The First Direct Limit on the t Quark Lifetime A dissertation presented by Ayana Tamu Holloway to The Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the subject of Physics Harvard University Cambridge, Massachusetts May 2006 c 2006 - Ayana Tamu Holloway All rights reserved. Thesis advisor Author M.E.B. Franklin Ayana Tamu Holloway The First Direct Limit on the t Quark Lifetime Abstract This dissertation presents the first direct limit on the t quark lifetime, measured in tt¯ pair production candidate events from a 318 pb−1 sample of p +p ¯ collisions recorded by the Collider Detector at Fermilab (CDF). Candidate events are identified by a leptonically decaying W ± boson and three or more jets, at least one of which contains a secondary vertex. In each event we measure the transverse track-to-beam displacement (impact parameter) of the lepton from the W ± decay, and compare the distribution to templates constructed from tt¯ event simulations with a variable t lifetime. Anticipated background distributions are measured with a combination of Monte Carlo event simulation and control samples, and included in the templates. We determine the following limits: cτt < 52.5µm @ 95% C.L. cτt < 43.5µm @ 90% C.L. We thus conclude that the top mean lifetime is less than 145 fs (175 fs) at 90% (95%) C.L. Contents Title Page . i Abstract . iii Table of Contents . iv List of Figures . viii List of Tables . xi Acknowledgments . xiii Dedication . xv 1 Introduction 1 1.1 Making and finding top . 1 1.2 Is the lifetime short? . 3 1.3 Detecting “large” top lifetimes . 4 1.4 Overview of the thesis . 5 2 Theoretical Predictions 8 2.1 Dynamics of the t quark . 9 2.1.1 Production . 9 2.1.2 Decay . 13 2.2 Indirect evidence for a short-lived t ................... 18 2.2.1 Indirect measurements of the t mass . 18 2.2.2 Direct measurements of the t mass . 22 2.2.3 Indirect measurements of the t weak couplings . 23 2.2.4 The top width and electroweak top production . 24 2.3 Evolution of a long-lived t state . 25 2.3.1 Hadronization and spin correlations . 26 2.3.2 Soft gluon radiation . 27 2.4 Secondary t quarks . 27 3 Experimental Apparatus 32 3.1 The Fermilab Tevatron accelerator . 33 3.1.1 Proton beam . 34 iv Contents v 3.1.2 Antiproton beam . 34 3.1.3 Collisions . 35 3.2 The Collider Detector at Fermilab . 37 3.2.1 Detector geometry . 39 3.2.2 Trackers . 40 3.2.3 Time-of-Flight . 49 3.2.4 Calorimetry . 49 3.2.5 Muon detectors . 55 3.2.6 Trigger . 59 4 Event Reconstruction and Simulation 66 4.1 Algorithms . 66 4.1.1 Tracking algorithms . 67 4.1.2 Jet reconstruction . 72 4.1.3 Secondary vertex tagging . 74 4.1.4 Lepton reconstruction . 76 4.2 Event simulation . 80 4.2.1 Monte Carlo datasets . 82 5 Data selection 85 5.1 The W +jets sample . 85 5.2 Event reconstruction requirements . 89 5.2.1 Detector status . 89 5.2.2 Interaction point requirements . 89 5.2.3 Background rejection . 90 5.3 Lepton selection . 92 5.3.1 Electron identification . 92 5.3.2 Muon identification . 95 5.4 Heavy flavor jet identification . 98 5.5 Neutrino identification . 99 5.6 Lepton track requirements . 99 5.6.1 Silicon hit cut scale factor . 100 5.7 Accepted events . 101 6 Backgrounds 106 6.1 Background event multiplicity . 107 6.1.1 Diboson background . 108 6.1.2 Z0 → ττ background . 109 6.1.3 Single top background . 110 6.1.4 Conversion electron background . 111 6.1.5 W/Z+jets background . 112 6.1.6 Cosmic ray muon background . 115 Contents vi 6.1.7 Summary . 116 6.2 Systematic uncertainties in background composition . 118 6.2.1 QCD background estimates . 118 6.2.2 Heavy flavor tagging . 119 6.2.3 Luminosity, trigger efficiency and scale factor uncertainties . 120 7 Templates 122 7.1 Track impact parameter resolution . 123 7.1.1 Prompt lepton track samples . 123 7.1.2 Z0 sample purity . 124 7.1.3 Resolution functions . 126 7.2 Signal sample d0 probability distributions . 127 7.2.1 Signal templates . 129 7.3 Background d0 distribution functions . 135 7.3.1 Prompt backgrounds . 135 7.3.2 Z0 → ττ and W ± → τν backgrounds . 135 7.3.3 Conversion electron background . 136 7.3.4 QCD jet background . 137 7.3.5 Background templates . 141 7.4 Systematic distortions of templates . 145 7.4.1 Signal templates . 145 7.4.2 Z0 selection effects on resolution function . 146 7.4.3 Background template shape systematics . 152 8 Limits 158 8.1 Maximum likelihood fit . 159 8.2 Pseudoexperiments . 160 8.2.1 Error inclusion . 164 8.3 Results . 167 8.4 Conclusions . 168 A Background and signal in W +jets 171 A.1 Missing energy and isolation . 172 A.2 Untangling contributions to the W +jets sample . 174 A.2.1 Systematics from W/Z + (2p) events in W/Z + 3j data . 177 B The conversion background 180 B.1 Efficiency of the conversion finder . 180 B.2 Tagged conversion events . 183 B.2.1 Systematic errors . 184 Contents vii C First hadron collider measurement of ττ 185 C.1 Event selection . 186 C.2 Sample composition . 189 C.2.1 Expected backgrounds . 189 C.2.2 Signal yield . 197 C.3 Template generation . 198 C.3.1 Detector resolution . 198 C.3.2 Background templates . 201 C.4 Template fit . 202 C.4.1 Template bias . 204 C.4.2 Errors from template fitting . 204 C.5 Results . 207 C.5.1 Tau lifetime . 207 C.5.2 Data-Monte Carlo comparisons . 207 Bibliography 211 List of Figures 1.1 Schematic diagram of tt¯ production. 2 1.2 Schematic diagram of the tt¯ decay products. 5 1.3 Illustration of the measurement of the W ± decay position. 6 2.1 Feynman diagram representing the t-gluon interaction term. 10 2.2 Feynman diagrams for leading-order t production processes at the Tevatron. 10 2.3 Parton distribution functions evaluated√ at the scale µ = 100 GeV. 13 2.4 The p+p ¯ → ttX¯ cross section at s = 1.96 TeV as a function of mt. 14 2.5 Feynman diagrams for t decay at tree level. 15 2.6 Radiative corrections to electroweak boson mass terms. 20 2.7 Radiative correction to the Z0 → b¯b partial decay width. 20 2.8 Sensitivity of electroweak observables to mt and mH . 21 2.9 Important diagrams for “single top” production at the Tevatron. 24 2.10 Top width dependence of the differential cross section for gluon radia- tion from tt¯ final states in e+e− → ttX¯ .................. 30 2.11 Feynman diagrams representing the leading contributions to gluino decay in “split” supersymmetry models. ..