~ [7 1 0 ( 1 -7 ) / f c ,
(14.66)
where 7 is the anomalous dimension of U which depends on the as yet not fully understood details of S'O(IO) gauge theories with spinors [83], but which seem to have infrared fixed points [57,83]. For 10 > k > 10(1 — 7) SUSY is broken. There are two composite generations corresponding to the spinor S. The composite squarks and sleptons have masses of order mcomp
F ?a = .
(14.67)
where F is the value of the F-component of U which breaks SUSY (i.e. the vacuum energy is |F| 2 ). This can be thought of as gauge mediation via the strong SO(10) interactions. The global SU{2) symmetry enforces a degeneracy that suppresses flavor changing neutral currents. The composite fermions can only get couplings to the Higgs fields from higher dimension operators, so they are light. The fundamental gauginos and third generation scalars get masses from gauge mediation. Since the superpartners of the the first two (composite) generations are much heavier than the superpartners of the third generation, the spectrum is very similar to that of the "More Minimal" Supersymmetric Standard Model [84]. Acknowledgments I would like to thank Howard Georgi and the Harvard and MIT students and post-docs who attended Physics 284 in the fall of 1999: they were the first guinea pigs for.these lectures. I would also like to thank the organizers, lecturers, and students from TASI 2002 who provided stimulating and fun environment for these lectures. Finally I must thank Csaba Csaki, Vladimir Dobrev, Josh Erlich, Shiraz Minwalla, Stavros Mouslopoulos, Ann Nelson, Ricardo Rattazzi, Yuri Shirman, and Witold Skiba for useful discussions and comments. J.T. is supported by in part by the US Department of Energy under contract W-7405-ENG-36, and during the original stages of
439
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KEITH R. DIENES
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N E W DIRECTIONS FOR N E W DIMENSIONS: A N I N T R O D U C T I O N TO KALUZA-KLEIN THEORY, LARGE EXTRA DIMENSIONS, A N D THE BRANE WORLD
KEITH R. DIENES Department of Physics, University of Arizona, Tucson, AZ 85721 E-mail: dienes@physics. arizona. edu This is the written version of an introductory self-contained course on theories with extra spacetime dimensions, delivered at the 2002 TASI summer school. No prior knowledge of such theories is assumed. The goal is to provide a short and practical introduction to this important field of modern research, focusing exclusively on the case of flat extra dimensions. Subsequent lectures at this school will focus on more advanced topics.
Contents 1
Lecture # 1 : Extra spacetime dimensions: General properties and bounds 451 1.1 Original Kaluza-Klein idea 451 1.2 The compactification procedure: Mode expansions, Kaluza-Klein reductions, and low-energy signatures 455 1.3 Orbifold compactifications 462 1.4 Types of extra dimensions 466 1.5 Bounds on extra dimensions 472 1.6 Embedding 4D theories into extra dimensions 478
2
Lecture # 2 : Large extra dimensions and the brane world 481 2.1 The Standard Paradigm for physics beyond the Standard Model: A quick review 482 2.2 Extra dimensions: Lowering the GUT scale 486 2.3 Extra dimensions: Lowering the Planck scale 491 2.4 Extra dimensions: Lowering the string scale 497 2.5 Extra dimensions: A unified picture for physics beyond the Standard Model? 498 447
448
2.6 Open questions
502
Lecture # 3 : Extra dimensions in action: Applications to particle phenomenology 503 3.1 Proton decay 504 3.2 The fermion mass hierarchy 508 3.3 Neutrino oscillations 511 3.4 QCD axion phenomenology 519 Lecture # 4 : Beyond the simple pictures: Lessons and warnings524 4.1 Shape, not just volume 524 4.2 Strings, not just effective field theories 534 4.3 Curved, not just flat 542
Introduction Within the past four years, the subject of theories with extra spacetime dimensions has undergone a profound transformation. What was once the almost exclusive province of practitioners of supergravity and string theory has exploded to become one of the dominant research topics for particle phenomenologists and even particle experimentalists. What began in the 1920's as an abstract mathematical attempt at unification near the Planck scale has become, in the short space of the past four years, one of the major contenders for possible physics at energy scales immediately beyond that of the Standard Model. Indeed, growing numbers of theorists and experimentalists are actively exploring the possibility that extra curled-up spacetime dimensions might soon be discovered, either in the next round of collider experiments, or in precision table-top experiments. One has only to glance at some of the headlines culled from the popular scientific press of the past few years (see Fig. 1) to appreciate the magnitude of the sudden interest in this subject. Looking further at these headlines, however, we can discern another theme at play. Certainly, many of the headlines focus on the possibility of large extra dimensions; for example, most the headlines in the upper half of Fig. 1 are in this category. But in the headlines in the lower half of Fig. 1, we instead perceive another theme entirely — the idea that fundamental ideas of grand unification and string theory may soon be directly testible! Since the energy scales of these fundamental theories are normally considered to be unreachably high (near the Planck scale ~ 1019 GeV), a direct examination of these theories can only happen if their characteristic energy
449
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450
scales are somehow significantly reduced, perhaps all the way to the TeV range. Of course, these two themes are related: theories with large extra spacetime dimensions tend to exhibit significantly reduced GUT, Planck, and string scales. The main goal of these lectures will be to explain this connection. We will begin, in Lecture # 1 , by discussing extra dimensions in and of themselves. How are such theories formulated? What are their lowenergy signatures? What bounds govern their allowed parameter ranges? Then, in Lecture # 2 , we shall discuss how and why such extra dimensions tend to reduce the characteristic energy scales of fundamental physics. Together, this picture of large extra dimensions and reduced energy scales gives rise to a paradigm commonly called the "brane world". In Lecture # 3 , we will then continue to see how extra dimensions can provide new mechanisms for addressing some long-standing phenomenological problems within the Standard Model or its extensions. To provide concrete examples, I have chosen such diverse topics as proton decay, fermion mass hierarchies, neutrino oscillations, and axion physics. Finally, in Lecture # 4 , we shall discuss some issues that are normally ignored in current treatments of the brane world, but which perhaps may play a role in more sophisticated developments of a complete higher-dimensional theory. I have chosen to raise such issues not only as a way of indicating caution concerning the standard treatments of higher-dimensional theories, but also to stimulate further thinking and research. These lectures were delivered at the 2002 TASI summer school. Since one of the main topics of this school was the physics of large extra dimensions, there were many lectures covering numerous advanced aspects of this subject. These lectures, by contrast, were requested by the TASI organizers to provide an overall, non-technical introduction to the subject. Consequently, there are clearly numerous topics which are omitted in these lectures, but which are as important as those which are included. Chief among these is the possibility of non-flat extra dimensions — i.e., extra dimensions which are warped or have non-trivial (e.g., non-factorizable) geometries. Such extra dimensions have played a huge role in recent discussions of higher-dimensional theories, but will not be covered here. By contrast, these lectures will focus primarily on the simpler case of flat, toroidally compactified extra dimensions. As we shall see, this will be sufficient in order to enable us to deduce most of the main properties that have excited the particle physics community in recent years.
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1. Lecture # 1 : Extra spacetime dimensions: General properties and bounds We begin with a general discussion of the properties and constraints on extra spacetime dimensions. These properties and bounds will apply for all scenarios involving flat extra spacetime dimensions. 1.1. Original Kaluza-Klein
idea
Why consider physics in more than four dimensions? As indicated in the Introduction, one of the first application of extra spacetime dimensions to fundamental physics dates back to the 1920's, in the original work of T. Kaluza and 0 . Klein. Kaluza and Klein pointed out that if extra dimensions of suitable size exist, then they might provide a possible means of unifying the two fundamental forces which were known at that time to exist, namely gravity and electromagnetism. The Kaluza-Klein argument rests upon one of the important properties of extra dimensions, namely that they provide a way of unifying particles with different spins. To see this, we must first recognize that since the D-dimensional Lorentz group SO(D — 1,1) is larger than the fourdimensional Lorentz group SO(3,1) for all D > 4, each representation of the Z?-dimensional Lorentz group is larger than the corresponding representation of the four-dimensional Lorentz group. This implies that a single representation of the ZJ-dimensional Lorentz group decomposes into several different representations of the four-dimensional Lorentz group all at once. However, different representations of the four-dimensional Lorentz group correspond to different physical particles of different spins. Thus, we see that certain combinations of different particles with different spins in four dimensions can be viewed merely as different components of a single particle in higher dimensions, with a single spin associated with the higher-dimensional Lorentz group. Let us consider a trivial example of this phenomenon. A vector field in five dimensions has five components: V^ = (V0, V\, V2, V3, V4). If we identify the first four components as the directions corresponding to our usual four-dimensional spacetime, then we can decompose this five-dimensional vector inito representations of the four-dimensional Lorentz group, yielding a four-dimensional vector V^ = (Vo, Vi, V2, V3) and a four-dimensional scalar $ = V4. Thus a single Lorentz vector representation in five dimensions has yielded both a spin-1 particle and a spin-0 particle in four dimensions.
452
This is, of course, a trivial example. However, let us now turn to the case originally considered by Kaluza and Klein, concerning Einstein gravity in five dimensions. This situation is sketched in Fig. 2. In such a theory, the fundamental field is the five-dimensional metric gMN, with (M,N) = 0,1,2,3,4. As a symmetric tensor field, gMN has 15 independent components. Decomposed into representations of the four-dimensional Lorentz group, however, we find that gMN yields three different representations: 4
5
,( ) -= g\v „( ) with 0 < /z, v < 3; • a spin-two symmetric tensor field g^J •5) • a spin-one massless vector field Ajj, = g^JA* with 0 < [i < 3; and (5)
• a spin-zero massless scalar field <£> = g\4 . While a spin-two symmetric tensor field gjjj is interpreted as the fourdimensional graviton, a spin-one massless vector field Ajj is naturally interpreted as a photon. (The remaining scalar field combines with the graviton to produce a modified version of Einstein gravity called "Brans-Dicke gravity".) While this examination of the field content is certainly suggestive of these interpretations, we really must verify that these interpretations hold at the level of the actions (or at the level of the equations of motion of the fields). However, beginning with the five-dimensional Einstein-Hilbert (5")
action for the field gMN and decomposing this action into representations of the four-dimensional Lorentz group, we indeed obtain not only the Maxwell action for Ajj, but also the Brans-Dicke action involving gjjj and $. Indeed, the four-dimensional gauge coupling
732
I-'•••'• J
where K§ is the five-dimensional gravitational coupling and R is the radius of compactification. (We will see how to derive relations such as this shortly.) We see, then, that four-dimensional gravity can indeed be unified with electromagnetism into pure five-dimensional gravity! Or, phrased slightly differently, four-dimensional gauge invariance (the symmetry underlying electromagnetism) can be derived from five-dimensional general coordinate invariance (the symmetry underlying Einstein general relativity). While this setup succeeds in unifying both of the fundamental forces known in the 1920's, we now know that there are at least four fundamental forces: gravity (mediated by gravitons); electromagnetism (mediated by
453
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902
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9 1 1
912
913
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9jxv 4D Brans-Dicke gravity
i 9 |LL4=
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Figure 2. The basic Kaluza-Klein observation: Four-dimensional Brans-Dicke gravity and four-dimensional electromagnetism can be unified within the framework of fivedimensional Einstein gravity.
photons); the weak nuclear force (mediated by the W^ and Z particles); and the strong nuclear force (mediated by gluons). Thus, unification along these lines a la Kaluza and Klein would require even more extra dimensions. Moreover, it turns out that generating non-Abelian gauge symmetries in this way would also require more complicated geometric compactifications than the simple circle-compactifications we will consider here. In general, the gauge group that is generated through a Kaluza-Klein (KK) reduction of this sort is the isometry group of the manifold on which the extra dimensions are compactified. Thus, if we have 5 extra spacetime dimensions, with each compactified on a circle, then the gauge group generated in four dimensions is U(l)s. (However, we shall see in Lecture # 4 that string theory provides an elegant way of generating non-abelian gauge symmetries even with simple toroidal compactifications.)
454
What is the current status of KK theory? Nowadays, the best modern candidate for unification of all forces and particles in nature is string theory. Needless to say, we will not have time in these lectures to provide a proper introduction to string theory. However, the basic premise of string theory is very simple: all elementary particles are really closed vibrating loops of energy called strings. The length scale of these loops of energy is on the order of 10~ 35 meters (corresponding to 10 19 GeV), so it is not possible to probe this stringy structure directly. This idea has great power, because it provides a way to unify all of the particles and forces in nature. Specifically, each different elementary particle can be viewed as corresponding to a different vibrational mode of the string. A pictorial representation of this idea is given in Fig. 3, where we are schematically associating higher vibrational string modes with string loops containing more "wiggles". From the point of view of a lowenergy observer who cannot make out this stringy structure, the different excitations each appear to be point particles. However, to such an observer, the states with more underlying "wiggles" appear to have higher spin. Thus, in this way we find that string theory predicts not only spin-1/2 and spin-1 states (which can be associated with the fermions and gauge bosons of the Standard Model respectively), but also a spin-2 state (which can naturally
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spin 1 gauge bosons
photon W,Z gluons
graviton antisymmetric tensor dilaton
=}
Figure 3. The basic hypothesis of string theory is that the different elementary particles correspond to the different vibrational modes of a single fundamental entity, a closed loop of energy called a string. In this way one obtains not only spin-1/2 and spin-1 states which can be associated with the matter and gauge bosons of the Standard Model, but also a spin-2 state which can be identified with the graviton. Thus, string theory provides a way of unifying the Standard Model with gravity.
455
be associated with the graviton). Thus, through string theory, we see that the gauge interactions, particles, and also gravity are unified into a common quantized description as corresponding to different excitation modes of a single fundamental entity, the string itself. This form of unification seems to be drastically different from the KK proposal. However, just like the original KK proposal, strings also require extra spacetime dimensions. Specifically, they require six extra dimensions (making a total spacetime dimensionality D = 10) if they are weakly coupled, and seven extra dimensions (making D = 11) if they are strongly coupled. Thus, in this respect, string theory is the modern incarnation of the original KK idea: instead of having particles vibrating in higher dimensions, we have strings vibrating in higher dimensions. Indeed, one of the ways in which string theories give rise to gauge symmetries is through a direct implementation of the KK idea. We shall discuss this in more detail in Lecture $ 4 . Thus, the original KK idea finds a natural home in string theory, and continues to play a role in modern theories of unification.
1.2. The compactification procedure: Mode expansions, Kaluza-Klein reductions, low-energy signatures
and
Given this motivation, let us begin our study of extra spacetime dimensions by understanding literally what an extra dimension entails. Clearly, in our everyday world, we are not familiar with any additional space dimensions beyond those of length, width, and height. These are dimensions of sufficient size that we can walk into them and experience them fully as spatial degrees of freedom. Therefore, if any extra space dimensions exist beyond those that we already know, they must be of sufficiently small size that they are not observable with the naked eye. (We will shortly define "naked eye" more scientifically.) In other words, they must be compactified down to a small length scale. What does it mean to compactify an extra dimension? The idea is simple: we imagine that our extra dimension takes some compact shape, such as a circle, such that by travelling along that extra dimension we quickly return to our original location. Thus, taking a circle compactification as our example, we imagine that at every point in spacetime, there exists an additional circle of radius R which is orthogonal to all of the known dimensions. This is illustrated in Fig. 4. In this example, spacetime has the topology A^4 x Si where M4 is four-dimensional Minkowski space and Si
456
is a circle of radius R. We then imagine that R is sufficiently small that that we cannot probe this additional structure in our spacetime. Thus, this extra dimensions is essentially unobservable.
0 I fi" 4D spacetime Figure 4. An extra dimension compactified on a circle of radius R. At every point in the four-dimensional spacetime (here collapsed onto a single line), there exists an "orthogonal" dimension compactified on a circle of radius R.
More generally, we can compactify 8 extra spacetime dimensions by formulating a theory on M.^ x K where M4 is our four-dimensional Minkowski spacetime and K is any (^-dimensional compact manifold. For example, for 5 = 2, we can take K = T^ (a two-torus) or K = S\ x S\ (two independent, orthogonal circles, one for each compactified dimension). Note that S\ x Si is a special case of Ti- However, we can also take K = S2 (a two-sphere). Many other topologies are possible, even for S = 2. For example, two-dimensional surfaces with boundaries include a rectangle and a cylinder. As we shall see later, consideration of manifolds with boundaries (more generally known as "orbifolds") will be necessary for phenomenological reasons. We should also mention that there are more general forms of compactification beyond the simple factorizable M4, x K structure. This product structure indicates that the properties of the extra spacetime dimension are the same no matter the location of this extra dimension in the physical four-dimensional Minkowski spacetime. However, this too is an assumption. Later, in Lecture # 4 , we shall see that the possibility of non-factorizable geometries leads to some extremely interesting physics. In general, the fact that K is compact means that we must impose boundary conditions appropriate for the specific choice of K. We shall generally let x^ indicate the coordinates on .M4, and let yl indicate the coordinates of K. While each of the a;M are valued along the infinite real line, our boundary conditions on the yl indicate the specific choice of compact-
457
ification manifold K. For example, in the case of a single extra dimension compactified on a circle, we have only a single y coordinate. Compactification on a circle then requires that we impose periodic boundary conditions under y —> y + 2irR. Indeed, it is this which defines the radius R of the circle. Note, in this connection, that the volume (i.e., the complete spatial extent) of the extra dimension is given by the circumference 2TTR. (By contrast, the quantity irR2 has no meaning here, since it ultimately depends on the manner in which this periodic extra dimension is embedded in some larger spacetime. Only the quantity 2irR has an intrinsic topological meaning.) Given the specific spacetime geometry, our next task is to construct suitable wavefunctions on this space. For example, in the circlecompactification case, all wavefunctions <&(x^,y) must be periodic under y —> y + 2irR, implying that ^(x,i,y) must have a mode expansion of the form oo
1
$(*",!/) = -/== V2irR
Y,
M^)eKP{iny/R).
(1.2)
n=_oo
This is the most general form for the wavefunction consistent with the compactification symmetries in this case. The normalization prefactor in Eq. (1.2) will be discussed shortly. In order to interpret the coefficients ^>n(a:'t), let us imagine that the five-dimensional field $(x M , y) is a complex Klein-Gordon field of mass mo. The five-dimensional action for such a field is given by TR
dy i(0 M *)*(0 M *)-^m§$*$
d4x
S5 =
(1.3)
where M is the five-dimensional spacetime index such that the range 0 < M < 3 corresponds to our ordinary four-dimensional Minkowski spacetime, with M = fi, and xM = y for M = 4. Our procedure is now to derive a corresponding four-dimensional action by substituting the mode expansion (1.2) into Eq. (1.3) and explicitly performing the integration over the compactified coordinate y. First, inserting the mode expansion (1.2) into Eq. (1.3), we obtain 1
S5 = ^
r
/-2TTR
Id dxAx I
2TTR J
dy
J0
(I
l^(-im\
f in
2^{ R J
KR
ran
v
'
'dyl^Yl3,&3"ie^n-m^/R ei(n-m)y/R
458
"
\rnlJ2
.
(1.4)
ran
The first term comes from the kinetic term in Eq. (1.3) with M — fi < 3, while the second term arises from the kinetic term with M = 4. However, we now recall that exponential wavefunctions with different frequencies are orthogonal: dy
ei(n~m)y/R
27TRSmn
=
.
(1.5)
'0 Jo
Performing the y-integrations thus gives rise to the four-dimensional action
S4 = Jd^xl1- 5>„03^„) " \^ 2 ^
2
l' 2 1
n
0 „ l . (1.6)
Interpreting this action is straightforward, since this is nothing but the action of an infinite tower of Klein-Gordon fields
m2 = ml + jp.
(1.7)
The lightest field is thus 4>o, with mass mo, while 4>n and _„ are degenerate for each n. As evident from the wavefunction decomposition in Eq. (1.2), the index n is the (discrete) quantum number corresponding to (quantized) momentum in the (compactified) fifth dimension, with p$ = n/R; we thus see that our mass formula in Eq. (1.7) is nothing but the statement that the total mass is given by m§ + p | . Note that if we had compactified S extra dimensions on circles of radii Ri (i = 1,..., 5), we would have obtained the same action (1.6) with the masses (1.7) replaced by s
2
,2
ESi=l
l
^
This is an example of a Kaluza-Klein reduction, and the procedure we have just outlined is completely general. We begin with our £>-dimensional action SD, written in terms of our D-dimensional quantum fields. These fields may be scalars, spinors, vectors, or any bona-fide representation of the D-dimensional Lorentz group. The higher-dimensional action may also contain arbitrary interaction terms between these fields. In general, this action should respect all of the symmetries (such as Lorentz invariance and gauge invariance) appropriate for the fundamental higher-dimensional theory. Given this parent theory, we then compactify it by choosing an arbitrary ^-dimensional compactification manifold K, where S = D - 4.
459
First, we decompose the Lorentz structure of our D-dimensional fields as discussed in Sect. 1.1, writing these fields as sums of irreducible representations of the /cmr-dimensional Lorentz group. This will yield a spectrum of fields of different four-dimensional spins. Next, we perform a mode expansion for each field as in Eq. (1.2), where the exponentials are generally replaced by the eigenfunctions f{yi,y2,---,ys) of the Laplace operator on the manifold K. We then substitute these mode expansions into our higherdimensional action Sp, and integrate over the compactification volume to obtain the corresponding effective four-dimensional action S4: S4 =
f d6y SD .
(1.9)
JK
In performing this integral, the Laplace eigenfunctions will continue to exhibit an orthogonality relation analogous to that in Eq. (1.5) since they continue to be the eigenfunctions of a Hermitian operator. This will have the net effect of ensuring conservation of higher-dimensional momenta for each term in the resulting four-dimensional Lagrangian, so that any terms m4>n...4>p in the resulting four-dimensional Lagrangian will satisfy m + n+ ... +p = 0. Given our resulting four-dimensional Lagrangian, we then can read off the complete set of fields, effective masses, and interaction terms appropriate for the effective theory in four dimensions. Note that in general, the four-dimensional KK mass terms will emerge from the kinetic terms in the original higher-dimensional Lagrangian. The resulting KK mass spectrum is thus determined by the eigenvalues of the appropriate Laplace operator on the manifold K. Given this procedure, it is now also easy to understand why we chose the overall (volume) normalization given in Eq. (1.2). As evident from the action (1.3), the $ field has mass dimension +3/2 in five dimensions; in general, the mass dimensions of scalar, fermion, and gauge fields in D dimensions are given by m
= [A] = \{D-2),
[*] = \{D-l).
(1.10)
However, even though the $ field has mass dimension +3/2, we wish the individual KK modes n to have the mass dimension + 1 , as appropriate for-a four-dimensional interpretation. This explains the factor of VR in the prefactor in Eq. (1.2). The remaining factor of y/2n is inserted so that the kinetic terms for the resulting n fields in four dimensions will be appropriately normalized after the integration over y. In general, the appropriate prefactor for a KK mode expansion is [Vol(K)] - 1 / 2 where K is the compactification manifold.
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The mass formula (1.7) simplifies in cases where the extra dimension is small compared toTOQ~. For R~1
mn
« m0 + —
(1.11)
where each excited mass level is doubly degenerate. We thus have an infinite tower of approximately equally spaced mass levels, with the ground state shifted by an overall constant mo- The lowest-lying state with n = 0 is called the Kaluza-Klein zero mode or ground state, whereas the <\>n for all n ^ 0 are the excited KK states. If the accessible energy in our experiment is much less than -R -1 , we will not have sufficient energy to excite the higher KK modes. In such cases, we would only observe the KK zero mode 4>o- This is therefore nothing but the the usual four-dimensional state, and TOO is nothing but the usual four-dimensional mass of the four-dimensional particle. The threshold energy for detecting extra dimensions is therefore ~ R~l, since this is the threshold for producing the first excited KK state. These results are in complete accordance with our expectations based on elementary quantum mechanics. An extra dimension compactified on a circle is mathematically equivalent to a particle placed into a periodic box of length L = 2TTR. In this analogy, the "compactification" is nothing but the imposition of the periodicity arising from the walls of the box. As we expect, the momenta inside the box are quantized in units of 1/L, and we do not expect to be able to probe the features of this box if our energy scale is much less than 1/L. Indeed, it precisely by mapping out the resonance pattern of momentum wavefunctions inside the box that we learn of the existence of the box, and eventually hope to determine its volume and shape. Thus, to summarize, we see that there are two primary signatures of extra spacetime dimensions: • the appearance of an infinite tower of Kaluza-Klein states for each known four-dimensional state; and • the appearance of new fields of different spins which combine with our known fields to fill out irreducible representations of the higherdimensional Lorentz group. The first feature implies that there will be an infinite tower of heavier KK excitations corresponding to each four-dimensional field. In other words, there will be a KK tower of electrons, a KK tower of quarks, a KK tower of photons, etc., with the ground state of each tower corresponding to our usual four-dimensional particle. Note that each state in the KK tower
461
will have exactly the same quantum numbers (same spins, same couplings, etc.) as those of their ground-state particle. These KK excitations are thus essentially "echoes" off the extra dimensions, with the higher excitations corresponding to the higher momentum modes in these extra dimensions and leading to a correspondingly higher KK mass in our effective four-dimensional theory. By contrast, the second feature implies we would naively also expect to see additional fields of varying spins, as necessary to fill out representations of our higher-dimensional Lorentz group. As expected, each of these new fields will also have its own KK tower of states. These conclusions hold in the general case where we compactify on a smooth manifold K. Thus, these conclusions apply for the simple case of circle-compactification that we have been discussing here. However, in many cases the second property (the existence of particles of new spins) causes phenomenological difficulties, since it tends to require the existence of light scalar particles associated with our usual four-dimensional fields. Unfortunately, the bounds on such scalar particles are very severe. Thus, for phenomenological reasons, it is often necessary to compactify in such a way that we evade certain aspects of these predictions. We shall discuss how this can be done in the next section. One might wonder why our original higher-dimensional action should respect D-dimensional Lorentz invariance. After all, the spacetime on which we are formulating our D-dimensional theory is not MD, but rather M4XK where K is presumably a compact space. This choice therefore already breaks the D-dimensional Lorentz invariance — certain directions of the D-dimensional geometry are being compactified while others are not. Why then should D-dimensional Lorentz invariance play such a critical role? For many applications, it turns out that higher-dimensional Lorentz invariance need not be imposed. For example, later in this lecture, we will consider different types of extra dimensions in the presence of branes; the presence of a brane will break the higher-dimensional Lorentz invariance explicitly. However, for the sorts of compactifications we are discussing here, a full D-dimensional Lorentz invariance should indeed emerge in the UV limit where our characteristic energy scales are large compared to all compactification radii (and also large compared any curvature scales inherent within K, if K is a non-flat manifold). In this UV limit, our particles have extremely small Compton wavelengths compared to the compactification and curvature radii, and thus experience K as though K were effectively uncompactified and flat. Phrased slightly differently, in the highenergy limit the KK spectrum appears relatively continuous, signalling the
462
effective emergence of an uncompactified extra dimension like those of .M4. In such cases, the full D-dimensional Lorentz invariance should be restored. Finally, there are other symmetries that our D-dimensional action might also exhibit. For example, if our four-dimensional action is to exhibit an N = 1 supersymmetry, then it turns out that our D-dimensional action will need to exhibit an extended N > 1 supersymmetry, with the precise value of N depending on D. We will see an explicit example of this in Sect. 1.6. In general, our D-dimensional action must exhibit whatever symmetries are necessary in order to yield a four-dimensional action of the sort that we desire; the particular choice of the D-dimensional action then describes how we choose to embed our four-dimensional theory into higher dimensions.
1.3. Orbifold
compactifications
In the previous section, we discussed compactifications where K is a manifold (i.e., a smooth surface with no special points or boundaries), and we concentrated on the explicit case of compactifications on circles. However, as we shall see later, another important class of compactifications are on manifolds with special points, such as endpoints or boundaries. For example, rather than compactify a single extra dimension on a circle, we might choose to compactify an extra dimension on a finite line segment of a given length. Since the two endpoints of the line segment are not identified, a line segment is topologically distinct from a circle; we shall see, moreover, that compactifications on line segments give rise to radically different low-energy phenomenologies than compactifications on circles. Likewise, if we have two extra dimensions, we could choose to compactify each extra dimension on a line segment (thereby taking the compactification space to be a bounded rectangle), or one extra dimension on a circle and the other on a line segment (thereby taking the compactification space to be a finite-length cylinder). In general, a manifold with special points such as endpoints or boundaries is called an orbifold. In this section, we shall discuss how the above Kaluza-Klein procedure is modified for compactifications on orbifolds. The proper mathematical construction of an orbifold is to begin with a manifold K, and then to "mod out" by a discrete symmetry T. Here the phrase "modding out" refers to imposing a further discrete identification between points on the manifold. The resulting quotient space K/T is the orbifold. As an example, let us consider our standard case of a circle compactification, where K is a circle of radius R. This implies the periodic boundary
463
condition y <-> y + 2irR. Given this boundary condition, we may continue to regard y as taking values along the whole real number line; alternatively, we can restrict our attention to a fundamental domain 0 < y < 2TTR, with the endpoints of this range identified. However, let us now impose the additional discrete Z2 identification Y : y <-> —y. (Note that as a result of the circle boundary condition, this is equivalent to Y : y <-> 2irR — y.) circle of radius R
line segment = circle/ Z, 2 fixed points
y=jrR y=7tR
y=0
Figure 5. The orbifolding process: the circle S1 defined by y <-> y + 2nR is subjected to a Z 2 identification y <-> —y, becoming a line segment S1 l~S-i with two fixed points at y = 0 and y = -KR.
What does this do to our original circle manifold K? Clearly, the resulting space K/Y no longer consists of the full range 0 < y < 2nR, since points in this range with y > -KR are now identified with points y < TTR. The fundamental domain is thus reduced to 0 < y < irR. Note that unlike the case of the circle, the endpoints of this smaller domain are not identified with each other by either the original circle identification or the new orbifold identification. Instead, they are "fixed points" with respect to the discrete symmetry Y : y —> — y: Y:
(y = 0) -» (y = 0) (y = nR) -> (y = -TTR)
<-> (y = nR)
(1-.12)
where we have used the circle identification in this last step. Thus, our resulting manifold K/Y has the topology of a line segment of length irR, formed by "folding" the circle in half on itself. This folding process is illustrated in Fig. 5. Since S1 is used to denote the circle, S1 /Z2 is often used to denote the line segment, where Z2 indicates any discrete Z2 symmetry Y. The presence of the special fixed-point boundaries is what prevents S1 )7Li from being a manifold in its own right. The above remarks are completely general. When we mod out a manifold K by a discrete symmetry T, there are usually a set of fixed points y,.
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The resulting space K/T would be a manifold except for these special fixed points. The resulting space is thus an orbifold. The number of fixed points is often called the "order" of the orbifold, and the volume of the orbifold is always reduced compared to that of the manifold. How then do we compactify theories on an orbifold? Once again, let us concentrate on the case of a line segment S1 jTLi. We have already seen how to compactify on a circle. We thus must now consider the effects of introducing the discrete identification T : y —* —y. Clearly, since we are now interested in the Z2 action T, our first step should be to rewrite our KK mode expansions in Eq. (1.2) in terms of eigenfunctions which are even (denoted +) or odd (denoted —) under T: <S>(x»,y)
-.
1
OO
i=4 (^) + E^ +) (^) cos (f)
I-KR
v
+)
n— 1
OO
^
r
^rV)sin(f) " v" ' — \R.
(1.13)
Note that this is nothing but a rewriting of the original KK mode expansion. In each case, we have a single KK zero-mode (in this case 0 ), as well as an infinite tower of doubly degenerate excited KK modes. The explicit mapping between the original KK modes cpn in Eq. (1.2) and the modes 4>n ' in Eq. (1.13) is given by: £o +) = 4>o ,
^ 0 = -ft (<j>„ 0™ + + (/>_„)
4>\^ ^ i > o0 = = --=^( ( / > „ - -n) . (1.14)
Since this is just a reshuffling of our KK modes, we have not yet affected the physics in any way. Specifically, this is still compactification on a circle. Let us now turn to the original five-dimensional action. It is here that we will now impose our orbifold Z2 identification, requiring the five-dimensional action to be even under the discrete parity symmetry y —-> — y. If $ denotes a five-dimensional Lorentz scalar field, then the five-dimensional Klein-Gordon action (1.3) does not require a particular parity for $. However, if this parity along the fifth dimension is to be a good symmetry, then we must choose $ (which presumably corresponds to a physically observable quantum field) to have a definite parity. Depending on our choice, this then implies that we are forced to eliminate roughly half of our Kaluza-Klein modes: •t ih • / e v e n if $ is < , , >- o d d .
{ ] ^ for all n > 0 ,-, , ^ + + / 4> n = 0 then we must set < ^,, N (1.15) I 0 .
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This "orbifold projection" thus removes roughly half of the degrees of freedom of the theory, signifying that we are cornpactifying on an orbifold rather than on a circle. The choice of whether $ is taken even or odd in this case is completely arbitrary, governed by our ultimate phenomenological goals for the field . We shall see explicit examples later in this lecture. However, in either case, our resulting Kaluza-Klein tower is now only singly degenerate. Moreover, if we choose $ odd, then we lose our zero-mode ground state as well. This orbifold projection also has another important effect on the Lorentz structure of our higher-dimensional fields. To see this, let us imagine that $ actually denotes a five-dimensional gauge vector field AM where 0 < M < 4. The kinetic term for such a field then takes the usual form d xdy where FMN = 8MAM
— 8MAM
1
771
TPMN
(1.16)
+ ••• is the five-dimensional field-strength
tensor. Let us consider the mixed component F^ = d^A^ — 84 A^ + .... Clearly, since c?M is even under our parity symmetry while 84 = 8y is odd, we see that A^ and A4 must be chosen to have opposite parities! Thus, even though A^ and A4 are part of the same Lorentz multiplet, either A^ can have zero modes or A4 can have a zero mode, but not both. Indeed, at the level of the zero modes, the higher-dimensional Lorentz symmetry is broken. Similar conclusions can be drawn for fields of higher spin as well. Note that the entire five-dimensional action must be invariant under the Z2 discrete symmetry. This includes not only kinetic terms, but also possible interaction terms. Thus, we see that imposition of an orbifold symmetry may be a powerful way of removing certain unwanted interaction terms from a higher-dimensional Lagrangian. Having chosen the parities of our fields, the rest of the KK reduction procedure follows as before. For example, we obtain exactly the same KK mass spectrum. The only subtlety is that the integral over the compactified dimensions is to be performed over the full circle 0 < y < 2-KR, and not over the reduced orbifold volume 0 < y < irR. The easiest way to see this is to realize that we require an integration over the full range 0 < y < 2TTR in order to maintain the orthogonality of our Kaluza-Klein modes:
466
for all rn, n € Z + . Note that these normalizations are also consistent with the normalizations in Eq. (1.13), guaranteeing that each of our KK modes ^n>o w ^ have a properly normalized kinetic term in the resulting effective four-dimensional action. Even though we have illustrated the orbifold procedure for the simple case of compactifications on a one-dimensional line segment S1 /~Z-2, these properties tend to be quite general and hold regardless of the manifold and the discrete symmetry. For example, if V corresponds to be a Z3 symmetry of the original manifold K, then compactification on the orbifold K/Y would tend to remove a third of the excited KK levels that would have arisen in a straightforward compactification on the manifold K. Moreover, the zero mode would survive for fields which are neutral {i.e., eigenvalue +1) under the action of V. Thus, to conclude, we see that compactification on an orbifold rather than a manifold generally has two important effects on the Kaluza-Klein spectrum: • a severe reduction in the total numbers of KK states; and • elimination of the zero mode for fields which transform non-trivially under the orbifold discrete action. As we shall see later, this elimination of the zero mode for certain fields is a powerful way of using compactification to break certain symmetries at the level of the zero modes (which, we recall, may be the only observable fields at low energies). Indeed, this procedure is often called "symmetry breaking by orbifolds", which stands in stark contrast to the Higgs mechanism but nevertheless retains many of the advantageous features of Higgs-induced spontaneous symmetry breaking. Moreover, as we shall see in subsequent lectures, the orbifold symmetry T can also be used to restrict the interaction terms which might otherwise exist in the original higher-dimensional Lagrangian. Thus, compactification on orbifolds rather than manifolds will play a large role in our attempts at realistic higher-dimensional modelbuilding.
1.4. Types of extra Thus far, we have sion, one in which of extra dimension forces and particles
dimensions
been considering the simplest type of extra dimenall forces and particles propagate. This is the type originally considered by Kaluza and Klein. Since all "feel" this extra dimension, we shall designate such an
467
extra dimension as "universal". For such an extra dimension, all particles accrue Kaluza-Klein excitations. There are, however, additional types of extra dimensions which may be considered. Specifically, recent developments in non-perturbative string theory predict the existence of solitonic membranes (or Dirichlet branes, abbreviated "D-branes") to which various gauge forces can be restricted. These branes may be imagined as dynamical, fluctuating hypersurfaces floating in the extended D-dimensional spacetime. These hypersurfaces may be of any dimensionality p < D - l ; when necessary, we shall specify the dimensionality of the brane by referring to a Dp-brane. (Here p traditionally refers to the number of space dimensions only, which is why p < D — 1 rather than p < D.) It is, of course, well beyond the scope of these lectures to discuss why and how such D-branes emerge in string theory, or how they manage to trap various forces on their surfaces (or "worldvolumes"). However, the important point is that such D-branes exist, and that they have this magic property. Given the existence of such D-branes, new types of extra dimensions become possible. For example, let us begin by considering what happens if there exists a D3-brane which is capable of trapping all of the gauge forces. In this case, all of the gauge bosons, as well as particles which carry gauge charges with respect to these gauge bosons, are restricted to the brane. They therefore fail to experience any extra dimensions beyond the four spacetime dimensions of the D3-brane; they are "trapped" on the brane, and cannot propagate off the brane into the so-called D-dimensional "bulk" of the full D-dimensional spacetime. Thus, in the effective four-dimensional theory, these gauge bosons and other charged particles do not accrue any infinite towers of KK excitations associated with these extra dimensions in the bulk. By contrast, states which are completely neutral with respect to these gauge forces, such as the graviton, are free to wander off the brane, and experience the full D-dimensional spacetime. For this reason, these sorts of extra dimensions which are perpendicular to a brane are often called "gravity-only" extra dimensions: the graviton accrues KK states from these extra dimensions, but the Standard-Model particles do not. These extra dimensions are illustrated in Fig. 6. Other possibilities exist if the D3-brane traps only some forces [e.g., the SU{2) color force associated with the strong interaction], while leaving other gauge forces [such as SU(2) x U(l) electroweak forces] free. Such a brane therefore traps gluons and quarks, but fails to trap leptons which are colorless. (Because mesons and baryons are composites of trapped quarks,
468
$
&
gauge forces and particles restricted to "brane"
only gravitational force can leave "brane" and propagate in "bulk'
Figure 6. A "gravity-only" extra dimension. Gauge forces and particles are restricted to the brane, while only the gravitational force can leave the brane and propagate in the bulk.
they are also presumably trapped.) However, gravitons would again be free to wander throughout the higher-dimensional space. It is also possible to imagine several D3-branes, one trapping SU(3) and another trapping SU(2) x U(l). Gluons would then be trapped to the SU(3) brane, while the W±, Z, and leptons would be trapped to the electroweak brane. Quarks would have to be simultaneously trapped on both branes — i.e., trapped at the location of their intersection. Indeed, having such branes intersect becomes a model-building requirement in order to give rise to light quarks. Thus far, we have only discussed D3-branes. In general, however, forces can be trapped on Dp-branes for arbitrary p < D — 1. In such cases, the particles trapped on the Dp-brane will accrue infinite towers of KK excitations associated with the p — 3 extra dimensions in the brane, but will not have any KK excitations associated with the D — p dimensions in the "bulk" perpendicular to the brane. (These are often called "longitudinal" and "transverse" extra dimensions respectively.) Of course, such a Dp-brane cannot be imagined as a flat, infinite hypersurface for p > 3, since ultimately the p — 3 extra dimensions in the brane will need to be compactified (just as the bulk extra dimensions also need to be
469
compactified). Thus, such a brane will typically take a "wrapped" shape {e.g., like a higher-dimensional cylinder, with the usual M4 running along the infinite direction of the compactified extra dimensions in the brane running around the circumference), or a truncated shape {e.g., like a semiinfinite rectangle where the infinite edge corresponds to M4 while the finite edge corresponds to a compactified space with a boundary). Note that since gravity is nothing but a reflection of the geometry of the spacetime, it is impossible to "trap" gravity except by curving the spacetime itself. We will discuss such theories briefly in Lecture # 4 . Such "Randall-Sundrum theories" give rise to very interesting phenomenologies as well as important theoretical insights, but are beyond the scope of these lectures. We see, then, that the existence of D-branes gives rise to a very rich set of geometric possibilities for higher-dimensional model-building. We can imagine branes of various types and dimensionalities, extending along different directions and wrapping and/or intersecting in a variety of different ways. By changing the sizes and compactification patterns of the overall spacetime and the branes that reside therein, we can construct models of incredible richness and variety. This, then, becomes the language of modern model-building in higher dimensions. We close this section with three important comments. First, it is important to realize that our D-branes are not infinitely thin objects; like any soliton, they have a characteristic thickness which is ultimately set by the underlying string scale. However, for many purposes it is legitimate to approximate such branes as being infinitely thin, and to incorporate the associated trapping of forces and particles into the higherdimensional Lagrangian by using Dirac (^-functions. For example, if our full spacetime is five-dimensional but there exists a D3-brane located at y = 0, we would write our action as S5 =
/ d4x / dy {£buik + CbTa,ne5{y)}
(1.18)
where £buik is the five-dimensional Lagrangian for the bulk fields £brane is the four-dimensional Lagrangian for the trapped brane fields. For example, if the D3-brane traps all of the gauge forces of the Standard Model, then £buik might be the Lagrangian for general relativity while £brane would be the Lagrangian for the Standard Model. This setup originally emerged in work of Hofava and Witten, and forms the basis of the so-called "ADD model" (due to Arkani-Hamed, Dimopoulos, and Dvali). Upon KaluzaKlein reduction as outlined above, the presence of the Dirac ^-function
470
then eliminates the KK excitations for the Standard-Model particles. Of course, in cases where the brane thickness is important (and we shall see some examples of this in Lecture #3), £brane would be replaced by a higherdimensional Lagrangian and the Dirac 5-function would be replaced by a brane profile function with the appropriate thickness. Our second comment is that D-branes are dynamical objects. While it is tempting to consider them as rigid and immovable, D-branes are actually fluctuating with degrees of freedom of their own which can be associated with their positions and momenta. Once again, the energy scale for such fluctuations is set by the underlying string scale, and thus may be sufficiently high that they the dynamics of the brane itself may be ignored for low-energy phenomenological applications. However, since the brane has its own stress energy, it warps the spacetime around it just as any massive object would. In these lectures, we shall ignore this effect and consider the extra dimensions perpendicular to brane to be flat (even though compactified); however, in some circumstances (such as the case where there is only one extra dimension perpendicular to the brane), this warping effect is actually quite significant and important. As indicated earlier, we shall discuss such Randall-Sundrum scenarios briefly in Lecture # 4 . Finally, even though we have presented D-branes as mysterious objects capable of trapping various kinds of particles, it is useful to have a rough sense of the underlying string-theoretic ideas that give rise to these pictures. In Fig. 3, we sketched the basic idea behind string theory, representing each fundamental particle as a different vibrational mode of a closed string. This picture is valid for closed-string theories (such as the Type II and heterotic strings), but there also exist string theories based on a mixture of open and closed strings. These are the so-called Type I strings. (It is not possible to formulate an interacting theory consisting only of open strings, since an open string theory always contains a string configuration in which the endpoints are joined together, forming a closed string.) In fact, as we shall discuss in Lecture # 2 , the most natural ways to realize our theories of extra dimensions in string theory are through Type I strings. In Type I string theory, particles which carry gauge charges are represented by open strings, and the actual charges themselves are associated with the endpoints of these open strings. This type of charge is often called a "Chan-Paton charge" in the string literature. Indeed, one can form an analogy with strong interactions by picturing a meson as an open string: the endpoints of the string correspond to the quark and antiquark, while the string itself corresponds to the gluonic flux tube. (It was, in fact, this
471
analogy that gave rise to string theory in the late 1960's as a theory of strong interactions.) By contrast, a closed string is one which contains no endpoints, and which must therefore be neutral with respect to these types of charges. For example, while all quarks and leptons and gauge bosons of the Standard Model carry gauge charges of one sort or another, and hence are typically represented within the Type I framework by open strings, gravitons are neutral and thus are represented by closed strings. a This contrast between open strings and closed strings is relevant for us because D-branes are the surfaces on which the endpoints of all open strings must end. In other words, open strings must be attached to (or "trapped" on) D-branes. Closed strings, by contrast, have no endpoints, and are thus free to wander off the D-brane into the bulk. Needless to say, there are many constraints which govern the types of D-branes and geometric configurations that can exist within a given Type I string model. These constraints govern all sorts of parameters such as the total number of D-branes, which gauge forces they may trap, the spectrum of Dp-brane dimensionalities (i.e., the allowed spectrum of values of p), and even which dimensions they may occupy, how they may overlap, and the angles, if any, of their intersections. All of these features must be chosen in order to satisfy numerous string-theoretic constraints, chief among them the cancellation of certain dangerous anomalies. Clearly, the art of Type I string model-building is a study unto itself which we cannot enter into here. Thus, in order to make progress, D > 4 model-builders can take either of two approaches. They may follow a "top-down" perspective, building a fully self-consistent Type I string model and then investigating its low-energy phenomenological properties. Such model-builders are typically a
Note that although the photon is neutral, it must also be represented by an open string. This arises because the photon, like all gauge bosons, actually transforms in the adjoint of the corresponding group; it is only an "accident" of the abelian nature of the C/(l) gauge group that the photon happens to be electrically neutral. Similarly, there may also be other particles (such as the right-handed neutrino) which are neutral with respect to all Standard Model gauge symmetries. Whether such particles are represented by open or closed strings depends on the particular Type I string model under study. If the Standard Model gauge symmetry is realized by breaking a grand unified (GUT) symmetry such as SO(10) or SU(5), the right-handed neutrino will carry appropriate gauge charges under this larger GUT symmetry and must therefore emerge as an open string state. However, if the Standard Model gauge symmetry is realized directly at the string scale without passing through an intermediate GUT symmetry, both options are possible and depend on the particular string construction. Similar comments also apply to other gauge-neutral particles such as axions. We shall discuss some of the phenomenological implications of these choices in Lecture # 3 .
472
string theorists or string phenomenologists. Alternatively, model-builders can follow a "bottom-up" perspective, studying various phenomenological properties of theories in extra dimensions, developing scenarios for solving phenomenological problems in low-energy particle physics using KaluzaKlein states, imagining various ad hoc brane and then determining whether there might exist a suitable brane configuration which might give rise to these hypothetical models. Such studies usually take place within the language of a higher-dimensional effective field theory, borrowing only those ideas from string theory (such as t h e notion of t h e brane and perhaps a few stringy constraints concerning allowed brane configurations) as necessary. Of course, one hopes t h a t these two approaches eventually meet, t h a t a given phenomenologically interesting brane configuration built'through a "bottom-up" analysis can ultimately be demonstrated to arise from a bonafide T y p e I string construction. Since higher-dimensional field theories are intrinsically non-renormalizable, one hopes t h a t such an embedding into a fully consistent Type I string model will be possible at some energy scale because string theory provides one of the few known finite theories in higher dimensions.
1.5. Bounds
on extra
dimensions
Having discussed general theoretical possibilities concerning the types and properties of extra dimensions, let us now t u r n to some experimental constraints. Perhaps the most important experimental fact concerning e x t r a dimensions is the following: at the present time, there exists absolutely no experimental evidence that any extra dimensions of any type exist. b Given our previous discussion in Sect. 1.2, what this means is t h a t the lightest Kaluza-Klein excitations have never been discovered for any particle. This refers not only to a direct discovery (e.g., through direct production of K K states), but also to an indirect discovery (e.g., through loop effects or new, effective interactions t h a t they may induce). However, turning this into a bound on extra dimensions depends on the type of extra dimension under study. "Universal" extra dimensions of the Kaluza-Klein type, felt by all forces and all particles, would in principle be accessible through collider experiments. Since colliders have probed the b
Of course, the same may also be said of supersymmetry, grand unification, string theory, or nearly any extension to the Standard Model. As a general rule, there is never any experimental evidence that something exists... until, of course, it is discovered!
473
structure of matter down to a length scale of approximately 10~ 18 meters, or equivalently probed the spectrum of particles up to an energy scale near the TeV range, our failure to have detected extra dimensions through collider experiments implies that any possible extra dimensions of this sort must have a radius R-1
> 0(1) TeV .
(1.19)
Here 0(1) denotes a factor of one to several (e.g., 3 or 4), depending on the particular model under study. If the Standard Model (or a subsector of the Standard Model) is trapped on a Dp-brane, this bound continues to apply for those p — 3 longitudinal dimensions in the brane. This is because those gauge bosons and charged particles trapped on the brane necessarily accrue Kaluza-Klein excitations corresponding to the extra dimensions in the brane, and thus could potentially lead to observable effects at colliders. Thus, the bound (1.19) applies quite generally for any longitudinal extra dimensions (i.e., extra dimensions in the brane). It is, however, sometimes possible to evade this constraint slightly. For example, there exist certain models in the literature in which there exists a conserved quantum number for KK states which implies that any excited KK states must be produced in pairs. In such cases, it therefore becomes twice as difficult to produce KK excitations, which implies that the experimental bound on the size of the corresponding extra dimensions becomes roughly half as severe. In such cases, one can then potentially tolerate i ? _ 1 > 500 GeV. However, the important point is that such extra dimensions are generally bounded in the TeV range. The remaining extra dimensions are of the "transverse" type, perpendicular to the brane(s) on which the Standard-Model gauge forces and particles are trapped. Since no Standard-Model forces or particles feel these extra dimensions, these extra dimensions are immune to collider probes. Instead, the primary effects of such extra dimensions are gravitational, since it is often only the graviton which can propagate in their bulk. We therefore must ask: down to what length scale have we tested gravity? Such tests could take the form of direct tests of the Newtonian gravitational inverse square law (e.g., through Cavendish-type experiments), or indirect tests through astrophysics or cosmology. In this case, not surprisingly, the experimental bounds are much weaker. Let us first consider Cavendish-type experiments. The normal four-dimensional graviton state is the particle responsible for ordinary
474
Newtonian gravity, with the inverse-square force law arising from graviton exchange. If the graviton is now considered to be merely the zero-mode of a potential KK tower of gravitons, we must consider how exchanges of the excited KK gravitons will affect the inverse-square law. In general, if there are gn graviton states with masses mn = n/R at the n t h KK level, the corresponding corrections to the Newtonian gravitational potential take the form
1
V(r)
+ XI 9ne~
(1.20)
where GN is the usual four-dimensional gravitational Newton constant. For length scales r S> R, the corrections from all but the first excited KK graviton will be exponentially suppressed. We therefore find
V(r) = -9lfMl
[i +
fllC-/«]
.
(1.21)
Precision laboratory Cavendish-type experiments have been performed, searching for deviations from Newtonian gravity of just this form. Although our main interest is in Kaluza-Klein gravitons as the origin of such deviations, there are many other effects which could potentially produce corrections of this form as well. Results are therefore usually quoted simply as excluded regions in the parameter space of (gi,R). The most recent plot summarizing the results of various experiments is shown in Fig. 7. Since our KK graviton theories have gi ~ 0(1) (depending on details to be discussed shortly), the excluded regions in Fig. 7 imply the constraint R < 0.1 millimeter
<=>
R'1
> O(10~ 4 eV) .
(1.22)
This may seem like a very weak constraint. Basically, this constraint indicates that despite years of intense, sophisticated experimental effort, we simply do not know much about the behavior of gravity at length scales shorter than fractions of a millimeter. However, it must be remembered that gravity is an extremely weak force. Indeed, by definition, the couplings of gravitons to matter are suppressed by powers of the Planck scale. It is an extremely subtle art to be able to design experiments capable of measuring such minute forces with such precision. Alternatively, it may seem that extra dimensions of this potentially large size could be ruled out by astrophysical or cosmological means. After all, if there exist excited KK gravitons which are really as light as 10 ~ 4 eV, it would seem that such light states could have a huge effect on processes such
475
Figure 7. Exclusion plots for deviations from Newtonian gravity. The heavy solid line denotes the most recent result from the so-called "Eot-Wash group" at the University of Washington; the entire range above this line is excluded at the 95% confidence level. Results from previous experiments, as well as important theoretical regions, are also shown. When comparing to Eq. (1.21), A denotes R and a denotes gi. The horizontal lines labeled as "two extra dimensions" indicate a = g\ = 3 and 4, corresponding to compactifications on a two-sphere and a two-torus respectively.
as stellar cooling (where extra KK gravitons could carry away energy and thereby increase stellar cooling rates beyond observational bounds). They could also potentially affect supernova dynamics, etc. However, detailed studies have been performed, and in all cases it is found that the constraints in Eq. (1.22) are not significantly altered. Discussions of these analysis can be found in the literature. The upshot, then, is that "gravity-only" extra dimensions could potentially be as large as fractions of a millimeter. Indeed, we might be living in a D3-brane universe, with an extra dimension of submillimeter size immediately adjacent, and yet have no experimental way of detecting this extra dimension. It is also possible to place constraints on the time-variation of the sizes of extra dimensions. To understand the procedure by which this is done,
476
let us consider a simplified picture in which the only forces are gravity and electromagnetism. The fundamental constants of nature thus appear to be GN (the four-dimensional gravitational Newton constant) with mass dimension (length) 2 , and a^4' (the four-dimensional electromagnetic finestructure constant) with mass dimension (length) 0 . In D > 4, however, these fundamental constants have different mass dimensions: [G™] = (length) 13 " 2 ,
[a] = (length) 0 " 4 .
(1.23)
As evident through a Kaluza-Klein reduction, the higher- and lowerdimensional constants are therefore related by powers of the compactification radius: /~i(D)
(D)
GN o* . G(4) _ (4) _ ( N D 4 D 4 { ~ R ' ~ R ~ ' ' But which are the true constants of nature? If our underlying theory is really D-dimensional, then it is the D-dimensional constants are the true constants of nature; the four-dimensional constants are only derived quantities once a particular compactification radius is specified. Therefore, if the compactification radius has a relative time rate of change R/R, then the apparent four-dimensional "constants" of nature will experience a relative rate of change ^ l - ^ G(4)
"
a (4)
= U-D)X ^
(125)
U
)
R
•
V-^)
The time variations of any possible compactification radii can thus be constrained by constraining the time variations of the fundamental constants of nature. Note that Eq. (1.25) holds for universal extra dimensions. In general, relations such as these hold for any gauge coupling as long as the corresponding gauge bosons experience the corresponding extra dimension^). The question then boils down to determining the constraints on time variations of the four-dimensional Newton constant GN and the fourdimensional fine structure constant a^4'. In general, such bounds come from many sources: • primordial nucleosynthesis and element abundances (which are sensitive to a through the proton/neutron mass differences and to az through nuclear stability bounds); • astrophysical observations {e.g., observations of high-redshift radio galaxies, whose signatures can provide information about a at earlier times); and
477
• terrestrial measurements (most notably, through evidence of a natural geochemical reactor in Oklo, West Africa, that went critical « 1.8 x 109 years ago and which thereby provides bounds on nuclear stability at that time). Putting all these constraints together, we find rather significant bounds on GN/GN: GN GN
< 10" 1 1 y " 1 ,
(1.26)
as well as even more stringent bounds on a/a: \a/a\ \a/a\
< 10_lr y_1 < 10
-16
y"
1
over 109 years over 10
10
years
(Oklo reactor) , (nucleosynthesis) . (1.27)
Together, these constraints imply that extra dimensions have been essentially fixed in size since nucleosynthesis (i.e., since the first three minutes after the big bang). There are, of course, methods by which these bounds can be evaded. For example, it is possible that R/R oscillates rather than changes monotonically; thus there would not be a discernable trend in the direction of evolution of these couplings. Similarly, there may be conspiracies: for example, even if the fine-structure constant changes, the strong coupling «3 might change in a compensatory way so that together, the couplings conspire to preserve nuclear properties. However, none of these scenarios are particularly likely. Is there any evidence for non-zero variations in the couplings? Using spectroscopic observations of the absorption lines of gas clouds against background quasars at redshifts in the range 1.0 < z % 1.6, there have recently been claims of a non-zero variation in the fine-structure constant of magnitude — = (-2.64 ± 0.35) x 10" 5 . (1.28) a Given the redshifts involved, this is just at the edge of the limits quoted above. However, these measurements tend to have large systematic calibration errors. Thus, further study is needed before a definitive conclusion can be drawn. Finally, let us consider whether there might be spatial variations in the fine-structure constant (i.e., variations in a observed across different
478
regions in the sky). The best bounds are currently — < 3 x 10" 6 for Ax « 3000 Mpc . (1.29) a Thus, once again we see that any possible variations are highly constrained. 1.6. Embedding
4D theories
into extra
dimensions
We now turn to a practical matter: given what we have learned about extra dimensions, how do we take a four-dimensional theory and embed it into higher dimensions? In this section, as an illustrative example, we shall describe the embedding of the Minimal Supersymmetric Standard Model (MSSM) into five dimensions. As we shall see, the issues that arise will be fairly typical for most models of interest. Recall that the MSSM matter content consists of gauge bosons (gluons, W^, Z, and photon); two Higgs fields (Hu and Hd); and three generations of chiral fermions. Moreover, because the MSSM has N = 1 supersymmetry, each of these fields comes in aa JV = 1 SUSY representation: the gauge bosons come in N = 1 vector supermultiplets (thereby including their gaugino superpartners); the Higgs fields come in N = 1 chiral supermultiplets (thereby including Higgsino fields); and the chiral fermions also come in N = 1 chiral supermultiplets (thereby including both the fermions and their sfermion superpartners). In order to embed this theory into five dimensions, at first glance it may seem that for each of these fields, we simply introduce a single KK tower of identical corresponding states. However, we can immediately see that this procedure runs into trouble. The first source of trouble concerns the fact that our four-dimensional theory (the MSSM itself) has N = 1 spacetime supersymmetry. Since our four-dimensional theory has N = 1 supersymmetry, this means that our five-dimensional theory must have at least this much supersymmetry as well. However, N = 1 supersymmetry in five dimensions is equivalent to N = 2 supersymmetry in four dimensions! The easiest way to see this is to realize that a single spin-3/2 gravitino in five dimensions decomposes into two spin-3/2 gravitino representations in four dimensions. Again, this is ultimately a reflection of the fact that our higher-dimensional fields typically have an expanded Lorentz (and thus also supersymmetry) structure compared with the structure in four dimensions. The second problem that we face concerns the chirality of the MSSM (and indeed, of the Standard Model itself). In five spacetime dimensions,
479
there is no such thing as chirality; chiral representations of the Lorentz algebra are possible only in even numbers of spacetime dimensions. Again, the reason for this is completely analogous to the reason for the expanded supersymmetry. A single fermion in five dimensions decomposes into two fermions in four dimensions — a chiral fermion, and its chiral "conjugate" which has opposite chirality. Thus, the five-dimensional theory can have no intrinsic chirality of its own. More mathematically, this follows from the fact that the (Euclidean) Lorentz (rotation) group in D dimensions is SO(D), yet only when D is even does this group contain chiral spinor representations. What this means, then, is that although our "zero-mode" theory (the MSSM) must be N — 1 supersymmetric and chiral, our field content at all excited KK modes must be N = 2 supersymmetric and chiral. What does this mean for embedding the MSSM into higher dimensions? Let us first consider the gauge fields. Rather than merely repeat the N = 1 supersymmetric vector supermultiplet at each excited KK level, we must also introduce a new N = 1 chiral supermultiplet at each excited level. Together, these representations combine to produce an TV = 2 vector supermultiplet at all excited KK levels. The corresponding situation is illustrated in Fig. 8(a). A similar situation exists for each Higgs field. Rather than merely repeat the N = 1 supersymmetric chiral supermultiplet at each excited KK level, we must also introduce an additional N = 1 chiral supermultiplet at each excited level. Together, these representations combine to produce an N = 2 hypermultiplet at each excited KK level, as illustrated in Fig. 8(b). Embedding each fermion field into five dimensions requires a similar situation. While we wish to retain only the original four-dimensional chiral supermultiplet fermion as our zero-mode field, we find that we must introduce an additional chiral supermultiplet (a so-called chiral-conjugate "mirror fermion") at each excited KK level in order to produce a non-chiral N = 2 hypermultiplet. In each case, we see that we are forced into a situation where the field content at each excited KK level is different from, and larger than, the field content of the zero modes. Clearly, there is only one way to achieve this. As discussed in Sect. 1.3, we must actually compactify on an orbifold (in this line segment) rather than on a circle. We then must choose this orbifold so that all of the MSSM fields are chosen even with respect to the Z2 orbifold symmetry, while all of the fields that we needed to introduce beyond the MSSM must be chosen odd. This guarantees that these extra
480
(a) gauge fields:
A, A,
(b) Higgs fields:
,V
H + ,v+
H_ .V-
A, X
H + .V+
H_ ,v_
A,X
H + .v+
H_ ,V-
A,X
H + .V+
(A, X): N=1 vector supermultiplet (0, V): N=1 chiral supermultiplet N=2 vector supermultiplet
(H + , V + ): N=1 chiral supermultiplet (H _, V_) : W= J c/i/'ra/ supermultiplet —^^-
N=2 hypermultiplet
Figure 8. Embedding the non-matter field content of the MSSM into five dimensions, (a) Gauge fields: The zero modes contain only the N = 1 vector supermultiplet, while each excited KK level contains this supermultiplet as well as an N = 1 chiral supermultiplet. Together, these representations combine to produce an N = 2 vector supermultiplet at each excited KK level, (b) Higgs fields: The zero modes contain only a single N = 1 chiral supermultiplet, while each excited KK levels contains two copies of such chiral multiplets. Together, these combine to produce an N = 2 hypermultiplet at each excited KK level.
fields beyond the MSSM will not have zero modes, which is in accordance with our original desire that our low-energy, observable, four-dimensional theory is merely the MSSM. The case of the MSSM is quite typical. Rather than compactify on higher-dimensional manifolds, we typically are forced to compactify on orbifolds for two reasons: • We must reduce the extended supersymmetry (N = 2 or N = 4 supersymmetry) that would arise higher dimensions, leaving us with only N — 1 supersymmetry at the zero-mode level; and • We must introduce chirality at the level of the zero modes. Once again, this is achieved through the (chiral) orbifold projection which removes the chiral-conjugate states.
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In other words, in the language of symmetry breaking, we would say that we must compactify on an orbifold rather than a manifold in order to (partially) break the supersymmetry as well as break the non-chiral spectrum into a chiral one. Having seen how to embed each individual MSSM field into five dimensions, we now must address the only remaining issue of deciding which fields should indeed feel the extra dimensions. As we discussed in Sect. 1.4, there are many different types of extra dimensions, some of which can be felt by some fields and not others. In general, assuming that we are not talking about "gravity-only" extra dimensions (which none of the MSSM fields would feel), we are left with various choices as to which gauge groups and which charged particles can propagate in which extra dimensions. If we are considering a five-dimensional theory, the most natural situation is to imagine that all of the gauge forces can feel this extra dimension. This means that all of the MSSM gauge bosons will propagate in the extra dimension. We then must decide whether our Higgs fields and/or fermion generations will feel this extra dimension as well. In general, we can let e = 0,1,2 denote the number of Higgs fields which feel the extra dimension; likewise, we can let r\ = 0,1,2, 3 denote the number of fermion generations which feel the extra dimension. (The remaining fields are imagined to be restricted to brane intersections or orbifold fixed points.) These parameters (e, r\) thus join the radius R as parameters which describe different possible embeddings of the MSSM into higher dimensions.
2. Lecture # 2 : Large extra dimensions and the brane world In the previous lecture, we discussed general properties of extra spacetime dimensions, and we noted that the experimental bounds on the sizes of such extra dimensions are actually quite generous. Extra "longitudinal" dimensions in the brane can potentially be as large as an inverse TeV, while extra "transverse" dimensions perpendicular to the brane can potentially be as large as tenths of a millimeter. Despite these bounds, extra dimensions were traditionally considered to be quite small, on the order of the Planck length. The reason for this was, indeed, naturalness: since extra dimensions emerge primarily in theories such as string theory which also contain gravity, their natural length scales are at the Planck length. Clearly, if these extra dimensions are so small, their corresponding Kaluza-Klein excitations will have masses ~ 10 19 GeV.
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These excitations will be essentially unobservable. For this reasons, it was not traditionally considered that such extra spacetime dimensions could ever have relevance for the low-energy world, or four our phenomenological ideas about physics beyond the Standard Model. In particular, it was felt that they could never alter our fundamental picture for physics beyond the Standard Model, which includes supersymmetry at the TeV scale; grand unification near 1016 GeV, and ultimately some sort of embedding into string theory near 10 1 8 " 1 9 GeV. However, all of this began to change dramatically in 1998. Specifically, high-energy theorists began to wonder what would happen if potential extra dimensions were not so small, or equivalently if their corresponding KK states were not so remotely heavy? If the KK states were to become sufficiently light, one could imagine that they could begin to have a direct effect on the physics beyond the Standard Model! What would these effects be? Surprisingly, it was found that large extra dimensions have the power to alter the fundamental high energy scales of physicsl In other words, supposedly "fixed" energy scales such as the GUT scale, the Planck scale, and the string scale are actually not fixed at all, but can be altered in theories with extra dimensions of suitable sizes. As we shall see, this has given rise to a dramatically new set of possibilities for physics beyond the Standard Model. In this lecture, I shall present this new picture and discuss how and why the presence of large extra dimensions can have these profound effects. Ultimately, taken together, these new possibilities have given rise to an alternative picture for physics beyond the Standard Model: this picture is often called the "brane world". It is our goal in this lecture to introduce this new paradigm.
2.1. The Standard Paradigm for physics Standard Model: A quick review
beyond
the
By way of contrast, let us first review the standard paradigm for physics beyond the Standard Model. This was the unique, prevailing picture which held sway until 1998, and it is still a compelling (and perhaps even correct) picture for physics at energy scales beyond those of present-day colliders. Our focus will be on the energy scales inherent in this picture. Our first energy scale of interest is the GUT scale. By measuring the strengths of the gauge couplings corresponding to the fundamental forces [strong SU(3), weak SU{2), and hypercharge U(l)] of the Standard Model and extrapolating them upwards within the framework of either the
483
Standard Model itself or its minimal N = 1 supersymmetric extension (the MSSM), one finds that they meet at approximately M GU T « 2 x 1016 GeV ,
(2.1)
with aouT ~ 0.04 = >
MPlanck = y / —
(2.2)
This is, of course, the Planck scale. Alternatively, we can view this as the energy scale at which the gravitational interaction becomes roughly as strong as the gauge interactions. If we view GJV as a "gauge coupling" for gravity, then the fact that it is dimensionful (with dimensions of inverse
60
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i
5 log 10
i
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i
i
.
.
[
.
10 15 (M/GeV)
.
.
•
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Figure 9. Gauge coupling unification within the framework of the MSSM. The three gauge couplings appear to unify at an energy scale MQTJT ~ 2 X 10 16 GeV. The thickness of the lines represents current experimental uncertainties in the low-energy measurements of these couplings.
484
mass squared) implies that GN "runs" with a classical scaling ~ y? where /j is the corresponding energy scale. In other words, the gravitational interaction gets dramatically stronger with increasing energy, ultimately becoming ~ 0(1) at the Planck scale. One therefore often associates the Planck scale with the scale at which quantum gravitational effects should become significant. The string scale is, strictly speaking, another independent energy scale governing physics beyond the Standard Model. In principle, this scale is set by the so-called "Regge slope" a' (which is in turn set by the string tension): one has
Mstring = - L = .
(2.3)
However, the relation between a' and other physical parameters such as GN depends on the particular string model under study. In the case of the weakly coupled heterotic string, we find that we must set ^string
where String is the string coupling (the strength of string interactions). The relation (2.4) must hold in order that the string states we wish to identify as gravitons actually induce gravitational interactions of the correct strength. Thus, combining Eqs. (2.2), (2.3), and (2.4), we find that -^string =
ffstringMpianck
•
(2.5)
Unfortunately, the value of String is not known; this value can only be determined by understanding the non-perturbative dynamics of a special string state known as the dilaton. However, based on the prejudice (coming from grand unified theories) that String should be approximately 0.7, we find that Mstring ~ 10 18 GeV. We emphasize, however, that this value for the string scale rests upon a number of important assumptions, including the choice of a particular string framework (namely, the weakly coupled heterotic string). For other types of string theories, the value of M s t r i n g may be quite different. We see, then, that we have a huge hierarchy of energy scales: the energy scales associated with "fundamental" physics (grand unification, quantum gravity, and string theory) are at least thirteen orders of magnitude beyond those that we currently probe through collider experiments. This is an important hierarchy, because the energy scales that we currently probe are associated with another important energy scale, namely the scale of
485
electroweak symmetry breaking. In order to stabilize this hierarchy against quantum corrections, the traditional approach is to introduce supersymmetry and imagine that this supersymmetry is broken at some energy scale beyond the scale of electroweak symmetry breaking. There are several considerations which go into the choice of this SUSYbreaking scale. Most importantly, since supersymmetry is meant to stabilize the gauge hierarchy, we must invoke this symmetry at an energy scale which is not too far beyond the scale of electroweak symmetry breaking. Otherwise, the protection that supersymmetry provides for quantities such as the radiative corrections to the Higgs mass will be lost. Moreover, it is found that supersymmetry just beyond the weak scale is also necessary in order to achieve the precise gauge coupling unification shown in Fig. 9. Thus, we are led to imagine that the scale for supersymmetry breaking (i.e., the energy scale at which the lightest superpartners will appear) should be somewhere not too far beyond the scale of electroweak symmetry breaking. This scale is typically taken to be in the TeV range.
The Standard Paradigm. (inverse) strengths of forces
Energy (GeV) 1000 SUSY? (TeV)
GUT scale
Planck (string) scale
fundamental physics
"The Desert" Figure 10.
The Standard Paradigm for physics beyond the Standard Model.
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Thus, combining these energy scales, we are led to the "Standard Paradigm" for physics beyond the Standard Model. This picture is sketched in Fig. 10. As we have stated above, this picture is highly successful, and leads to a number of elegant solutions for many outstanding problems ranging from proton decay and neutrino physics to dark matter and cosmological evolution. However, our purpose in the rest of this lecture will be to present another, alternative picture. As we shall see, this new picture will be radically different from the standard one, providing new benefits but also simultaneously introducing new challenges. 2.2. Extra dimensions:
Lowering
the GUT
scale
Let us now see how the- standard paradigm may-be affected by the presence of large extra dimensions. We shall first focus on the case of the GUT scale.
Figure 11. One-loop wavefunction renormalization diagram. This diagram contributes to the one-loop running of the gauge couplings, where the entire field content of the theory runs in the loop.
In the usual MSSM calculation, one derives the one-loop running of the SU(3) x SU(2) x U(l) gauge couplings by evaluating the one-loop wavefunction renormalization diagram shown.in Fig. 11, where all MSSM states can propagate in the loop. This then leads to the logarithmic renormalization group equation (RGE)
ar\^) = a-\Mz)-^\n(j^}
,
(2.6)
where bi are the one-loop MSSM beta-function coefficients. Given the experimentally measured gauge couplings at the Z-scale, one can use this RGE to extrapolate the gauge couplings upwards in energy. This then
487
leads to the conventional unification near M Q U T ~ 2 X 10 16 GeV, as shown in Fig. 9. How do we extend this calculation into higher dimensions? First, let us imagine that there exist 5 = D — 4 "longitudinal" extra spacetime dimensions, each compactified on a circle of radius R. Here S = 1,2,... can take any integer value, and likewise .R - 1 , which sets the energy threshold for the extra dimensions, can range anywhere from the TeV scale all the way up to the usual GUT scale M Q U T ~ 2 X 10 16 GeV. Corresponding to each MSSM field, there will be an infinite tower of Kaluza-Klein states whose masses are separated by R~l. Of course, for the reasons discussed in Sect. 1.6, we actually must compactify on orbifolds rather than circles, and likewise our states at the excited levels of the Kaluza-Klein towers must fall into N = 2 supermultiplets (even though the ground-state zero-mode MSSM states are only N = 1 supersymmetric). It also turns out not all of the Standard Model particles need have Kaluza-Klein excitations. Thus, the particle content at each excited Kaluza-Klein level can be different from the content of the MSSM zero-mode ground state. Given this setup, let us now calculate the higher-dimensional "running" of the gauge couplings. To do this, we must simply re-evaluate the standard one-loop wavefunction renormalization diagram in Fig. 11. However, we now have not only the usual MSSM states, but also their their corresponding Kaluza-Klein excitations propagating in the loop. What is the effect of these Kaluza-Klein excitations? Evaluating the one-loop diagrams, we find the general result
(2.7) In this equation, the Jacobi theta-function $3(1-) = J^^L-oo exp(-7riTn2) reflects the sum over the Kaluza-Klein states. The b\ are the beta-function coefficients describing the matter content at the excited Kaluza-Klein levels (which depend on the particular embedding of the MSSM into higher dimensions, as discussed in Sect. 1.6). The numerical coefficient r is given by r = TT(XS)-2/5 where Xs = 2ns/2/6T(S/2); this factor is needed as an overall normalization, and corresponds to the volume of a <5-dimensional unit sphere. While the final term in Eq. (2.7) reflects the contributions from the Kaluza-Klein towers, the penultimate term reflects the fact that the zero-mode MSSM states and their Kaluza-Klein towers need not be identical.
While the result in Eq. (2.7) emerges from an exact summation over the Kaluza-Klein states, it is possible to simplify this expression in most cases of interest. For energy scales below R"1, we find that the excited KK states do not substantially contribute to Eq. (2.7); thus, at energy scales below i ? _ 1 , it is legitimate to use the standard four-dimensional RGE in Eq. (2.6). However, at energy scales above -ff-1, we can approximate A 3> R~x and evaluate the Jacobi-theta-function integral explicitly, obtaining the simplified RGE ar\A)
* a-\R-')
-
^
ln(Ai?) -
| J
[(AR)S - l] .
(2.8)
While this result is derived only in the limit A » -R -1 , it is easy to verify that it actually holds with extreme accuracy all the way down to AR « 1. We see, then, that the net effect of the Kaluza-Klein states running in the loop in Fig. (11) is to change our logarithmic evolution of the gauge couplings into a power-law evolution! There are several ways to interpret this result. From a four-dimensional perspective, we simply have a theory endowed with infinite towers of KK states. Thus, our gauge couplings continue to experience logarithmic evolution, but with a beta-function coefficient which is constantly changing as new KK thresholds are crossed at higher and higher energies. This combination of logarithmic running with a scale-dependent beta-function coefficient thus gives rise to an effective power-law evolution. But perhaps even more fundamentally, this is really a higherdimensional theory. As we have already seen in Eq. (1.23), gauge couplings in D — 4 + S spacetime dimensions are no longer dimensionless; they have mass dimensions [a] = S. Thus, we expect that they should accrue a power-law scaling with exponent S in addition to their quantum running. The Kaluza-Klein states thus manage to achieve, from a fourdimensional perspective, exactly what we expect to observe in our underlying five-dimensional theory. Note that the coefficient of the power-law scaling piece within Eq. (2.8) is indeed nothing but bi, which depends on the field content at the excited KK levels. So, we see that the presence of extra dimensions changes the evolution of the gauge couplings, introducing power-law evolution rather than the traditional (four-dimensional) logarithmic evolution. But what does this do the unification that exists in the four-dimensional case? To see this, let us first examine an extreme case. Let us choose R~l = 1 TeV, and take 6=1. Let us also imagine that only the gauge bosons
489
and a single Higgs gauge field propagate in the extra dimensions (i.e., we take e = 1 and rj = 0 in the language of Sect. 1.6). This implies the values (61,62,63) = (33/5,1,-3) and (61,62,63) = ( 3 / 5 , - 3 , - 6 ) . We then find the evolution of gauge couplings illustrated in Fig. 12. Note that although the running of the gauge couplings is profoundly altered, becoming power-law rather than logarithmic, the unification of the gauge couplings is nevertheless preserved! Thus, we see that extra dimensions are consistent with the emergence of a grand unified theory in the TeV range. In other words, the GUT scale has been lowered all the way to the TeV range! However, this is not the only possibility. For example, let us see what happens if we adjust the radius R of the extra dimensions. Clearly, taking smaller radii corresponds to increasing the energy scale associated with the KK states. We thus expect the power-law behavior to emerge later in the evolution of the gauge couplings. Nevertheless, regardless of the value of R, we see in Fig. 13 that we continue to have unification, with a GUT scale that depends on the value of R. Indeed, we now see that the usual unification in Fig. 9 is nothing but the special case of Fig. 13 when R-1
- • M G U T « 2 x 1016
GeV.
60
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2
3 log10(/x/GeV)
4
Figure 12. Unification of gauge couplings at the new unification scale MQ-^^ RJ 20 TeV, assuming the appearance of a single extra spacetime dimension of radius R~ = 1 TeV.
490
Similarly, one can ask whether this unification persists for different numbers of extra spacetime dimensions. However, the parameter 8 = D — 4 governs the power inherent in the power-law evolution. Thus, increasing 5 merely has the effect of increasing the rate at which the unification occurs. Nevertheless, the unification is preserved. This reduced-scale unification leads to many important quantitative questions. How predictive is this unification? How perturbative is it? How sensitive is it to unification-scale effects? What about higher-loop correc-
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491
tions? These issues are discussed in some detail in the original references. The upshot, however, is that this sort of unification scenario is predictive, perturbative, and not unreasonably sensitive to unification-scale effects. It is important to emphasize that not all embeddings of the MSSM into higher dimensions automatically lead to an acceptable gauge coupling unification. While increasing the value of r\ (the number of fermion generations which propagate in the extra dimensions) preserves the unification, this increase tends to drive the unified unified gauge coupling towards stronger and stronger values, thereby invalidating the accuracy of the above oneloop results. Moreover, changing the value of e (the number of Higgs fields experiencing the extra spacetime dimensions) also has a dramatic effect on the unification, and can spoil it entirely unless further new states are introduced. However, this is no different than the situation in the MSSM. As we know, gauge coupling unification in the usual MSSM is also extremely sensitive to particle content and shifts in energy thresholds. Nevertheless, we see that extra dimensions have broadened our perspective considerably. Indeed, through extra dimensions, it becomes possible for the first time to contemplate grand unification occurring at an intermediate scale — indeed, even as low as the TeV range. In other words, we now see that it becomes possible to replace a four-dimensional GUT at 1016 GeV with a higher-dimensional GUT at the TeV-scale. 2.3. Extra dimensions:
Lowering
the Planck
scale
Let us now turn to the fundamental scale associated with gravity, namely the Planck scale. As we shall see, a similar phenomenon occurs here as well. Recall that the Planck scale is defined through the value of Newton's constant GN, as in Eq. (2.2). However, as we have already seen in Eq. (1.24), if D — 4 extra spacetime dimensions exist, then the four-dimensional Newton's constant GN which defines the usual fourdimensional Planck scale Mpianck is ultimately related to a £>-dimensional Newton's constant GN : G = RD-*G%>,
(2.9)
or equivalently G™
= RD-AM^nck.
(2.10)
In other words, in the presence of D — 4 extra spacetime dimensions, it
492
is GN which is the fundamental constant of nature, not GN or Mpi anc k. Thus, depending on the value of R, we see that Mpi anc k can also be adjusted! In order to write this result in its more common form, note that GN has mass dimension ( m a s s ) 2 - 0 . Thus, we can define an effective higherdimensional Planck scale M pla ' nck M(D)
iW
Planck
__
\MD)
=
\ N
-,1/(2-13)
(2.11)
U
Inserting this into Eq. (2.10), we thus have m
Mh^JRD~A
(2-12)
= M2lanck/vn
(2.13)
Planck
or equivalently M{D) JW
n+2
Planck
where n = D — 4 and where Vn = Rn is (proportional to) the n-dimensional compactification volume. Since the D-dimensional Planck scale is the fundamental energy scale, we now have a new possible explanation of why M p l a n c k appears to be so large in four dimensions. Rather than imagine that Vn is small and M p l a n c k is also large, we can instead imagine that M p l a n c k is relatively small but that Mpi anc k appears large in four dimensions only because Vn is large! Thus, once again, we see that if we have large extra spacetime dimensions, our fundamental higher-dimensional Planck scale M p l a n c k can indeed be considerably lower than previously imagined. As an example, let us consider how large these extra dimensions would have to be if we were to attempt to take M p ] a n c k in the TeV range. With Mpi anc k « 1019 GeV, we find from Eq. (2.13) that R-1
« 10" 3 2 /" TeV
<=^
R P» io 32 /™- 19 meters .
(2.14)
Thus, we see that the required extra dimensions would have to have sizes R w 10 13 meters R w 1 millimeter i ? « 1 0 f e r m i « (10 MeV)- 1 .
(2.15)
Clearly, according to the discussion in Sect. 1.5, the case with n = 1 is ruled out; an extra dimension of this size would clearly disturb obvious phenomena such as planetary orbits. Indeed, the case with n = 2 is just at the edge of the bounds depicted in Fig. 7 coming from precision Cavendishtype experiments [note that we have not been careful with overall factors
493
of 0(1), which is why we cannot make a definitive claim about this case]. However, we see that the cases with n > 2 are comfortably within the range allowed by precision gravitational experiments, as well as all astronomical and cosmological data. Of course, this assumes that such extra dimensions are transverse to the brane on which all gauge forces are restricted, so that they are of the "gravity-only" type. This is a major result. We see that the Planck scale need not be unreachably high at all — indeed, the fundamental energy scale associated with quantum gravity may be accessible within the next round of collider experiments. Like our previous result concerning the lowering of the GUT scale, such a result thus calls for a drastic rethinking of the possibilities for physics beyond the Standard Model. Despite its importance, however, it is important to recognize that this result does not provide a solution to the gauge hierarchy problem. It may seem, at first glance, that since the fundamental energy scale M p l ^ k may be in the TeV range, the hierarchy between Mpi anc k (the effective fourdimensional Planck scale) and the electroweak symmetry breaking scale is eliminated. However, it is not eliminated: Mpi anc k continues to be at 1019 GeV. Or, phrased another way, the reason that Mpi anc k can differ so greatly from M p i a n c k is that we have an unusually large volume Vn = Rn. Thus, our previous hierarchy in energy scales has merely become a hierarchy in geometry: why are the extra dimensions so much larger than the fundamental energy scales in the theory? For example, if M p l k ~ O(TeV) with n = 2, then we require J ? - 1 « 1 0 - 4 eV. The hierarchy between the compactification scale R^1 of the extra dimensions and the electroweak scale is thus exactly as large as the hierarchy between the electroweak scale and the previous four-dimensional GUT scale. In other words, we have not eliminated our big number (the hierarchy); we have merely hidden it. Although this therefore not a solution to the hierarchy problem, this still represents a tremendous breakthrough in our attempts to solve this problem. We now have an additional perspective, one resting on geometry, which did not exist previously. Indeed, as will be discussed in subsequent lectures at this TASI school, there does exist a way of truly solving the hierarchy problem in the context of extra dimensions; the solution is to warp the geometry and thereby soak up the large number within a relatively small deformation of the compactification space. Because of the importance of Eq. (2.13), we conclude this subsection by providing three further ways of interpreting this result.
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gravitational flux lines "leak out" into the extra dimensions
Figure 14. If there exist extra spacetime dimensions perpendicular to the brane, then the gravitational flux lines between two masses on the brane will leak out into the extra dimensions, leading the gravity on the brane to appear weaker than it would otherwise have been.
First, geometry. Let us consider two masses located on a fourdimensional brane, as shown in Fig. 14. Without extra dimensions perpendicular to the brane, the gravitational flux lines all remain within the brane, giving rise to the familiar Newton force law F = GN MxMijr1 where r is the distance between the masses. However, if there exist n extra dimensions perpendicular to the brane, then the flux lines can leak off the brane into these extra dimensions as well. By Gauss' law, this changes the Newton force law to become F = G^ ' MiM2/r2+n. This leakage of the gravitational flux lines into the extra dimensions implies that the gravitational interaction will appear weaker than we would have naively expected in four dimensions. Thus, we can compensate by allowing gravity in higher dimensions to be intrinsically stronger than in four dimensions. In other words, the higher-dimensional Planck scale (where gravity gets strong) can be lower than the four-dimensional Planck scale. This geometric argument also explains why gravity appears to be fourdimensional until we probe distances on the order of the compactification radius R. Let us imagine that our extra dimension has only a finite "width" R perpendicular to the brane. If the'distance r between the point masses is much smaller than R, then our flux lines are effectively "spherical" in the full, higher-dimensional space. This gives rise to the higher-dimensional force law outlined above, which implies that our higher-dimensional gravity
495
is effectively stronger than expected. However, if the distance r between the point masses is much larger than R, then our flux lines, even though extending into the small width R, are still forced to travel along the constricted hypervolume parallel to the brane. They thus cannot effectively spread much beyond the four dimensions of the brane itself (or regions closely parallel to it), and consequently they reproduce the usual four-dimensional force law. Indeed, by matching the four- and higher-dimensional force laws smoothly in the region r « R, we can immediately derive our starting relation in Eq. (2.9). Our second way of understanding the emergence of this modified force law is through the exchange of Kaluza-Klein graviton states. Recall that the contributions from the exchange of KK graviton modes are already given in Eq. (1-20) where mn denote the corresponding KK masses. As we have seen in Sect. 1.4, for r » R the KK graviton modes are heavy, and produce only small corrections to the usual four-dimensional Newtonian inverse square law. However, in the opposite limit r < i { , many of the KK graviton modes become light and we must explicitly perform the sum over the KK states in Eq. (1.20). For D — 4 extra dimensions, and for r
J29n<
(2.16)
n=0
Thus, we find that the gravitational potential becomes
This is indeed the expected Gauss-law result in D dimensions, provided we make the identification given in Eq. (2.9). Finally, let us understand this result one final way, as a "running" of the gravitational coupling. This is closest in spirit to our discussion of the method by which extra dimensions can be used to lower the GUT scale. Since GN' is a dimensionful quantity, we really should define a dimensionless gravitational coupling GJV(M) = fi2GN . It is this quantity which is the closest analogue to the dimensionless four-dimensional gauge couplings cti(ii). However, because of the underlying mass dimension of GN , we see that the dimensionless gravitational coupling Gjv already experiences a power-law running, scaling like fx2 (or equivalently, with the inverse coupling GNl scaling like /x~2, as shown in Fig. 15). The energy scale at which GN ~ O(l) defines the Planck scale.
496
G N (M)= H 2 G N
AGJJ(p)
-, R-1
n=2:
(10~4eV)
i M'p|
(TeV)
i Mp,
•
log(u)
(lO'^GeV)
Figure 15. In higher dimensions, the effective, dimensionless gravitational coupling GJV(M) runs even more rapidly than in four dimensions, reaching order one (and thus denning MpiJ^ k) at a lower energy scale than expected.
However, if there are n extra dimensions which "open up" (i.e., become accessible) at a radius R, then the combined effect of the gravitational KK modes above this energy scale is to provide a further power-law growth for GN, turning the /i2 scaling into /z 2 + n . As illustrated in Fig. 15, this causes the gravitational interactions to get stronger at an even lower energy scale, thereby effectively reducing the (now higher-dimensional) Planck scale. Indeed, one can even deduce graphically that for n = 2, one requires R~x fa 1CP4 eV in order to bring the reduced Planck scale to the TeV range down from 10 19 GeV. The picture that emerges, then, is relatively simple. Ordinarily, the gravitational interaction is weak because it is mediated by a graviton whose couplings to ordinary matter are Planck-scale suppressed. (This is indeed the definition of the Planck scale.) However, in the presence of extra spacetime dimensions, the gravitational interaction is mediated not only by the single Planck-suppressed graviton, but also by its infinite towers of
497
Planck-suppressed KK gravitons as well. The combined effect of the exchange of this KK graviton tower thus leads to an enhancement in the strength of the resulting gravitational interaction, or equivalently a reduction in the Planck scale suppression with which the single higherdimensional graviton state couples to ordinary matter. 2.4. Extra dimensions:
Lowering
the string
scale
Finally, large extra dimensions can also be used to lower the string scale. This is important because higher-dimensional gauge theories are intrinsically non-renormalizable, and consequently they experience divergences which cannot be absorbed through shifts in the bare parameters in their effective Lagrangians. Consequently, if we construct a theory in which our GUT and Planck scales are reduced, we must somehow manage to embed this theory into a finite, higher-dimensional theory at an energy scale which is not too far above the reduced GUT or Planck scale. Since the best candidate for such a theory is string theory, we thus are faced with the need to find a way to lower the string scale at the same time we lower the GUT and Planck scales. Recall that in the perturbative heterotic string, the string scale is tied to the usual four-dimensional Planck scale as indicated in Eq. (2.3), and thus remains at the perturbative heterotic string value near 10 18 GeV. Therefore, unless the perturbative heterotic string coupling is extremely suppressed, with String <S 0(1), we cannot appreciably lower the string scale below the usual Planck scale. For heterotic strings at strong coupling, however, this behavior changes. Specifically, it turns out that various (closed) heterotic strings at strong coupling can be equivalently described as (open) -Type I strings at weak coupling. Thus, many non-perturbative features of heterotic string theory can be studied by analyzing weakly coupled Type I strings. Remarkably, Type I string theory offers the interesting possibility of lowering the fundamental string scale, for Eq. (2.3) no longer continues to apply. Instead, we find the relation M^ing ~ e* /2 ggaUge Mpi anck
(2-18)
where <\> represents the so-called ten-dimensional dilaton field andffgaugeis the Type I gauge coupling. Thus, simply by adjusting the VEV of the tendimensional dilaton, one can lower M str i n g relative to Mpi anc k. Of course, the value of the dilaton indirectly affects the values of the gauge and gravitational couplings, implying that 5gauge and Mpi anc k themselves change
498
their apparent values when the dilaton is changed. However, using other string relations it is possible to eliminate this dependence algebraically, and relate Mstring and Mpianck directly to each other without exhibiting the dependence on the dilaton. We then find the general relation String ~ J
^
V-1^
(2.19)
y agaugeJWpi a n c k
where agauge = ffgauge/^71") a n d where (2n)6V is the (normalized) sixdimensional volume of compactification. In writing Eq. (2.19), we have ignored (and will continue to ignore) all numerical factors of order one, since our goal will merely be to obtain order-of-magnitude estimates. The relation (2.19) thus represents the non-perturbative counterpart of Eq. (2.3). We see, then, that within the context of Type I string theory, we can again achieve a dramatic reduction in the value of M s t r i n g ; all that is necessary is to have a large compactification volume V. 2.5. Extra dimensions: A unified picture for beyond the Standard Model?
physics
Given these results, let us now attempt to construct a unified scenario for physics beyond the Standard Model utilizing extra dimensions. If all of the gauge forces of the Standard Model are restricted to a brane, we have already seen that extra "longitudinal" dimensions in the brane can lower the corresponding GUT scale while extra "transverse" dimensions perpendicular to the brane can lower the corresponding Planck scale. String theory, however, requires at least six (or seven) extra spacetime dimensions. Thus, in order to embed our reduced-scale theories into string theory, we should choose some of these extra dimensions to be longitudinal, and the remainder to be transverse. If we choose the sizes of these dimensions correctly according to Eq. (2.19), we can also lower simultaneously the string scale, and thereby produce a self-consistent Type I string theory without any high energy scales whatsoever. Let us give an explicit example of how this can be done. In order to embed our grand unification scenario into Type I string theory, we shall attempt to identify agaUge with the gauge coupling a'GXJT at the lowered GUT scale M Q U T . We shall also attempt to identify M str i n g with this lowered GUT scale MGVT, and seek to build a scenario where MGVT — 10 TeV. Moreover, let us assume that this is accomplished through the use of 8 extra longitudinal dimensions of common radius R. As we have
499
seen, these are the dimensions that cause the gauge couplings to experience power-law corrections. Given the relation (2.19), we can now easily determine the required common radius r for the remaining 6 — <5 compactified dimensions. Writing the normalized compactification volume as V ~ R5r6~5
,
(2.20)
we find
P ^ -
~ «'GUT {M'GmRf/2 ( M ^ r ) 8 - * / 2 .
(2.21)
AtPlanck
Let us begin by considering the case 5 = 1. Taking M Q U T = 10 TeV, we see from Fig. 12 that this requires MQVTR « 20 and a'GVT « 1/50. (Note that Fig. 12 is drawn for M Q U T RS 20 TeV rather than 10 TeV, but approximately the same results hold in both cases.) This in turn implies that MQVTr ss 10" 6 , which implies that the radius r of the five extra dimensions must be smaller than the string length scale! Rather than view this as an inconsistency, we instead recognize that this is actually a signal that we should pass to a slightly different description (the so-called Type I' description) of the physics. Technically, this procedure of passing from one description to the other is called a T-duality, and in general a Type I theory with a compactified radius r is the T-dual of an equivalent Type I' theory with a compactified radius r' = (M?tring r )~ 1: T-duality:
Mstlingr
<-> (Mstring/)^ 1 •
(2.22)
We therefore pass to a Type I' description by T-dualizing our five extra dimensions. We then find ( r ' ) " 1 ~ 10" 6 M Q U T ~ 10 MeV .
(2.23)
Note, however, that extra dimensions of this size transverse to the brane are precisely what are required to lower the Planck scale! Thus, putting the pieces together, we see this combination of extra spacetime dimensions longitudinal and transverse to the brane has managed to self-consistently lower the GUT, Planck, and string scales simultaneously. Indeed, we see the GUT-lowering scenario and the Planck-lowering scenario actually require each other when they are embedded within the contet of string theory. This makes sense, since string theory is a unified theory containing not only gauge forces but also gravity. Thus, to summarize our results, we see that we can naturally associate the scale of gauge coupling unification at 10 TeV with the string scale of
500
G"N^) Type 1' string
efft3CtlVe field thee>ry\
i
theory
V a
2
PaT1-
!
r
^string
«3^
10MeV 1/r'
gauge: gravity:
4D 4D
100 GeV Mz
4D 9D
0.5 TeV 1/R
5D 10D
10 TeV M GUT
log p,
10D 10D
Figure 16. The unification of gauge and gravitational couplings within the framework of a Type I' string theory, assuming a string scale at M s t r i n g = 10 TeV. Note that the running of Gj^(fi) is not shown to scale in this figure. This figure should be contrasted with Fig. 10.
a Type I' theory in which one dimension has radius R^1 ~ 0.5 TeV and the five remaining dimensions have radii r' ~ (10MeV) - 1 . The resulting scenario is sketched in Fig. 16. Above the string scale Mstring = 10 TeV, the physics is described in terms of a full Type I' string theory. Below 10 TeV, by contrast, the physics is described by a series of effective field theories in which the gauge and gravitational forces feel different numbers of spacetime dimensions. Together, everything is balanced so as to produce a self-consistent simultaneous lowering of GUT, Planck, and string scales. Needless to say, other configurations are also possible. However, the main point is that this picture provides a completely different alternative when compared with the usual paradigm sketched in Fig. 10. Rather than have fundamental energy scales which are very high and which are separated
501
from the electroweak symmetry breaking scale through a large "desert", we instead now have a scenario in which there are no high energy scales whatsoever. Instead, the desert in energy scales has been completely eliminated, and has been replaced by a large extra-dimensional volume. Thus is thus a completely different scenario for physics beyond the Standard Model.
X6,x7,x8,x9,x10
Figure 17. A D-brane configuration which can accommodate the scenario sketched in Fig. 16 within the context of Type I' string theory.
It is also interesting to understand the spacetime geometry associated with such a model. One such possibility is sketched in Fig. 17. At low energies, the Standard Model (or the MSSM) lives on the four-dimensional intersection of different "branes" (here shown as the intersection between the D4-brane and the D6-brane). At higher energies, however, the gauge forces feel the extra longitudinal dimension inherent in the D4-brane (here sketched as a cylinder with radius R), and the gauge bosons and Higgs fields propagate on this extra dimension as well. This is the dimension whose size is restricted to be no larger than an inverse TeV. Note that even though we have sketched this extra dimension as a cylinder, in order to break our extended supersymmetry down to N = 1 supersymmetry and introduce chirality, this circular extra dimension might be replaced by a line sigment of finite length. Also, depending on the value of 77, our chiral fermions may or may not be restricted to the intersection of the D4-brane and the D6brane. Thus, at this energy scale, the gauge world has effectively become a cylinder. As described in Sect. 2.2, this can lower the GUT scale and ensures gauge coupling unification.
502
Of course, there are also additional transverse directions which are felt only by the gravitational interactions. These directions are, for example, those that extend along the D6-brane. These extra dimensions lower the Planck scale, and thereby explain the weakness of gravity. Finally, as we have seen, choosing these combinations of extra dimensions correctly manages to simultaneously achieve a lowered string scale. This makes possible an embedding into string theory which ultimately ensures the consistency and calculability of such a theory. Loosely speaking, this type of scenario is called a "brane world". Of course, the various types of branes and geometric configurations is virtually limitless, and thus the potential for model-building in this new language is extremely rich as well. Ultimately, of course, we would like to be able to realize a given brane configuration as emerging from a bona-fide string model; work along these directions is much more difficult than the relatively simpler "bottom-up" constructions which we have described thus far. In any case, we see that the brane world provides a new and tantalizing alternative perspective in which to view the theoretical possibilities for physics beyond the Standard Model.
2.6. Open
questions
Needless to say, the brane world opens up whole new vistas that await exploration. Virtually all aspects of physics beyond the Standard Model must now be considered in a new light. For example, what is the role, if any, of supersymmetry and of supersymmetry-breaking? What are the consequences of low-scale GUT physics and string physics? What sorts of solutions do these problems provide for long-standing problems such as the fermion mass hierarchy problem, as well as general issues in flavor physics? How can we understand neutrino oscillations in this context? Indeed, for each of these questions, we now must consider the possible role of large extra dimensions, and the effects of fundamental energy scales which are now radically altered. There are also numerous questions of a more phenomenological, even experimental, nature. If the fundamental energy scales are really down in the TeV range, then many striking signals will be possible at future colliders. These include the direct detection of Kaluza-Klein states; decays of KK states into (s)fermions; Drell-Yan production of KK states in pp collisions; new effective four-fermi contact interactions that emerge via exchange of gauge-boson KK states; and even direct detection of GUT particles, Regge
503
states, and signatures of quantum gravity (such as TeV-scale black holes). Thus, it becomes possible possible, for the first time, to contemplate probing the properties of GUTs and strings experimentally, which would lead to a new experimental approach to string phenomenology! Of course, this new physics is not merely limited to elementary particles, for large extra dimensions would also have profound effects for astrophysics and cosmology. New issues that immediately emerge in this framework include the role of light stable KK states, possible new dark-matter candidates (such as stable low-lying KK states), reheating and thermal regeneration of KK states, new brane-related scenarios for inflation in higher dimensions, phase transitions in higher dimensions, topological defects, and density perturbations. One can also ponder whether there might be cosmological methods for generating the large radii that are required in the brane world, or whether there might even be cosmological explanations for the dimensionality of spacetime (i.e., a reason why only four dimensions became uncompactified and large, while the others remain compact). There may even be new brane-related approaches to the cosmological constant problem. Clearly, then, there is a lot to think about. In Lecture # 3 , we shall discuss several new approaches towards addressing some of these problems in more detail. Then, in Lecture # 4 , we shall raise some new issues which have not received much attention in the literature, but which should serve to broaden our perspective concerning some of the fuller implications of studying theories with extra spacetime dimensions.
3. Lecture # 3 : Extra dimensions in action: Applications to particle phenomenology In this lecture, we shall address some phenomenological issues that arise within the context of the brane world. As we shall see, most of these issues arise within the standard paradigm as well, but their resolutions within the brane world take on a wholly new character. The set of topics I have chosen to address in this lecture is, of necessity, quite limited. However, it is my hope that by examining these topics, it is possible to get a sense of how actual model-building in extra dimensions proceeds. Indeed, at this point, most of the major phenomenological issues facing the brane world scenario have been studied quite extensively; we therefore refer the interested reader to the original literature for more details.
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3.1. Proton
decay
Perhaps the most immediate question that arises within any extension to the Standard Model is the issue of proton decay. In a nutshell, the proton is known to have a lifetime exceeding 10 33 years, yet most extensions to the Standard Model introduce new interactions which tend to provide baryon non-conserving channels for proton decay into leptons. In the standard paradigm, such new physics occurs at a high energy scale, however, and thus these theories may be able to maintain proton stability to a sufficient degree. By contrast, in the case of the brane world scenarios in which all fundamental high energy scales are eliminated, this issue becomes even more severe. However, in this lecture, we shall show that there exist various intrinsically higher-dimensional mechanisms which enable us to cancel the usual proton-decay diagrams to all orders in perturbation theory. Let us begin by considering the scenario outlined in the previous lecture wherein MQUT is lowered to a new smaller scale M'GVT. Any grand unified theories, in and of themselves, already tend to introduce severe complications for proton decay, for proton decay can be mediated by the X bosons of the GUT gauge group as well as new, colored Higgs triplets. The relevant diagrams are illustrated in Fig. 18.
¥.
Figure 18. Typical diagrams that can mediate proton decay. Here the external lines correspond to the MSSM (s)quarks and leptons, while the internal ^ fields correspond to X-bosons (as in the diagram on the left) and Higgsino fields (as in the diagram on the right).
In a traditional GUT, these diagrams are suppressed by the large GUT mass ~ 1016 GeV associated with these new particles; this mass appears in the intermediate propagators and can often suppress these amplitudes to an acceptable degree.
505
In the brane world scenarios, however, this situation is drastically altered: although the unified gauge coupling is typically weaker than the usual GUT gauge coupling, the unification scale is significantly lower! This means that the usual proton decay amplitudes are apparently increased by a factor
This would seem to indicate immediate trouble for these scenarios. The important point, however, is that these are actually grand unified theories in higher dimensions. Therefore, we should ask whether there might exist intrinsically higher-dimensional solutions to the proton decay problem. One possible mechanism proceeds as follows. Recall that all of the extra GUT states beyond the MSSM should not be observable below the unification scale. What this means, then, iis that such states should not have zero modes, or equivalently, such states should be chosen odd under the same Z2 orbifold projection that we used to break our extended supersymmetry to N = 1 supersymmetry and to introduce chirality at low energies. Indeed, such states must have wavefunctions which behave as sm(ny/R) in the higher-dimensional space. How does this help us? Recall that in bany scenarios, the three generations of chiral fermions are restricted to the orbifold fixed points located at y = 0 and y = irR. In such scenarios, then, we see that all GUT states beyond the MSSM have wavefunctions which necessarily vanish at the higher-dimensional fermion locations! These states therefore do not couple to the MSSM fermions, and consequently are not able to mediate proton decay along the lines of the diagrams in Fig. 18. This is actually a rather powerful mechanism. It completely suppresses the dangerous interactions not only for the lowest modes of the X-bosons and Higgs triplets, but also all of their infinite towers of KK excitations, as illustrated in Fig. 19. Thus, such proton decay diagrams vanish to all orders in perturbation theory. Although we have chosen to illustrate this mechanism as the failure of a wavefunction to reach a localized state, we should stress that this pictorial representation is only meant to be suggestive. In truth, we must remember that all such localizations are effective only down to a characteristic length scales associated with the string scale; indeed, as we know from elementary considerations of quantum-mechanical wavepackets, no perfect localizations
506
n=1 (A £
0)
o 0)
I I
CO CO
y=0
y=7TR
n=2 (0
c .2 E i_
//
/
s
\\
CO
s
•^
\
2
'
CO (0
\
N
^Ss / "-^^ ^,'
2
y==0
/
y= n:=3 ,-
w c o
I0)
1 1
k.
M—
:
s
(0 CO
s y=0
-v
/
/ \\ 1 \\ 1 \ 1
' \ / \ \
1
1
i
1
! \
\
\ '
1 1
/
/
' ^x
(0
CO
\
i i
I i
i
»/
x'
i
i
\
*x /
y=7i;R
Figure 19. The orbifold projection requires the GUT particles beyond the MSSM to have odd wavefunctions under y —> —y, which in turn prevents these particles from coupling to the MSSM fermions which reside at the orbifold fixed points. This applies not only for the lowest-lying KK mode (top figure), but all excited KK modes as well (lower figures).
are possible. Consequently there is always a little bit of overlap. However, our conclusion that these diagrams vanish to all orders in perturbation theory continues to hold. This is because our pictorial representations really
507
illustrate something deeper: the Z 2 orbifold projection that we have been discussing actually requires that all terms in our underlying Lagrangian must be Z 2 invariant, and it is this requirement that eliminates the direct couplings between these GUT states and the MSSM fermions. It is tempting to view this suppression mechanism as a Kaluza-Klein selection rule whereby an excited n > 1 KK state cannot couple directly to zero modes. Indeed, such KK selection rules would represent momentum conservation in the higher-dimensional space. However, it is important to realize that the presence of our orbifold fixed points (or branes) at y = 0 and y = irR actually breaks the higher-dimensional translation invariance that would have given rise to such a higher-dimensional momentum conservation. Thus, this orbifolding procedure breaks the KK selection rules, but (as we have seen) supplies a new discrete symmetry which ends up having the same effect. We see, then, that this is an intrinsically higher-dimensional solution to the proton decay problem. It is valid for all radii R and spacetime dimensions D, and instead makes use of a higher-dimensional (orbifold) symmetry to eliminate the coupling rather than a heavy mass scale to suppress a propagator. Such a mechanism therefore has no analogue for ordinary four-dimensional grand unified theories. There are, of course, many other contributions to proton decay. For example, there are still a plethora of additional, non-renormalizable interactions which can emerge, and which will require their own methods of suppression. In general, these can be handled by introducing further discrete symmetries which can eliminate whatever proton decay diagrams might destabilize the proton below experimental limits. There may also be non-perturbative effects leading to proton decay, such as instanton contributions which scale like ~ e x p ( - l / g 2 ) . These are generally small. If the scale of quantum gravity is also reduced, then there exist additional effects that can lead to observable proton decay. For example, black holes can typically mediate proton decay. Suppressing such contributions would therefore require not merely discrete symmetries, but gauged discrete symmetries. Finally, we should mention that there also exist other mechanisms for stabilizing the proton. For example, even though our MSSM fermions may be localized relative to the extra dimensions transverse to the brane, the brane itself may still have extra longitudinal dimensions. If so, then it may be possible to localize the quarks and leptons at different points within these longitudinal extra dimensions. (These scenarios are commonly referred to
508
as having "split fermions".) Thus, even though our GUT particles may couple to both the quarks and the leptons equally, the required three-point vertex will be suppressed by the exponentially small wavefunction overlap between the quarks and leptons within the brane. This mechanism works not only for quark/lepton localization along longitudinal extra dimensions within the brane, but also within any small (string-scale) transverse width that the brane itself may have (a so-called "fat brane"). This mechanism will be discussed in more detail by other lecturers. 3.2. The fermion
mass
hierarchy
We now address the question of understanding the fermion mass hierarchy within the context of a higher-dimensional theory. First, recall that the fermion mass hierarchy refers to the hierarchical pattern of masses for the fermions of the Standard Model. For example, our lightest first-generation fermions have masses me
« 0.5 MeV = 5 x 1CT4 GeV
mu
«
md «
5 MeV = 5 x 1CT3 GeV 10 MeV = 1 x 1 0 - 2 GeV ,
(3.2)
while our third-generation quarks have masses mb « mt
5 GeV = 5 x 10° GeV 180 GeV « 2 x 10+2 GeV .
(3.3)
Thus, spanning all of the fermions, we have an overall hierarchy ^ )
« 1012 ,
(3.4)
meJ which is quite substantial indeed. Note that we have not even included neutrinos in this discussion (we shall discuss the case of neutrinos in the Sect. 3.3). This hierarchy is exceedingly difficult to explain within the usual Standard Model or the MSSM. In these models, these masses arise from interactions with the Higgs fields parametrized by Yukawa couplings, yet these Yukawa couplings, like the gauge couplings, also run logarithmically. Can extra dimensions provide a solution? If we naively consider the MSSM in higher dimensions and evaluate the standard Yukawa wavefunction diagrams, then we have Kaluza-Klein states of our MSSM particles
509
running in the loops. We then find that our Yukawa couplings ctp for each fermion F runs according to
«F(MO)
iFjp \XsJ
\Moy
We thus see that the Yukawa couplings are driven rapidly towards weak values with power-law dependence. This might be useful for avoiding problems with Landau poles, even for the top quark, and might even be useful for string theory (where we can potentially avoid dilaton runaway problems with exceedingly weak couplings at the string scale). However, this cannot be used to explain the fermion mass hierarchy, sinice the power-law dependence in Eq.(3.5) is universal for all fermions. Another possibility is to introduce a new singlet S with a superpotential coupling W = \HuHdS. In this case, we then find that our Yukawa couplings become string rather than weak in the UV, with the couplings ultimately sharing a common Landau pole. However, while this scenario can explain certain pairwise hierarchies between up- and down-type fermions, this still cannot provide a satisfactory solution to the fermion mass hierarchy problem. Clearly, in order to explain the full hierarchy between all fermions, we require a flavor-dependent coupling. It is important to note, however, that this flavor dependence does not have to be large, since power-law running can amplify the effects of even small flavor-dependent couplings. To illustrate this, let us consider the a variant of the Frogatt-Nielsen scenario by imagining an extra scalar field $ with its own KK tower, coupling to the fermions F through a superpotential interaction term of the form W = Y, VF $™F FFHu,d .
(3.6)
F
Here rip depends on the specific flavor F (indeed, this is the input flavor dependence). However, unlike the Frogatt-Nielsen scenario, we shall treat $ as fully dynamical. There will also be no need for an arbitrarily small vacuum expectation value for <J>. To see the role that extra spacetime dimensions can now play, let us consider the Yukawa wavefunction diagram shown in Fig. 20. We see that the number of internal KK towers in the loop is flavor-dependent. Thus, each fermion feels a different effective number of spacetime dimensions: AF = S(l+nF)
+ 2nF .
(3.7)
510
t,b
J
/
H
u,d
\
t_
Figure 20. Yukawa wavefunction renormalization diagram corresponding to the interaction given in Eq. (3.6). This setup induces each fermion to feel a different effective number of spacetime dimensions, leading to different power-law scalings for the different fermions.
This can then be used to explain the fermion mass hierarchy. For example, for S = 1 (one extra dimension) and R~l = 1 TeV, the entire mass hierarchy between the electron and the top quark can be accommodated simply with rit = 0 a n d fit — 3. There is thus no need for the usual large Frogatt-Nielsen values of ripNeedless to say, this is only one scenario for addressing the fermion mass hierarchy. However, all higher-dimensional scenarios for addressing this issue generally revolve around the same basic idea: arranging our model in such a way that the different generations of fermions (and even the different flavors of fermion within a single generation) feel the higher-dimensional spacetime in a flavor-dependent way. Other scenarios by which this can be done include trapping different flavors on different branes with different transverse volumes, and/or trapping the different fermions on subspaces of different dimensionalities. One can then exploit volume effects and/or different power-law scalings in order to achieve a variety of effects. Of course, this is not a solution to the various flavor puzzles. In particular, the brane world scenario doees not explain the flavor-dependent positions or volumes that the different fermions experience; it merely can accommodate them. To provide an explanation would require a proper, "topdown" string-theoretic construction of a particular self-consistent Type I string model. In any case, the important point is that the brane world scenario has clearly given us a new language in which to think about these issues: the language of geometry. Moreover, thanks to power-law running
511
and/or exponentially small wavefunction overlaps, the brane world has also given us at least partial tools for addressing flavor hierarchies, since we now only require small input parameters at high energies in order to generate potentially significant hierarchies at lower energies.
3.3. Neutrino
oscillations
As we have seen, the lesson from the brane world has been that heavy mass scales in four dimensions can be replaced by lighter mass scales in higher dimensions. However, low-energy neutrino data seem to provide independent evidence for yet another heavy mass scale, namely the seesaw scale. Let us first recall how the usual seesaw mechanism works. GUT models based on SO (10) gauge symmetries or larger generically predict the existence of a right-handed neutrino N for each generation. Let us therefore assume a Yukawa coupling of the form yuULHuN, which leads to a Dirac massTO= y^(Hu) « O(10 2 ) GeV. However, let us also assume a Majorana mass for N (which might arise from GUT breaking via the 126 representation receiving a GUT-scale vacuum expectation value): M « O(10 16 ) GeV. The total mass terms for our left- and right-handed neutrinos then take the form
Diagonalizing to find the mass eigenvalues, we obtain 2 771
A_ «
- — ,
A+ * M .
(3.9)
Thus, we see that the lightest eigenvalue (which we associate with our observable neutrinos) "seesaws" against heavy mass scale. Indeed, if we wish the lightest eigenvalue to fall into the approximate range near 10~ 2 eV, this indicates that we must have M « 10 1 4 ^ 1 6 GeV. Thus, light neutrino masses seem to provide further evidence for a high fundamental GUT scale. The question then emerges as to whether it is possible to generate light neutrino masses without the introduction of a heavy mass scale, perhaps by some intrinsically higher-dimensional mechanism. To date, there have been a number of proposals concerning how this might be accomplished. The vast majority of these proposals, however, originate with the same observation: because the right-handed neutrino is a Standard-Model gauge singlet, it need not be restricted to a "brane"
512
with respect to the full higher-dimensional space. It is therefore possible for this field to experience extra spacetime dimensions and thereby accrue an infinite tower of Kahiza-Klein excitations. In other words, the right-handed neutrino state may emerge within the context of Type I string theory as an open, rather than closed, string state. This then leads to a number of higher-dimensional mechanisms for suppressing the resulting neutrino mass without a heavy mass scale. For example, if the right-handed neutrino exists in the bulk, then its Kaluza-Klein excitations can give rise to power-law running for the neutrino Yukawa coupling through the diagram in Fig. 21. We see, then, that the neutrino Yukawa coupling can be driven to very small values over short energy interval, implying that the neutrino mass will be power-law suppressed relative to masses of all other fermions!
N VL
(
\
\
Hu Figure 21. Neutrino Yukawa renormalization diagram leading to power-law suppression of the left-handed neutrino relative to all other fermions.
Another observation concerns a possible suppression by the so-called bulk "volume" factor. If the right-handed neutrino N is in the bulk, then its wavefunction must be normalized (suppressed) by the bulk volume factor (RM*)n where M* is the fundamental energy scale associated with the bulk physics (e.g., the Planck or string scale) and where n is the number of extra spacetime dimensions associated with the bulk. However, this volume factor in turn suppresses the Dirac coupling m between the left-handed neutrino on the brane; basically, the wavefunction of the right-handed neutrino is so spread out over the relatively large bulk that its overlap with the lefthanded neutrino on the brane is exceedingly small. This, too, can therefore lead to a small effective physical neutrino mass. Both of these mechanisms suppress the mass of the resulting light neutrino state by suppressing the Dirac coupling m. But is there a higherdimensional analogue of the seesaw mechanism?
513
In order to construct such a higher-dimensional seesaw mechanism, let us again begin by assuming that the right-handed neutrino feels extra dimensions, while the left-handed neutrino VL, does not. For concreteness, we consider a Dirac fermion <]/ in five dimensions, and work in the Weyl basis in which \& can be decomposed into two two-component spinors: vf = (^ 1) -i/)2)T. When the extra spacetime dimension is compactified on a Z2 orbifold, it is natural for one of the two-component Weyl spinors, e.g., ipi, to be taken to be even under the Z2 action y —> —y, while the other spinor -02 is taken to be odd. If the left-handed neutrino VL is restricted to a brane located at the orbifold fixed point y = 0, then 1^2 vanishes at this point and so the most natural coupling is between VL and if>i. This then results in a Lagrangian of the form
£ = fdtxdy Ms j ^ i i a ^ V i + V w / ^ 2 j
./*,{,
VLia^D^VL + (mi'Lip1\v=o + h.c.) I
(3.10)
where y is the coordinate of the extra compactified spacetime dimension and where Ms is the mass scale of the higher-dimensional fundamental theory (e.g., a reduced Type I string scale). Note that the last term represents the Dirac brane/bulk Yukawa coupling between VL and i/>i • Next, we compactify the Lagrangian (3.10) down to four dimensions by expanding the five-dimensional \& field in Kaluza-Klein modes. Imposing the orbifold relations Vi,2(—2/) — ±^1,2(2/) implies that our Kaluza-Klein decomposition takes the form
M*,y)
= ^=$>in)(*)cos(m//i?)
(3.11)
and a similar result for ^2 with cosine replaced by sine. For convenience, we shall also define the linear combinations N^ = (f/'i + C ) / V 2 and M(") = (ip[n) - ip^n))/V2 for all n > 0. Inserting this decomposition into Eq. (3.10) and integrating over the compactified dimension, we then obtain an effective four-dimensional Lagrangian in which the StandardModel neutrino v^ mixes with the entire tower of Kaluza-Klein states of
514
the higher-dimensional ^ field with a mass mixing matrix of the form
M
m m m
m M0 0 0
m 0 0
m 0
M0 + i
Mo
(3.12) R
In Eq. (3.12), we have defined the basis
{uL,1,[0),NW,MW,NW,M™,...).
(3.13)
Note that m = m/y/2irRMs is the Dirac coupling suppressed by the volume factor corresponding to the extra spacetime dimension. The parameter Mo in Eq. (3.12) represents an additional contribution that may arise if we implement a Scherk-Schwarz twist upon compactification. Moreover, it turns out that topological considerations permit only two values: Mo = 0 and Mo = (2R)"1. Let us here consider the non-trivial possibility Mo = (2.R) -1 . It is then possible to solve for the eigenvalues and eigenstates of the mass mixing matrix in Eq. (3.12). Remarkably, it turns out that for any value of mR, there exists an exactly zero eigenvalue, with a corresponding mass eigenstate given exactly by
\H)
1 V l + TT2m2R2
\vL)-mRj2
fc=i
|AT(^i)) _ |M(fc>)
fc-1/2 (3.14)
,.(0) where we have denned A^°) = tp^'. Even though this result is exact for all mR, in most realistic scenarios we have mR
515
higher-dimensional scenarios, neutrino oscillations do not necessarily require neutrino masses! It is easy to determine the shape of the resulting neutrino oscillations. In general, the neutrino survival probability is given by 2
Uuk\ exp
.(*) k
(3.15)
2p
where U is the matrix that diagonalizes the mass matrix M. in Eq. (3.12). Eq. (3.15) represents the cumulative effects of the pairwise oscillations between VL and each of the Kaluza-Klein states associated with the righthanded neutrino. However, since A4 is non-diagonal, U will also be nondiagonal. We thus obtain the non-trivial neutrino oscillations shown in Fig. 22.
10
20 t/(2R z p)
30
10
20 t/(2R z p)
30
40
Figure 22. Neutrino oscillations without neutrino masses in five dimensions. The lefthanded neutrino on the brane oscillates with each of the Kaluza-Klein modes of the righthanded neutrino in the bulk. The infinite numbers of resulting individual oscillations (the first two components of which are shown in the left plot) add constructively to produce the resulting oscillation pattern shown in the right plot.
It is evident from Fig. 22 that our resulting neutrino oscillations are quite sizable. Moreover, our neutrino oscillations are effectively periodic, just as in four dimensions. Interestingly, in this scenario, the neutrino regeneration is never total, but the deficits can be total. In general, the approximate oscillation "length" is set by i ? _ 1 , since this determines the mass spacings of the right-handed neutrino KK states in the bulk.
516
It is important to note t h a t these neutrino oscillations arise from the same left/right neutrino mixing which gives rise t o the neutrino mass matrix and thereby suppresses the physical neutrino mass. This is an important distinction relative to the usual four-dimensional seesaw mechanism, where the left/right mixing has such a small effective mixing angle 6 ss 1 0 - 1 6 t h a t no observable oscillations result. Just as in the usual four-dimensional case, however, this type of higherdimensional seesaw mechanism can easily be generalized to include the case of flavor oscillations. Most flavor models in the literature thus far introduce one bulk neutrino for each of the three brane neutrinos, thereby extending flavor into the bulk. However, in such cases, the b r a n e / b u l k couplings become arbitrary 3 x 3 mixing matrices whose parameters are undetermined. Moreover, the three bulk neutrinos can in principle correspond to different e x t r a spacetime dimensions with different radii. T h u s , one obtains a scenario with many undetermined parameters governing neutrino masses and mixing angles. Perhaps more interestingly, one can instead consider a "compact" model in which there is only one bulk neutrino \1/ for all three flavor brane neutrinos Vi (i = 1,2,3) on the brane. Moreover, let us imagine t h a t each of these brane neutrinos has a corresponding Majorana mass m;, but for simplicity we will take the brane theory (i.e., the S t a n d a r d Model) to be completely flavor-diagonal. Likewise, we shall also take our b r a n e / b u l k couplings to be completely flavor-neutral (parametrized by a single parameter m as before). W i t h Mo = 0 we then obtain a mass m a t r i x of the form / m i
0 0 m m m
M
0 m2 0 m m m
0 0 m3 m m m
m m m 0 0 0
m m m 0 l R
m m m 0 0
(3.16)
l R
0
V It t u r n s out t h a t the eigenvalues A of this matrix are given exactly as the solutions to the transcendental equation t a n 7rAi?
2
*E
i
(3.17)
For each solution A to Eq. (3.17), the corresponding mass eigenstate \i>.\) is
517
exactly given by \ ~l 3
l*A>=
\
3
-.
oo
,
l
/Nx
J= l
J
/
1=1
fc=
—OO
(3.18) where iV* is an overall normalization constant, and where we have defined f iV« #(*) EE i M « |^0)
if k> 0 if jfc < 0 if jfc = 0 .
(3.19)
Note that the different flavor eigenstates \v/) in Eq. (3.18) consist of different linear combinations of the different mass eigenstates \i/\} for each A. Thus, the different flavor eigenstates will experience a relative oscillation with each other. This oscillation is entirely "bulk-mediated" in the sense that there are no explicit flavor mixings on the brane; it is the presence of the higher-dimensional bulk which is completely responsible for inducing the flavor oscillations on the brane. As an explicit example, let us consider the case with mR = 0.01, 7TI3R S> 1, and rtii^R = 1 — dm/2 for various dm. We then obtain the neutrino oscillations shown in Fig. 23 as functions of a dimensionless time parameter i = (1/2R2)(L/E). Here the subscripts (1,2,3) correspond to (z/1,^2,^3) = (yeiV^v^) respectively. Note that these flavor oscillations arise even without flavor mixing angles on the brane. Although this special case assumes nearly degenerate neutrinos on the brane, it is also possible to achieve significant flavor oscillations even when the brane neutrinos are non-degenerate. This illustrates that in higher dimensions, it may not be necessary to have large flavor mixing angles on the brane in order to accommodate experimental data; small (or even vanishing) flavor mixing angles on the brane may suffice. Furthermore, even though this model contains only five free parameters, it yields a surprisingly rich neutrino oscillation phenomenology, and exploits the higher-dimensional nature of the bulk in an essential way. This model also exhibits another intriguing feature. When the brane/ bulk coupling m is large, one might have naively expected to lose all initial probability of brane neutrinos into bulk neutrinos due to the strong brane/bulk mixing. However, it turns out even as m —> 00, we lose only 1/3 of the initial probability into bulk neutrinos. In other words, 2/3 of the original probability always remains "on the brane" even when m —> 00.
518
Figure 23. Bulk-mediated neutrino flavor oscillations with mR.= 0.01 and m i ^ f i = l=F<5m/2, for various 5m. As discussed in the text, these flavor oscillations are essentially "maximal" even though all mixing angles on the brane vanish identically.
519
This property holds regardless of the values of R or m;, and is intimately connected with the structure of the model in which we have taken only a single bulk neutrino. (By contrast, if we had taken a separate bulk neutrino for each brane neutrino, we would have found that all initial neutrino probability would be lost into the bulk as m —> oo.) This surprising observation suggests that in this model, the large-m case may be able to evade various four-dimensional bounds (such as those from supernova cooling rates) which ordinarily restrict the sizes of mixings with sterile neutrinos. In this lecture, we have merely sketched a number of ideas which play a role when considering neutrino oscillations in higher dimensions. However, over the past few years this has become a rather large industry, and many detailed models have been constructed with a variety of realistic phenomenologies. In general, one seeks to find parameters governing the brane/bulk couplings and mixing angles in such a way as to yield neutrino flavor oscillations in accordance with observational data from solar neutrinos, atmospheric neutrinos, and even laboratory neutrinos (e.g., from the LSND experiment). One must also be careful that conversion into bulk neutrinos not affect observed supernova cooling rates. One important constraint which tends to limit the success of such models is the recent result from the SNO collaboration, which reports flavor conversion in the solar neutrino flux from electron neutrinos to muon and tau neutrinos, with apparently no losses into sterile neutrinos. Since the right-handed bulk Kaluza-Klein states we have been discussing act as sterile neutrinos from the four-dimensional point of view, these constraints require that any higher-dimensional treatment of the solar neutrino flux not leave too great a neutrino probability in the bulk neutrino states. While one can develop a higher-dimensional version of the MSW effect, one must be sure to choose parameters so that most, if not all, of the intermediate-stage bulk neutrino probability is ultimately returned to the active neutrino flavors on the brane. This is perhaps the most severe constraint currently facing models of neutrino oscillations in higher dimensions.
3.4. QCD axion
phenomenology
Many of the above ideas are completely general, and apply to other bulk fields as well. Towards this end, let us now discuss how extra spacetime dimensions may contribute to the invisibility of the QCD axion. The possibility of large extra dimensions may provide a new approach towards solving the strong CP problem. One of the standard approaches
520
to the strong CP problem is to introduce a Peccei-Quinn (PQ) symmetry with a corresponding axion. Unfortunately, the experimentally allowed parameter space for the mass and couplings of the axion have become very narrow, and it is not clear how to generate the high energy scale associated with the breaking of PQ symmetry or to explain the "invisibility" of the resulting QCD axion. Within the framework afforded by large extra dimensions, however, this situation may be radically altered. The basic idea is that since the QCD axion is a singlet under all Standard-Model symmetries, it is free to leave the D-brane to which the Standard-Model particles are restricted and propagate into the higher-dimensional bulk. In other words, in theories with extra dimensions, the QCD axion can accrue an infinite tower of Kaluza-Klein axion states. Can this be used to lower the fundamental PQ symmetry-breaking scale? As in the neutrino case, it is possible to exploit the large volume factor associated with the extra dimensions in order to realize a large effective four-dimensional PQ scale from a smaller, higher-dimensional fundamental PQ scale. Specifically, if / P Q is the underlying higher-dimensional PQ scale, the effective PQ scale / P Q in four dimensions is given by / P Q = (VMS)1'2 f?Q ^ / P Q - It is the volume-renormalized scale / P Q which parametrizes the couplings between the four-dimensional axion and the PQ-charged Standard-Model particles on the brane. Therefore, this volume-renormalization of the brane/bulk coupling can be used to obtain a sufficiently suppressed axion even if the underlying PQ scale is near the TeV range. This is therefore one method of generating an apparent high PQ symmetry-breaking scale in a natural way. However, the presence of the Kaluza-Klein axions can have important and unexpected effects on axion phenomenology. In theories with extra dimensions, it turns out that the four-dimensional axion no longer is a mass eigenstate; instead, this axion ao mixes with the infinite tower of Kaluza-Klein axions an, with a mass mixing matrix given by ^/2 ^/2 V2 2 2 V2 2 + y 2 2 2 V2 2+ V 2 2 2 + 9y2 V2
/ 1 AT
m
PQ
(3.20)
V where TOPQ ~ A Q C D / / P Q and y = (mpQR) of interesting consequences.
1
. This mixing has a number
521
First, under certain circumstances the mass of the axion essentially decouples from the PQ scale, and instead is set by the radius of the extra spacetime dimension. This occurs because the eigenvalues A of the matrix (3.20) are given as solutions to the transcendental equation 7ri?Acot(7riZA) = A2/mpQ where R is the radius of the extra compactified dimension. Note that the lightest eigenvalue necessarily satisfies A < l/(2i?); this result holds regardless of the value of TOPQ. This implies that axions in the 10~ 4 eV mass range are consistent with (sub-)millimeter extra dimensions. More interestingly, however, this implies that in higher dimensions, the size of the axion mass is set by the radius R and not by the Peccei-Quinn scale / P Q when mpQ > 1/(2R). This decoupling implies that it may be possible to adjust the mass of the axion independently of its couplings to matter. This is not possible in four dimensions. Second, since the usual four-dimensional axion is no longer a mass eigenstate, it should undergo laboratory oscillations as it propagates. Such oscillations are entirely analogous to neutrino oscillations. Moreover, because the axion is now a higher-dimensional field, Standard-Model particles couple not only to the usual four-dimensional axion zero mode, but rather to the entire linear superposition a' ~ Yln an of Kaluza-Klein modes. Therefore, the quantity of phenomenological interest is the probability Par^a>(t) that a' is preserved as a function of time. This probability is shown in Fig. 24. Unlike the case of neutrino oscillations, we see that this probability drops rapidly from 1 (at the initial time t = 0) to extremely small values (expected to be « 10~ 16 in realistic scenarios). At no future time does this probability regenerate. Thus, we see that in higher dimensions, the axion state a' rapidly "decoheres" and becomes invisible with respect to subsequent laboratory interactions! This decoherence is therefore a possible higher-dimensional mechanism contributing to an invisible axion. Finally, one can investigate the effects of Kaluza-Klein axions on cosmological relic axion oscillations. Recall that as the universe cools and passes through the QCD phase transition at T K AQCD, instanton effects suddenly establish a non-zero axion potential where none previously existed. In the usual four-dimensional situation, the axion can therefore find itself displaced relative to the newly-established minimum of the potential, and begin to oscillate around it with a "damping" term due to the expansion of the universe (the Hubble term). The equation governing this evolution is given by d2a +
TT da 9 3H-+mla
= 0
, (3.21)
522
where H is the Hubble constant and ma is the axion mass. Some of the most severe upper bounds on the PQ scale come from demanding that the energy trapped in these oscillations today be less than the critical energy density to overdose the universe. In higher dimensions, by contrast, all of the axion Kaluza-Klein modes are coupled together via the mixing matrix (3.20). This means that the usual equation (3.21) is now generalized to become d2ak
(3.22)
=0
• SH^+Mhat
~dt?~
for each individual axion mode a^, where the mass matrix MM is given in Eq. (3.20). Thus, even if we assume that only the zero-mode axion starts out initially displaced when the axion potential is generated, the coupling wj
1 1 1 1
0.8 --\
0.6
OH
1
1
!
1
i
,
i
|
i
1
(e)
i
i
|
i
i
-
Vb) \
\(c)
\
\(d)
0.2
i
(^T^-^
\
- 1 \
0.4 -
1 '
W
|\\.VVViW/iW
0
0.001
0.002
0.003
0.004
(m£Q/2p) t Figure 24. The axion preservation probability P 0 '_> a '(i) as a function of the number "max of axion Kaluza-Klein states which are included in the analysis. In this plot we show the results for (a) n m a x = 1; (b) n m a x = 2; (c) n m a x = 3; (d) n m a x = 5; and (e) "max = 30. As n m a x increases, the axion probability rapidly falls to zero as a result of the destructive interference of the Kaluza-Klein states, and remains suppressed without significant axion regeneration at any later times. This "decoherence" of the axion implies that there is negligible probability for subsequently detecting the original axion state at any future time.
523
inherent in the equations (3.22) is sufficient to trigger the excited KaluzaKlein modes into oscillation. This situation is illustrated in Fig. 25(a). It is therefore important to understand whether the infinite towers of Kaluza-Klein axion states accelerate or retard the dissipation of the cosmological energy density associated with these relic oscillations. Remarkably, one finds that the net effect of these coupled Kaluza-Klein axions is always either to preserve or to enhance the rate of energy dissipation. This behavior is shown in Fig. 25(b). Because these Kaluza-Klein states accelerate the rate at which this relic oscillation energy is dissipated, the usual relic oscillation bounds are loosened in higher dimensions. This suggests that it may be possible to raise the effective PQ symmetry-breaking scale beyond its usual four-dimensional value, thereby further contributing to axion invisibility.
time -.
y = (mp,R)-'
Figure 25. (a) Coupled Kaluza-Klein cosmological relic axion oscillations. Although the excited Kaluza-Klein modes have vanishing initial displacements, they are triggered into oscillation as a result of the initial displacement of the zero-mode. The superimposed dashed line shows the behavior of the usual four-dimensional axion zero-mode when no Kaluza-Klein modes are present, (b) The energy-dissipation ratio factor p(t)/p4D(*) where P4D (t) is the normalized energy density today in the usual four-dimensional case and p(t) is the analogous quantity in the higher-dimensional case. The different curves correspond to different input parameters. As y —» 0 (implying a large extra dimension), we see that the cosmological relic energy density is dissipated more rapidly than in four dimensions.
Together, these results suggest that it may be possible to develop a new, higher-dimensional approach to axion phenomenology. In this lecture, we have chosen to discuss only a small subset of the many phenomenological issues that have been examined within the context of the
524
brane world. For further details concerning these or other approaches, we refer the interested reader to the original literature. 4. Lecture # 4 : Beyond the simple pictures: Lessons and warnings In this final lecture, we shall discuss some issues which are normally neglected in current treatments of the brane world, but which perhaps may play a role in more sophisticated developments of a complete higherdimensional theory. I have chosen to raise such issues not only as a way of indicating caution concerning the standard treatments of higherdimensional theories, but also to stimulate further thinking and research. 4.1. Shape, not just
volume
Thus far in our discussions of large extra dimensions, we have seen that large extra dimensions have the potential to lower the fundamental energy scales of physics. Indeed, in all cases, the degree to which these scales may be lowered depends on the volume of the compactified dimensions. However, compactification geometry is not merely endowed with volume; to borrow a phrase from Shakespeare, it is also "beautified with goodly shape." 0 Indeed, both volume (or "Kahler") moduli and shape (or "complex") moduli are necessary in order to fully describe the geometry of compactification. This distinction has phenomenological relevance because the shape moduli also play a significant role in determining the experimental bounds on such scenarios. Unfortunately, in most discussions of large extra dimensions in the literature, relatively little attention has been paid to the implications of these moduli. In this lecture, we shall discuss some of the phenomenological implications of non-trivial shape moduli. First, as we shall see, shape moduli can have dramatic effects on the corresponding Kaluza-Klein (KK) spectrum: they can induce level-crossings and varying mass gaps, they can help to eliminate light KK states, and they can alter experimental constraints in such scenarios to the extent that the bounds on the largest extra dimension can be completely eliminated. In other words, we shall see that large extra dimensions can be essentially invisible (even if they are flat). Second, we shall see that shape casts "shadows". Specifically, we shall show that non-trivial shape moduli can distort our experimental percepC
W. Shakespeare, The Two Gentlemen
of Verona: IV, i.
525
tions of compactification geometry at low energies. Indeed, we shall find that spacetime geometry is essentially "renormalized" as a function of the energy scale with which it is probed. Finally, we shall briefly discuss the effects that non-trivial shape moduli can have on string winding modes. We shall show, in particular, that toroidal compactifications exist for which all KK states as well as all winding modes are heavier than the string scale. Thus, in the context of low-scale string theories, it is possible to cross the string scale without detecting any string states associated with spacetime compactification. In order to illustrate these points, let us begin by considering the case of compactification on a general two-torus, as illustrated in Fig. 26. Such a torus is realized as the space K subject to the two discrete identifications 2/1
2/1 + 27ri?!
2/1 -*
2/2
2/2
2 / 2 - ^ 2 / 2 + 2nR2 sin 9 .
2/i + 2KR2
COS 9
(4.23)
Thus, the flat two-torus is specified by three parameters: R\, R2, and 9. The volume of the torus is V = ATT2RIR2 sin 9, while the shape parameters are R2/R\ and 9. Most studies of large extra dimensions in the literature focus.on the effects of V and R2/Ri, essentially fixing 6 = TT/2. But what are the phenomenological implications of non-trivial 9?
27CR,
271R1
Yl
Figure 26. General two-dimensional torus with shift angle 9.
Clearly, we need to analyze the KK wavefunctions on this space. Demanding invariance under the torus periodicities in Eq. (4.23), we find that the KK wavefunctions take the form * r
exp lR-^Vl yi
( ~tan £e) 6J
.n2
y2
R2 sin 9
Applying the (mass) 2 operator —{d2/dy\ + d2/dy2),
n^TL
. (4.24)
we then find the KK
526
masses
Note that these masses are no longer invariant under n\ —> — ri\ or n2 —> —ri2 individually unless combined with 8 —> TT — 8. We can therefore restrict our attention to values of 8 in the range 0 < 8 < TT/2 without loss of generality. The case with 8 = TT/2 corresponds to the usual "rectangular" torus, but we are interested here in the behavior of the KK spectrum as 8 is varied while holding the radii (R\,R2) fixed. We then find dramatic changes in the KK spectrum, as illustrated in Fig. 27 (upper plots) for the cases with Rx = R2 and Ri = 4R2. We observe that in general, the mass gap /J, (defined as the mass of the first excited KK state) depends on 8, and tends to increase as 8 becomes smaller than TT/2. Note that in the case of KK gravitons, this increase in the mass gap implies that the deviations from Newtonian gravity will be exponentially suppressed as 8 becomes smaller, even though the radii of the extra dimensions are held fixed. Another possibility, of course, is to hold the volume of the torus fixed (as well as the ratio R2/Ri) while we vary 8. We then find the results illustrated in Fig. 27 (lower plots). In this case, the mass gap can vary, but need not necessarily grow as 8 gets smaller. It is important to note that modular symmetries imply a redundancy in whether we interpret changes in the shape moduli as changes in 8 or as changes in R2/R\. In other words, the effects of taking 8 —> 0 can equivalently be viewed as keeping 8 relatively large and instead varying R2/R1. However, the important point is that the Kaluza-Klein spectrum is indeed sensitive to changes in the shape moduli, and this feature may be used to evade experimental constraints even when the radii (or volume) of the extra dimensions may be large and/or fixed. In fact, there even exists a small window of parameter space in which, thanks to the shape moduli, the usual experimental constraints on the radii of the largest extra dimensions become, instead, constraints merely on the volume of the compactified space, regardless of the size of the largest radii. In other words, the experimental bounds becomes independent of the size of the largest extra dimension! Thus, under certain narrow circumstances, a large, flat extra dimension can be essentially invisible, even though the volume of compactification is held fixed.
527
\
A2-' ' ' //
7(2.-2)
,
|
1 ,
// I
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:
/X y^^>^L /
(2.1)//
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51
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,
,
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0.8
,
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0.6
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0.4
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sinfl
Figure 27. The masses of the lowest-lying KK states (tii, 712) as functions of the shape parameter d for Ri = R2 = R (upper left plot) and R\ = 4R2 (upper right plot), holding .Rl and R2 fixed. The lower plots represent the same scenarios where we vary 9 holding both the volume of the torus and the ratio R2/R1 fixed.
Let us now consider another surprising aspect of shape moduli: this is the phenomenon called "shadowing". To understand this, let us consider general tori in one, two, and three dimensions. These tori are illustrated in Fig. 28. The one-dimensional torus, of course, is nothing but a circle and has no corresponding shape moduli. By contrast, the two- and threedimensional tori are described not only by radii but also by the shift angles 6 and ctij which mix the periodicities associated with translations along the corresponding directions. Note that we shall now use the symbol p to indicate the radius of the one-torus; lowercase r\, r^ to indicate the radii of
528
(3)
Figure 28.
27Cp
General one-, two-, and three-dimensional tori with arbitrary shape angles.
the two-torus, with 8 serving as the shape angle; and uppercase i?i, R2, R3 to indicate the radii of the three-torus, with 0:12,011.3, «23 denoting the corresponding shape angles. Our goal is to study the extent to which various low-energy observers can determine the shapes of these tori by studying their associated KK spectra. Towards this end, let us assume that the "true" compactification geometry is given by the three-torus shown in Fig. 28(c). Furthermore, let us assume that there is a hierarchy of length scales such that R3 0(R^1), the KK spectrum will reveal the presence of all three dimensions of the torus. Such an observer can then determine all three radii Ri and shape angles Qy through a detailed spectral analysis of the KK states. However, for an observer with access to only intermediate energies 0(R^ ) the third dimension will be inaccessible; the compactification manifold would then appear to be a two-torus, as
529
illustrated in Fig. 28(b). Finally, for the low-energy observer with access to energies 0(R{1) <S £ m a x
,
r2=R2
,
0 = a12 .
(4.26)
After all, at energies much below R^1, we expect all remnants of the third dimension to vanish, so that the two-torus experienced by the intermediateenergy observer is merely the base of the original three-torus in Fig. 28(c). Likewise, it is natural to expect that the lowest-energy observer would perceive a circle with radius p = n, which by Eq. (4.26) implies p = n
= Hi .
(4.27)
Once again, this would be the naive expectation, given that we have only sufficient energy to probe the largest dimension of the original three-torus. However, as we shall now demonstrate, the above relations in Eqs. (4.26) and (4.27) are incorrect, even in the presence of a large hierarchy R3
530
seen in E q . (4.25) t h a t t h e K K s p e c t r u m is given by TliTlj
M2 1=1
«
(4.28)
t^tj
where ra» G 2 . Finally, for t h e general three-torus shown in Fig. 28(c), t h e K K s p e c t r u m is given by
M2
1 ~K
3
En
2
'H2 2 J?2bjk
V ^ njn "» 7 Z^ P . RiR 1-Ll.J i^i
CikCjk)
(4.29)
where k ^ i,j, where Cij = c o s a ^ a n d s^- = s i n a ^ , a n d where K [the dimensionless squared volume of t h e parallelepiped in Fig. 28(c)] is given by
« =l-E4+2]!^ i<3
= E 4 + 2 n^' - ! i<3
i<3
(4.30)
\i<3
Given these results, we can now determine the appropriate correspondence relations. In deriving these relations, we shall assume a hierarchy i?3 < i?2 € R\ so that we can successively integrate out small extra dimensions when passing to larger length scales. Our procedure will be to disregard all KK states whose masses exceed the appropriate reference energy (either high, intermediate, or low) and are therefore inaccessible to the corresponding observer. The observer at highest energy clearly sees three dimensions worth of KK states, and deduces the "true" geometry of the compactified space by comparing the measured KK masses with Eq. (4.29). However, the observer at intermediate energy cannot perceive excitations in the R3 direction, since functionally R3 —> 0 for this observer. His attention is therefore restricted to states with 77,3 = 0, and he attempts a spectral analysis of the remaining states. This leads to the identifications 1
1
b
3k
(i = l , 2 ) sin C12 - C13C23 cos# 1 (4.31) sin 2 6>rir 2 KRiR2 This observer therefore deduces t h a t t h e compactified space is a two-torus parametrized by ( r i , r2, 0) given by KR\
(n
= si3Ri
\cos0
= (C12 - C13C23VS13S23 •
(4.32)
531
Note that both the radii r* and the shape angle 8 are affected, leading to apparent values for (ri,T2,0) which are not present in the original threetorus. The lowest-energy observer, by contrast, misses the i%i excitations as well. Upon comparing with the circle KK spectrum M2 = n\/p2, he therefore concludes that the compactified space is a circle of radius p = (sin0)n = —
i?i .
(4.33)
S23
Once again, this radius does not correspond to any periodicity in the original three-torus. Mathematically, these results reflect the geometric "shadows" that successive smaller extra dimensions cast onto the larger extra dimensions when they are integrated out. As such, they indicate that the low-energy observer can see only those "projections" of the compactification space which are perpendicular to the extinguished dimensions. But given the assumed large hierarchy of length scales, the physical implications of this shadowing effect are rather striking. A small extra spacetime dimension — even one no larger than the Planck length! — is able to cast a huge shadow over all other length scales and their associated dimensions, completely distorting our low-energy perception and interpretation of the compactification geometry. Indeed, the physics which we would normally associate with the Planck scale (such as the angles that parametrize the shape of the Planck-sized extra dimensions relative to the larger extra dimensions) fail to decouple at low energies. Of course, no observer at any energy scale can use this shadowing phenomenon in order to deduce the existence of an extra spacetime dimension beyond his own energy scale. Nevertheless, the observer's interpretation of that portion of the compactification geometry accessible to him is completely distorted, leading him to deduce geometric radii and shape angles that have no basis in reality. Since the existence of an even smaller extra dimension beyond those already perceived can never be ruled out, this shadowing effect implies that one can never know the "true" compactification geometry. Even when light KK states are detected and successful fits to the KK mass formulae are obtained via spectral analyses, the presence of further additional dimensions with appropriate shape moduli can always reveal the previous successes to have been illusory. We are not claiming that no "true" compactification geometry can ever exist. Indeed, if one t