The discovery of the top quark Claudio Campagnari University of California, Santa Barbara, California 93106 Melissa Franklin Harvard University, Cambridge, Massachusetts 02138 Evidence for pair production of a new particle consistent with the standard-model top quark has been reported recently by groups studying proton-antiproton collisions at 1.8 TeV center-of-mass energy at the Fermi National Accelerator Laboratory. This paper both reviews the history of the search for the top quark in electron-positron and proton-antiproton collisions and reports on a number of precise electroweak measurements and the value of the top-quark mass that can be extracted from them. Within the context of the standard model, the authors review the theoretical predictions for top-quark production and the dominant backgrounds. They describe the collider and the detectors that were used to measure the pair-production process and the data from which the existence of the top quark is evinced. Finally, they suggest possible measurements that could be made in the future with more data, measurements that would confirm the nature of this particle, the details of its production in hadron collisions, and its decay properties. [S0034-6861(97)00101-3] CONTENTS 2. Z tt background 174 → 3. Drell-Yan background 174 4. WW background 175 I. Introduction 137 5. bb¯ and fake-lepton backgrounds 175 II. The Role of the Top Quark in the Standard Model 138 6. Results 175 III. Top Mass and Precision Electroweak Measurements 140 B. Lepton 1 jets + b tag 176 A. Measurements from neutral-current experiments 140 1. Selection of lepton 1 jets data before b 1. Asymmetries at the Z 141 tagging 177 2. Lepton tagging in CDF and D0 178 2. Rb 142 B. W mass 142 3. Displaced-vertex tagging in CDF 180 4. Summary and cross checks of the tagging- C. Global fits for Mtop and MHiggs 142 IV. Top-Quark Production 144 background calculation on Z1 jets 184 A. Production mechanisms and cross sections 144 C. Lepton + jets 184 B. Top-quark hadronization 148 1. D0 lepton + jet kinematic analysis 185 C. Underlying event 148 2. CDF lepton + jets kinematic analysis 187 D. Modeling of top-quark production 149 3. Summary of lepton 1 jets kinematic analyses 188 V. Top-Quark Signatures 151 D. Significance of the top signal 189 E. Measurement of the pp¯ tt¯ cross section 190 A. Standard-model top-quark decay modes 151 → B. Detection of the top decay products 151 IX. Measurement of the Top-Quark Mass 191 1. Detection of electrons and muons 151 A. Direct measurements of the top-quark mass 2. Detection of quarks 152 from lepton 1 jets events 192 3. Detection of neutrinos 155 1. Constrained fits, combinatorics, and top-mass 4. Detection of tau leptons 155 resolution 192 C. All-hadronic mode 156 2. CDF and D0 top-mass measurements 194 D. Dilepton mode 156 3. Jet-energy corrections and systematics on the E. Lepton + jets mode 157 Mtop measurement 195 1. W + jets background 157 4. Updated CDF and D0 top-mass measurements 197 2. Separation of W1 jets and tt¯ for B. Top mass from dilepton events and kinematic distributions 198 Mtop,MW1Mb 159 3. Kinematic differences between tt¯ and W1 C. Reconstruction of the W mass from hadronic jets 160 decays in lepton 1 jets events 200 4. b-quark tagging 162 X. Future Prospects 201 VI. Early Searches for the Top Quark 164 A. Accelerator and detector upgrades 201 A. Searches in e1e2 collisions 164 B. Improving the top-mass measurement 202 B. Early searches in pp¯ collisions assuming C. Probing the Wtb vertex 203 standard-model top-quark decay 165 D. Further tests of the standard model and searches C. Searches for non-standard-model top-quark for new physics in the top-quark sector 205 decay modes 168 XI. Conclusion 207 VII. The Fermilab Proton-Antiproton Collider 169 Acknowledgments 207 A. Linear accelerators and synchrotrons 169 References 208 B. Antiproton source 170 C. Collider 170 I. INTRODUCTION VIII. Discovery of the Top Quark 171 A. Dilepton analysis results from CDF and D0 172 The intensive experimental efforts in the search for 1. Z ee/mm background 173 the heaviest fundamental fermion culminated in 1995 → Reviews of Modern Physics, Vol. 69, No. 1, January 1997 0034-6861/97/69(1)/137(75)/$21.25 © 1997 The American Physical Society 137 138 Campagnari and Franklin: The discovery of the top quark with the discovery of the top quark in proton-antiproton annihilations at the Fermilab Tevatron Collider. Obser- vation of the top quark is the latest in a long series of triumphs for the standard model of particles and fields. The top quark is the last fundamental fermion and the next-to-last fundamental particle predicted by the stan- dard model. Only the Higgs boson remains unobserved. The search for the top quark started in the late seven- ties, soon after discovery of the companion bottom quark. It has been a long and arduous process because the top quark turned out to be much more massive than was originally expected. The mass of the top quark is remarkably large, approximately 200 times larger than the mass of the proton and 40 times higher than the mass of the next-lightest quark. Whether this property of the top quark is a mere accident or a manifestation of a deeper physical process is an unanswered question in particle physics. The discovery of the top quark has been made pos- FIG. 1. Leptons and quarks in SU(2)3U(1) (standard model). sible by the technological progress in high-energy phys- Also shown are the values for the SU(2) weak isospin (I3), ics in the past fifteen years. In particular, the develop- U(1) weak hypercharge (Y), and electric charge (Q, in units ment of proton-antiproton colliders, pioneered first at of the electron charge). The subscripts L and R refer to the CERN and then at Fermilab, has been a crucial ingredi- left- and right-handed components, respectively. ent in the discovery of the top. In this article we review the discovery of the top quark as well as the developments that led to it. In Sec. three generations of quarks and leptons in the model, II we discuss the top quark within the framework of the with identical quantum numbers but different masses. standard model. While the top-quark mass is a free pa- Within each generation, quarks and leptons appear in rameter in the standard model, its value enters in calcu- pairs (see Fig. 1). The left-handed quarks form weak- lations of a number of electroweak observables. The isospin doublets, with the electric charge Q512/3 and top-mass dependence of the theoretical predictions is in Q521/3 quarks having weak isospin I3511/2 and general rather weak. However, many of these measure- 21/2, respectively. ments are now accurate enough that meaningful con- The tau lepton (t) was the first particle of the third straints on the top-quark mass can be obtained by com- generation to be discovered (Perl et al., 1975). A short paring them with theoretical predictions. These time later, in 1977, the Y was discovered at Fermilab constraints constitute a test of the predictive power of (Herb et al., 1977) as a resonance in the m1m2 invariant- the standard model and are reviewed in Sec. III. The mass spectrum in the reaction p1nucleon m1m21X. → top-production mechanisms in proton-antiproton colli- This resonance was interpreted as a bb¯ bound state (the sions and the experimental signatures of top events that Y), which subsequently decays into muon pairs. As will are crucial to the understanding of the experimental re- become abundantly clear in the remainder of this paper, sults are discussed in Secs. IV and V. Early searches for the top signature in hadron collisions is much more com- the top quark are described in Sec. VI. The Tevatron plicated. Collider, whose remarkable performance played a very In the past fifteen years a tremendous amount of ex- important role in the discovery of the top quark, will be perimental data on the properties of the b quark and of described briefly in Sec. VII. The data that finally led to b-flavored hadrons has become available, mostly from the discovery of the top quark are reviewed in Sec. VIII. 1 2 The value of the top mass is of fundamental importance, experiments at e e colliders. Both the charge and the and it is needed to complete precise tests of the standard weak isospin of the bottom quark are by now well estab- model. Furthermore, from an experimental point of lished (Qb521/3 and I3521/2). view, the techniques developed to measure the top mass The value of the charge was first inferred from mea- are new and particularly interesting. The top-mass mea- surements of the Y leptonic width (Ch. Berger et al., surement is described in Sec. IX. Historically, discover- 1978; Bienlein et al., 1978; Darden et al., 1978) at the ies of new leptons or quarks have opened up new fields DORIS e1e2 storage ring. This width is proportional to of inquiry that have enhanced our understanding of el- the square of the charge of the b quark (see Fig. 2), and ementary particles and their interactions. Consequently, can be quantitatively estimated from heavy quark- this article concludes in Sec. X with a discussion of the experimental prospects for top physics. II. THE ROLE OF THE TOP QUARK IN THE STANDARD MODEL Quarks and leptons constitute the basic building blocks of matter in the standard model (SM).