UNIVERSITY of CALIFORNIA SAN DIEGO Observation of Higgs Boson
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UNIVERSITY OF CALIFORNIA SAN DIEGO Observation of Higgs boson production in association with a top quark-antiquark pair in the diphoton decay channel A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Physics by Samuel James May Committee in charge: Professor Avraham Yagil, Chair Professor Garrison Cottrell Professor Aneesh Manohar Professor Julian McAuley Professor Frank Würthwein 2020 Copyright Samuel James May, 2020 All rights reserved. The dissertation of Samuel James May is approved, and it is acceptable in quality and form for publication on microfilm and electronically: Chair University of California San Diego 2020 iii DEDICATION To my mom and dad, who have always encouraged my curiosity and supported my education. iv EPIGRAPH Sometimes science is more art than science. —Rick Sanchez v TABLE OF CONTENTS Signature Page . iii Dedication . iv Epigraph . v Table of Contents . vi List of Figures . ix List of Tables . xiii Acknowledgements . xiv Vita . xvii Abstract of the Dissertation . xviii Chapter 1 Introduction . 1 Chapter 2 Theory . 3 2.1 Introduction . 3 2.2 Quantum Field Theory . 5 2.2.1 Classical Field Theory . 5 2.2.2 Quantum Mechanics . 6 2.2.3 The Klein-Gordon Field . 6 2.2.4 Spinor & Vector Fields . 9 2.3 The Standard Model of Particle Physics . 10 2.3.1 Gauge Fields . 11 2.3.2 Quantum Electrodynamics . 12 2.3.3 Quantum Chromodynamics . 13 2.3.4 Spontaneous Symmetry Breaking & The Higgs Mechanism . 15 2.3.5 Electroweak Interactions . 18 2.3.6 Shortcomings of the Standard Model . 21 2.3.7 The Higgs Boson . 22 Chapter 3 Physics of Proton-Proton Collisions . 24 3.1 The Parton Model . 24 3.2 Proton-Proton Collisions . 25 3.2.1 Cross Sections . 25 3.2.2 Parton Showers, Hadronization, and Jets . 28 3.2.3 Underlying Event and Pileup . 29 3.3 Monte Carlo Simulation . 30 Chapter 4 Compact Muon Solenoid . 32 vi 4.1 The Large Hadron Collider . 32 4.2 The Compact Muon Solenoid Detector . 35 4.2.1 General Design Concepts . 36 4.2.2 Solenoid . 38 4.2.3 Tracker . 40 4.2.4 Electromagnetic Calorimeter . 42 4.2.5 Hadronic Calorimeter . 44 4.2.6 Muon System . 45 4.2.7 Trigger System . 47 4.3 Acknowledgements . 49 Chapter 5 Event Reconstruction and Selection . 50 5.1 Introduction . 50 5.2 The Particle Flow Algorithm . 50 5.3 Vertex Reconstruction . 52 5.4 Photon Reconstruction . 53 5.4.1 Variable Definitions . 54 5.4.2 Energy Scale & Resolution Corrections . 56 5.4.3 Shower Shape & Isolation Corrections . 59 5.4.4 Photon Identification BDT . 59 5.4.5 Selection Criteria . 60 5.5 Jet Reconstruction . 62 5.6 Lepton Reconstruction . 65 5.7 Missing Transverse Momentum Reconstruction . 66 5.8 Acknowledgements . 66 Chapter 6 t¯tH (H gg) Analysis . 68 6.1 Introduction! . 68 6.1.1 The Top Quark Yukawa Coupling . 68 6.1.2 Landscape of t¯tH Measurements . 70 6.2 Overview of Analysis Strategy . 70 6.2.1 The H gg Decay Mode . 70 6.2.2 Estimates! of Expected Sensitvity . 72 6.3 Preselection . 73 6.4 Background Description . 73 6.4.1 Challenges of MC Description . 74 6.4.2 Data-Driven Description of Multi-jet and g + jets Backgrounds . 74 6.5 Machine Learning Algorithms . 83 6.5.1 High-Level Features . 83 6.5.2 Deep Neural Networks for gg + jets and t¯t + gg Backgrounds . 85 6.5.3 Top Tagger BDT . 91 6.5.4 BDT-bkg . 94 6.6 Event Categorization . 97 6.7 Signal & Background Models . 98 6.7.1 Signal Models . 98 6.7.2 Background Models . 99 vii 6.8 Systematic Uncertainties . 100 6.8.1 Theoretical Uncertainties . 102 6.8.2 Experimental Uncertainties . 103 6.8.3 Impact of Systematic Uncertainties . 106 6.9 Results . 106 6.9.1 Statistical Analysis . 106 6.9.2 Cross Section, Signal Strength, & Significance . 110 6.9.3 CP Measurement . 112 6.10 Acknowledgements . 113 Chapter 7 Conclusion . 116 Appendix A Plots of input features to BDT-bkg . 118 Appendix B Observed Diphoton Mass Distributions . 134 Bibliography . 137 viii LIST OF FIGURES Figure 3.1: Parton distribution functions for the proton, shown for Q2 = 10 GeV (left) and Q2 = 104 GeV (right), as calculated by the MSTW group. The width of each band indicates the 68% C.L. Taken from [102]. 26 Figure 3.2: Cross sections for typical processes of interest in pp collision experiments, shown as a function of the center-of-mass energy, ps. Taken from [20]. 27 2 Figure 3.3: The strong coupling constant as of QCD as a function of Q . Different colored lines correspond to various renormalization schemes. Taken from [62]. 28 Figure 3.4: Schematic of a hadron-hadron collision. Taken from [90]. 30 Figure 4.1: Mean number of interactions per bunch crossing recorded by the CMS detector during Run 2 of the LHC. Taken from [51]. 34 Figure 4.2: Total luminosity delivered by the LHC (blue) and total luminosity recorded by the CMS detector (yellow) during Run 2 of the LHC. Taken from [51]. 35 Figure 4.3: Schematic of the various components of the CMS detector. Taken from [33]. 36 Figure 4.4: Depiction of a transverse slice of the CMS detector, along with trajectories of particles of different types. Taken from [42]. 39 Figure 4.5: Comparison of tracker performance before and after the upgrade to the pixel detector, performed in between the 2016 and 2017 data-taking periods. Taken from [14]. 40 Figure 4.6: Schematic of the CMS tracker from a cross-sectional viewpoint. TIB, TOB, TID, and TEC represent the tracker inner barrel, tracker outer barrel, tracker inner disk, and tracker endcap components, respectively. Taken from [33]. 41 Figure 4.7: The material budget for the CMS tracker shown for both the characteristic radiation lengths of electromagnetic interactions (left) and the characteristic nuclear interaction lengths of hadronic interactions (right), with the contributions of each of the tracker subcomponents shown individually. Taken from [37]. 42 Figure 4.8: Tracking efficiency as a function of pT for muons (left), charged pions (middle), and electrons (right), shown separately for the barrel (black), transition region (blue), and endcap (red). Taken from [37]. ..