Discovery of Single Top Quark Production (eBook)

Discovery of Single Top Quark Production 3
Supervisor’s Foreword 5
Preface 7
Acknowledgments 8
Contents 9
1 Introduction 13
References 14
2 Theoretical Background 15
2.1…The Standard Model 15
2.1.1 Matter Particles 15
2.1.2 Particle Interactions 15
2.1.3 Gauge Theories 16
2.2…The Top Quark 17
2.2.1 Discovery 17
2.2.2 Properties 17
2.2.3 Decay 18
2.3…Electroweak Top Quark Production 18
2.3.1 Introduction and Motivation 18
2.3.2 Production Modes 19
2.3.3 Measurement of |V_{tb}| 20
2.3.4 Single Top Kinematics 21
2.3.5 Polarization 21
2.3.6 New Physics 23
References 24
3 Experimental Setup 25
3.1…The Accelerator Chain 25
3.1.1 Luminosity and Cross Sections 26
3.2…The DØ Detector 27
3.2.1 The DØ Coordinate System 27
3.2.2 The Central Tracking 29
3.2.3 Silicon Microstrip Tracker 30
3.2.4 Central Fibre Tracker 31
3.2.5 Preshower Detectors 31
3.2.6 The DØ Calorimeters 31
3.2.7 Muon System 34
3.2.8 Triggers 34
References 35
4 Event Reconstruction 36
4.1…Tracks 36
4.2…Primary Vertices 37
4.3…Calorimeter Clusters 37
4.4…Electrons 38
4.5…Jets 39
4.6…Muons 42
4.7…b Jets 42
4.8…Missing Transverse Energy, {/not/!{/!E}_T} 43
References 44
5 Analysis: Event Selection 45
5.1…Strategy 45
5.2…Data Set 46
5.3…Background Processes 46
5.4…Signal and Background Modeling 48
5.4.1 Monte Carlo Simulation 48
5.4.2 Monte Carlo Signal Samples 48
5.4.3 Monte Carlo Background Samples 49
5.4.4 Monte Carlo Corrections 52
5.4.4.1 Primary Vertex Position 52
5.4.4.2 Instantaneous Luminosity Reweighting 52
5.4.4.3 Z p_{T} Reweighting 53
5.4.4.4 Electron Identification Efficiencies 53
5.4.4.5 Muon Efficiency Correction 53
5.4.4.6 Jet Corrections 54
5.4.4.7 b Jet Identification Corrections 54
5.4.4.8 W+jets Reweighting 55
5.4.5 Monte Carlo Sample Normalization 55
5.4.6 Multijets and W+jets Normalization 56
5.5…Event Selection Criteria 57
5.5.1 General Selection 57
5.5.2 {/usertwo b}-Tagging Selection 58
5.5.3 Electron Channel Selection 58
5.5.4 Muon Channel Selection 58
5.5.5 Multijet Reduction Criteria 58
5.6…Event Yields 60
5.7…Data-Background Model Comparison 62
5.8…Systematic Uncertainties 63
References 70
6 Analysis: Decision Trees 71
6.1…Motivation 71
6.2…Overview of Decision Trees 72
6.2.1 History and Usage 72
6.2.2 What is a Decision Tree? 72
6.2.3 Advantages and Limitations 73
6.3…Growing a Tree 74
6.3.1 Node Splitting 75
6.3.2 Impurities 76
6.4…Pruning the Tree 77
6.4.1 Pre-Pruning 78
6.4.2 Post-Pruning 78
6.4.2.1 Reduced Error Pruning 78
6.4.2.2 Cost Complexity Pruning 78
6.5…Forests of Decision Trees 79
6.5.1 Bagging 79
6.5.2 Random Forest 80
6.5.3 Boosting 80
6.6…Decision Tree Options 81
6.7…Evaluating the Performance 82
6.7.1 Cross Section Significance, {{{/cal S}}}_{/sigma} 83
6.7.2 Excess Significance, {{{/cal S}}}_{s} 83
6.8…Thoughts and Suggested Improvements 84
6.8.1 Weighted Events and Pre-Pruning 85
6.8.2 Impurity Optimization 85
6.8.3 Consideration of Systematic Uncertainties 86
6.8.4 Logging 86
References 86
7 Analysis: Measurements 88
7.1…Decision Tree Analysis 88
7.1.1 Input Samples 88
7.1.2 Discriminating Variables 89
7.1.3 Choice of Decision Tree Parameters 91
7.1.3.1 Boosting Parameters 93
7.1.3.2 Impurity Measures 93
7.1.3.3 Minimal Leaf Size 95
7.1.3.4 Pruning 96
7.1.3.5 Summary 97
7.1.4 Output Transformation 98
7.1.5 The Final Decision Trees 102
7.1.6 Cross Checks 102
7.2…Cross Section Measurement 104
7.2.1 Bayesian Analysis 105
7.2.2 Numerical Calculation 106
7.2.3 Ensemble Tests 107
7.2.4 Observed Results 108
7.3…Event Kinematics 111
7.4…Signal Significance 111
7.4.1 Combined Significance 114
7.5…Measurement of |V_{tb}| 115
References 119
Appendix A 120
A.1…Event Displays 120
Appendix B 127
B.1…Systematic Uncertainties 127
B.1.1…Systematics Affecting Normalization Only 127
B.2…Shape-Changing Systematics 134
Appendix C 139
C.1 Decision Tree Outputs 139
Appendix D 144
D.1…Cross Check Samples 144
Appendix E 147
E.1 Combined Results 147
References 148