High jet multiplicity physics at the LHC : doctoral thesis accepted by the University of Oxford, UK

This Book Describes Research In Two Different Areas Of State-of-the-art Hadron Collider Physics, Both Of Which Are Of Central Importance In The Field Of Particle Physics. The First Part Of The Book Focuses On The Search For Supersymmetric Particles Called Gluinos. The Book Subsequently Presents A Set Of Precision Measurements Of “multi-jet” Collision Events, Which Involve Large Numbers Of Newly Created Particles, And Are Among The Dominant Processes At The Large Hadron Collider (lhc). Now That A Higgs Boson Has Been Discovered At The Lhc, The Existence (or Non-existence) Of Supersymmetric Particles Is Of The Utmost Interest And Significance, Both Theoretically And Experimentally. In Addition, Multi-jet Collision Events Are An Important Background Process For A Wide Range Of Analyses, Including Searches For Supersymmetry.div Supervisor’s Foreword 8 Preface 10 Abstract 11 Acknowledgements 12 Contents 14 Introduction 17 1 Theoretical Overview 19 1.1 The Particle Content of the Standard Model 20 1.2 Dynamics of the Standard Model 22 1.2.1 Gauge Theories in the Standard Model 23 1.2.2 Renormalisation 24 1.2.3 The Description of Mass 25 1.3 Quantum Chromodynamics 26 1.3.1 Structure of a Hadronic Event 27 1.3.2 The Perturbative Regime 28 1.3.3 Non-perturbative Effects 31 1.4 Going Beyond the Standard Model 31 1.4.1 Foundations of SUSY 32 1.4.2 Implications of SUSY 35 1.4.3 Experimental Constraints 37 1.5 Conclusion and Motivation 39 References 39 2 The Large Hadron Collider and the ATLAS Experiment 43 2.1 The Large Hadron Collider 43 2.2 ATLAS Detector Overview 45 2.2.1 Coordinate System 46 2.2.2 Inner Detector 47 2.2.3 Calorimetry 48 2.2.4 Muon Spectrometer 50 2.3 Trigger 51 2.3.1 Jet Triggers 52 2.4 Detector Simulation 53 2.5 Object Reconstruction 53 2.5.1 Tracks 54 2.5.2 Electrons 54 2.5.3 Muons 54 2.5.4 Jets 55 2.5.5 b-Tagging 57 2.5.6 Missing Transverse Momentum 58 References 59 3 Search for New Phenomena in Events with Large Jet Multiplicities 61 3.1 Introduction 61 3.1.1 Overview of the Analysis 62 3.2 Analysis Strategy 64 3.2.1 Signal and Backgrounds 64 3.2.2 Dataset and Trigger 68 3.2.3 Physics Object Definitions 70 3.2.4 Kinematic Variable Definitions 71 3.2.5 Event Cleaning 72 3.2.6 Optimisation of the Signal Region Definitions 73 3.3 Estimation of the Standard Model Background 77 3.4 Estimation of the Multi-jet Background 80 3.4.1 Soft Energy Correction 82 3.4.2 The Template Method in Events with b-Jets 83 3.4.3 Sensitivity to Pile-Up Effects 85 3.5 Systematic Uncertainties of the Multi-jet Background Prediction 86 3.5.1 Closure Systematic Uncertainty 86 3.5.2 Heavy-Flavour Systematic Uncertainty 86 3.5.3 Soft Energy and Pile-Up Systematic Uncertainties 89 3.5.4 Trigger Systematic Uncertainty 91 3.5.5 Leptonic Background Subtraction 92 3.6 Estimation of the Leptonic Backgrounds 92 3.6.1 Estimation of the Leptonic tbart and W+Jets Backgrounds 93 3.6.2 Estimation of Other Minor Backgrounds 95 3.7 Systematic Uncertainties on the Leptonic Backgrounds 101 3.7.1 Experimental Systematic Uncertainties 102 3.7.2 Theoretical Systematic Uncertainties 103 3.8 Results and Interpretation 105 3.8.1 Fitting Procedure 105 3.8.2 Fit Results and Interpretation 111 3.9 Summary and Outlook 115 References 117 4 Measurement of the Cross Section of Four-Jet Events 120 4.1 Introduction 120 4.1.1 Overview of the Analysis 121 4.1.2 Useful Concepts 122 4.2 Object Definitions and Event Cleaning 123 4.2.1 Jets 123 4.2.2 Event Cleaning and General Cuts 123 4.3 Trigger 125 4.3.1 Single-Jet Triggers 126 4.3.2 Four-Jet Triggers 127 4.3.3 Combination 129 4.4 Variables of Interest 130 4.4.1 Definitions and Truth-Level Distributions 132 4.4.2 Truth- to Reconstructed-Level Correlations 139 4.5 Binning 146 4.5.1 Statistical Uncertainties and Rounding 148 4.6 Unfolding of Detector Effects 149 4.6.1 Method Description 149 4.6.2 Choosing the Nominal Monte Carlo 151 4.6.3 Shape Systematic Uncertainty 154 4.6.4 Selection Efficiency Systematic Uncertainty 157 4.6.5 Statistical and Systematic Uncertainties 158 4.6.6 Optimisation of the Unfolding Algorithm 159 4.7 Theoretical Calculations and Uncertainties 160 4.7.1 Leading-Order Predictions 163 4.7.2 Next-to-Leading-Order Predictions 164 4.7.3 Other Predictions 164 4.8 Results 165 4.9 Summary and Outlook 183 References 183 Conclusion 186 Appendix ATrigger Efficiencies 188 Appendix BDeriving Variable-Width Binnings 190 About the Author 194 Front Matter....Pages i-xxii Theoretical Overview....Pages 1-24 The Large Hadron Collider and the ATLAS Experiment....Pages 25-42 Search for New Phenomena in Events with Large Jet Multiplicities....Pages 43-101 Measurement of the Cross Section of Four-Jet Events....Pages 103-168 Back Matter....Pages 169-177