ATLAS Measurements of the Higgs Boson Coupling to the Top Quark in the Higgs to Diphoton Decay Channel (Doctoral Thesis accepted by University of California, Berkeley, USA)

During Run 2 of the Large Hadron Collider, the ATLAS experiment recorded proton-proton collision events at 13 TeV, the highest energy ever achieved in a collider. Analysis of this dataset has provided new opportunities for precision measurements of the Higgs boson, including its interaction with the top quark. The Higgs-top coupling can be directly probed through the production of a Higgs boson in association with a top-antitop quark pair (ttH). The Higgs to diphoton decay channel is among the most sensitive for ttH measurements due to the excellent diphoton mass resolution of the ATLAS detector and the clean signature of this decay. Event selection criteria were developed using novel Machine Learning techniques to target ttH events, yielding a precise measurement of the ttH cross section in the diphoton channel and a 6.3 $\sigma$ observation of the ttH process in combination with other decay channels, as well as stringent limits on CP violation in the Higgs-top coupling. Supervisor’s Foreword Abstract Acknowledgments Contents 1 Introduction References 2 Phenomenology 2.1 Theory of Strong Interactions 2.1.1 Factorization of Proton–Proton Collisions 2.2 Theory of Electroweak Interactions 2.2.1 The Higgs Mechanism 2.2.2 Fermion Masses and Interactions 2.3 Higgs Boson Physics 2.3.1 Production Mechanisms 2.3.2 Decay Channels 2.3.3 Coupling Measurement Framework 2.4 Beyond the Standard Model Higgs 2.4.1 CP Violation in the Higgs Sector References 3 The ATLAS Experiment 3.1 The Large Hadron Collider 3.1.1 Luminosity 3.2 The ATLAS Detector 3.2.1 Inner Detector 3.2.2 Calorimeters 3.2.3 Muon Spectrometer 3.2.4 Trigger References 4 Object Reconstruction 4.1 Tracks and Vertices 4.2 Photons 4.2.1 Photon Identification 4.2.2 Photon Isolation 4.2.3 Primary Vertex Selection in Diphoton Events 4.3 Electrons 4.3.1 Electron Identification 4.3.2 Electron Isolation 4.4 Muons 4.4.1 Muon Identification 4.4.2 Muon Isolation 4.5 Hadronic Jets 4.5.1 Flavor Tagging 4.6 Missing Transverse Energy References 5 Data and Monte Carlo Samples 5.1 Data: LHC Run 2 5.1.1 Data Control Samples 5.2 Simulated Samples 5.2.1 SM Higgs Boson Samples 5.2.2 BSM Higgs Samples 5.2.3 Background Samples References 6 Statistical Model 6.1 Motivation for Categorization 6.1.1 A Short Proof 6.2 Signal and Background Shapes 6.2.1 Signal Model 6.2.2 Background Model 6.3 Likelihood Model 6.3.1 Treatment of Systematics 6.3.2 Generation of Asimov Datasets 6.4 Sources of Systematic Uncertainty 6.4.1 Theoretical Uncertainties 6.4.2 Experimental Uncertainties 6.5 Weighting Data for Presentation References 7 Machine Learning Techniques 7.1 Multivariate Analysis 7.1.1 Classification 7.2 Boosted Decision Trees 7.2.1 Boosting 7.2.2 Multiclass BDT 7.3 Hyper-Parameter Optimization 7.3.1 Grid Scan 7.3.2 Gaussian Processes Minimization References 8 Reconstruction of Top Decays 8.1 Reconstruction Strategy 8.1.1 Hadronic Channel 8.1.2 Leptonic Channel 8.2 Training of the Top Reco BDT 8.2.1 Constructing Signal and Background Samples 8.2.2 Training Variables 8.3 Performance 8.3.1 Hadronic Channel 8.3.2 Leptonic Channel References 9 Selection of tbartH(γγ) Events 9.1 Poisson Number-Counting Significance 9.2 Categorization of Events 9.2.1 Hadronic Channel 9.2.2 Leptonic Channel 9.3 Decomposition of Continuum Background 9.3.1 Hadronic Channel 9.3.2 Leptonic Channel References 10 First Observation of tbartH Production 10.1 Combination of Higgs Decay Channels 10.1.1 Input Analyses 10.1.2 Systematic Uncertainties 10.1.3 Results of Combined Analysis 10.2 tbartH in the Diphoton Decay Channel 10.2.1 Signal and Background Shapes 10.2.2 Systematic Uncertainties 10.2.3 Results References 11 CP-Sensitive Categorization of tbartH(γγ) Events 11.1 Number-Counting Limit on CP Mixing 11.2 Sensitive Observables 11.3 Multivariate Categorization Strategy 11.3.1 Hadronic Channel 11.3.2 Leptonic Channel 11.4 Determination of Category Boundaries References 12 Measurement of CP Properties in tbartH and tH 12.1 Treatment of Loop Processes 12.1.1 Kinematics of Gluon Fusion 12.1.2 Effective Loop Couplings 12.2 Signal and Background Model 12.2.1 Signal and Background Shapes 12.2.2 Signal Yield Dependence on Mixing Angle 12.3 Systematic Uncertainties 12.4 Results 12.4.1 Sensitivity to SM tbartH and tH 12.4.2 Constraints on CP Mixing References 13 Higgs Couplings in Hrightarrowγγ 13.1 Event Categorization 13.1.1 Leptonic VH 13.1.2 Vector Boson Fusion and Hadronic VH 13.1.3 Gluon Fusion 13.2 Signal and Background 13.3 Systematic Uncertainties 13.4 Results (79.8 fb-1) 13.4.1 Higgs Cross Section by Production Mode 13.4.2 Measurement of STXS References 14 Conclusions References Appendix A Search for TeV-Scale Gravity Appendix B Multijet Modeling with MG5_aMC@NLO Appendix C Radiation Damage in the IBL C.1 Single Event Upset C.2 Measurement of Leakage Current