A Beauty-ful Boson: Search for the Higgs Boson Produced in Association with a Vector Boson and Decaying into a Pair of b-quarks Using Large-R Jets with the ATLAS Detector (Springer Theses)

The analysis described in this thesis is the search for the Higgs boson, decaying into bb pair, in the associated production with a vector boson, in the extreme Higgs boson transverse momentum region where the Higgs boson is reconstructed using the large- R jet technique. The use of the large-R jets allows to add a part of the phase space unexplored so far, which is particularly sensitive to possible new physics. The analysed data have been collected at LHC by the ATLAS detector between 2015 and 2018 at a centre-of-mass energy of √ s = 13 TeV. The same dataset has been used to perform the differential pp → ZH and pp → WH cross-section measurements used to extract the information on the Higgs couplings and to put limits on Beyond the Standard Model effects. Furthermore the analysis has been re-used to perform a cross-section measurement of the diboson ZZ and WZ processes because the diboson and the Higgs processes have a similar topology. For the first time the ZZ( bb ) and WZ ( bb ) cross-sections are measured at √ s = 13 TeV and the observed cross-section measurements are consistent with the Standard Model predictions. Supervisors’ Foreword Abstract Preface Structure of the Thesis and Personal Contribution References Acknowledgments Contents 1 Theoretical Aspects and Analysis Overview 1.1 The Standard Model 1.2 The Higgs Mechanism 1.3 The Higgs Boson at LHC 1.3.1 Higgs Boson Production Modes 1.3.2 Higgs Boson Decays 1.4 State of the Art of the VH(bbarb) Analysis 1.4.1 Overview of the VH(bbarb) Resolved Analysis 1.5 The Simplified Template Cross-Section Measurements 1.5.1 STXS Measurements in the VH(bbarb) Resolved Analysis 1.6 The Effective Field Theory 1.6.1 EFT Interpretation in the VH(bbarb) Resolved Analysis References 2 The Large Hadron Collider and the ATLAS Experiment 2.1 The LHC 2.2 The ATLAS Experiment 2.2.1 Magnet System 2.2.2 Inner Detector 2.2.3 Calorimetric System 2.2.4 Muon Spectrometer 2.2.5 Trigger and Data Acquisition References 3 Dataset and Simulated Event Samples 3.1 Dataset and Trigger 3.1.1 ETmiss Trigger 3.1.2 Single Electron Trigger 3.2 Simulated Samples 3.2.1 Monte Carlo Generators 3.3 Signal Process Simulation 3.4 Background Process Simulation 3.4.1 Vector Boson + Jets Production 3.4.2 Top Pair Production 3.4.3 Single-Top Production 3.4.4 Diboson Production References 4 Object Reconstruction 4.1 Tracks and Primary Vertex 4.2 Leptons 4.2.1 Electrons 4.2.2 Muons 4.3 Jets 4.3.1 Topological Cluster Formation and Calibration 4.3.2 Jet Reconstruction Algorithms 4.3.3 Small-R Jets Calibration 4.3.4 Large-R Jets 4.3.5 Large-R Jet Mass 4.3.6 Large-R Jets Grooming and Calibration 4.3.7 Track-Jets 4.4 Missing Transverse Momentum 4.5 Overlap Removal References 5 Event Selection and Categorization 5.1 Object Identification 5.2 Event Selections 5.2.1 Higgs Candidate Reconstruction and Selection 5.2.2 0-Lepton Channel Selection 5.2.3 1-Lepton Channel Selection 5.2.4 2-Lepton Channel Selection 5.3 Event Categorization 5.3.1 pTV Splitting 5.3.2 Signal Region Splitting 5.3.3 Control Region Definition 5.3.4 Summary of the Signal and Control Regions References 6 Systematic Uncertainties 6.1 Introduction 6.2 Experimental Uncertainties 6.3 Modelling Uncertainties 6.3.1 Truth Tagging Strategy 6.3.2 Signal Modelling 6.3.3 Top Pair Production Modelling 6.3.4 Single-Top Production Modelling 6.3.5 Vector Boson + Jets Production Modelling 6.3.6 Diboson Production Modelling 6.3.7 QCD Multi-jet Modelling References 7 Statistical Treatment 7.1 General Statistical Treatment 7.1.1 The Likelihood Function 7.1.2 Profile Likelihood Ratio and Test Statistic 7.2 Fit Input 7.3 Nuisance Parameters 7.4 Tool for the Validation of the Fit Results 7.4.1 Pull Plots 7.4.2 Correlation Matrix 7.4.3 Ranking Plot 7.4.4 Breakdown of the Uncertainties References 8 Results of the VH(bbarb) Boosted Analysis 8.1 Results of the Simultaneous Fit to VH and VZ 8.1.1 Signal Strengths and Significances 8.1.2 Post-fit mJ Distribution 8.1.3 Nuisance Parameter and Constraints 8.1.4 Correlation Matrix 8.1.5 Breakdown of Uncertainties 8.1.6 Nuisance Parameter Ranking 8.2 Results Within the STXS Framework 8.2.1 Comparison Between Truth and Reconstructed Categories 8.2.2 Cross-Section Measurements 8.2.3 Effective Field Theory Interpretations 8.3 What's Next? References 9 Cross-Section Measurements of the VZ Production 9.1 Motivation and VZ Composition 9.2 VZ Simplified Template Cross-Section Bins 9.2.1 Comparison to Reconstructed Categories 9.3 Theoretical Cross-Sections Uncertainties 9.3.1 QCD Scale Uncertainties 9.3.2 PDF Uncertainties 9.3.3 αS Uncertainties 9.3.4 Summary 9.4 Results of the Multi-PoIs Fit 9.4.1 What's Next? References 10 Conclusions Appendix A EFT Eigenvector Results in the VH(bbarb) Resolved Analysis Appendix B The Tile Calorimeter Design, Calibration Methods and the Photomultiplier Stability B.1 The Tile Calorimeter Layout B.2 Calibration Methods B.2.1 Laser Calibration System B.3 PMT Response Stability B.3.1 PMT Ageing Appendix C Efficiency Studies in the Event Selection C.1 Efficiency Studies in 0-Lepton Channel Appendix D Pull Plots