Search for Leptoquarks in Electron-Proton Collisions by Frederic Benard A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physics University of Toronto Copyright © 1995 by Frederic Benard Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Search for Leptoquarks in Electron-Proton Collisions by Frederic Benard Graduate Department of Physics University of Toronto P h.D . 1995 A b stract Leptoquarks carry lepton number as well as baryon number and could provide an explanation for the observed symmetry between the quarks and leptons. Leptoquarks coupling to first-generation quarks and leptons were searched for in electron-proton colli­ sions at HERA using the ZEUS detector. In a sample of e~p —> v X events corresponding to an integrated luminosity of 27.5 nb-1, no leptoquark candidates were found. Limits on leptoquark couplings are derived for leptoquark masses ranging from 50 GeV/c2 to 225GeV/c2. Scalar (vector) isosinglet leptoquarks coupling to a left-handed electron and a tz-quark (d-quark) with electroweak coupling strength are ruled out at the 95% confidence level for masses below 154 G eV/c2 (108 GeV/c2). ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgements I am very grateful to my supervisor, David Bailey, for his enthusiasm, his patience, and his continuous support. I am happy to have been his first graduate student. Being part of the ZEUS Third Level Trigger group has been an exciting and enriching adventure. I am indebted to Robert Orr, Sampa Bhadra, Dinu Bandyopadhyay, and Gerd Hartner, as well as the students on the project: Frank, Mike, Cortncy, and Richard. I wish to thank the members of the ZEUS Exotics physics group, in particular Frank Sciull’-, Steve Ritz, Bruce Straub, and Jurgen Schroeder. I also wish to thank Sacha Davidson for answering my many qu .rations on leptoquarks. David Bailey and •onathan Labs reviewed many drafts of the thesis and their comments and suggestions have been invaluable. In addition, their questions prepared me well for my Ph.D. defences. The Canadian ZEUS group at DESY, under the leadership of John Martin, provided intellectual and social support during my stay in Hamburg. In addition to the aforemen­ tioned ZEUS members, I would like to thank Milos, Burkhard, Wai, Laurel, and Larry. I thank my friends, Sophie, Frederic, Jason, Charles. Mieke, Heather, Lysiane, Thomas, and Robert, without the support of whom I would have had difficulty maintaining my sanity during the last five years. The support of Charles, Heather, and Robert this last year is especially appreciated. Un gros merci a ma soeur Isabelle, ma mere Rolande et mon pere Raymond pour leurs encou-agements et leur amour. Finally, I would like to thank Jonathan for his tremendous support, for his love, and for having waited for me so long in rainy Hamburg while I finished my thesis in Toronto. iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Personal Contributions to the ZEUS Experiment I joined the ZEUS Third Level Trigger group in the fall of 1989. During the following two years I was involved with Monte Carlo trigger studies. I also set up and maintained a network of five computer workstations used by TLT members at the University of Toronto. I moved to Hamburg in the spring of 1991. As a member of the TLT group, I continued doing trigger studies and developed analysis software for the TLT processors. In particular, I interfaced various versions of the VCTRAK track reconstruction pr^ram to the TLT environment. I also set up a large network of computer workstations currently used by the Canadian ZEUS group in Hamburg. In 1992, I contributed to the conception and development of Funnel , the platform now used for almost all ZEUS Monte Carlo event generation. I joined the ZEUS Exotics physics group in the fall of 1991 and over the following two years made many contributions to that group. The first collisions were provided by the accelerator during the summer of 1992. A collective effort was made to extract a possible leptoquark signal from the data. I worked on the decay to neutrino plus jets channel and helped design criteria to select the deep inelastic charged current data. I also studied the effects of trigger acceptance, je t reconstruction, and track and vertex reconstruction. No leptoquark signals were found and lim its on leptoquark couplings were derived. The work of '.he Exotics physics group led to the publication “Search for Leptoquarks with the ZEUS Detector”, Phys. Lett. B 3 0 6 (1993) 173. iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C ontents Introduction 1 1 Theoretical Framework 3 1.1 The Standard M odel ................................................................................................... 3 1.2 Leptoquarks Beyond the Standard M odel ............................................................. 6 1.2.1 Grand Unification ............................................................................................ 0 1.2.2 Technicolour ................................. 7 1.2.3 Compositeness ................................. 8 1.3 A Model Independent Approach .............................................................................. 0 1.4 Experimental Limits on Leptoquarks .................................................................... 9 1.4.1 Searches in Electron-Positron Collisions .................................................. 9 1.4.2 Searches in Proton-Antiproton Collisions ....................... 12 1.4.3 Leptonic Pion Decays ..................................................................................... 13 1.4.4 Unitarity of the CKM M a trix .................................................................... 15 1.4.5 Atomic Parity V iolation ............................................................................... 10 1.5 Kinematics of Electron-Proton Scattering ............................................................. 18 1.6 Leptoquark Production and Decay at HERA ................................................... 20 2 Experimental Setup 27 2.1 The HERA Electron-Proton C ollider .................................................................... 27 v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.2 The ZEUS Detector ...................................................................................................... 30 2.2.1 O verview .......................................................................................................... 30 2.2.2 C a lo r im e te r ....................................................................................................... 34 2.2.3 Central Tracking D etector ........................................................................... 37 2.2.4 Co D e te c to r ....................................................................................................... 37 2.2.5 Luminosity Monitor ...................................................................................... 39 2.3 The ZEUS Trigger and Data Acquisition System ................................................. 40 2.3.1 C o n ce p ts .............................................................................................................. 40 2.3.2 First Level Trigger ......................................................................................... 42 2.3.3 Second Level Trigger ...................................................................................... 43 2.3.4 Third Level Trigger ......................................................................................... 44 2.4 The ZEU S O ffline E n v iro n m e n t ............................................................................... 48 2.4.1 Event R e co n stru ctio n ...................................................................................... 4S 2.4.2 Offline Analysis ................................................................................................ 49 3 Data Analysis 52 3.1 Description of the Problem ......................................................................................... 52 3.2 Analysis T o o ls ................................................................................................................. 54 3.2.1 C5 T im e .............................................................................................................. 54 3.2.2 Track and Vertex Reconstruction ............................................................. 57 3.2.3 Calorimeter Tim e ............................................................................................. 59 3.2.4 Calorimeter Energy Islands ........................................................................ 60 3.2.5 M uon F i n d e r .................................................................................................... 64 3.2.6 Reconstruction of Kinematic V ariables ................................................... 66 3.2.7 Luminosity Calculation ................................................................................... 67 3.3 Data S e le c tio n ................................................................................................................. 68 3.3.1 T he Level One F i l t e r ..................................................................................... 6S 3.3.2