The Beauty and the Boost: A Higgs Boson Tale : Measurements of Higgs Boson Production at High Energy in Decays to Bottom Quarks and Their Interpretations with the ATLAS Experiment at the LHC

Precision measurements of the Higgs boson’s properties are a powerful tool to look for deviations from the predictions of the Standard Model (SM) of particle physics. The 139/fb of proton-proton collision data which have been collected by the ATLAS experiment during Run 2 of the LHC, offer an opportunity to investigate rare Higgs-boson topologies, which are particularly sensitive to new physics scenarios but experimentally difficult to access. Several such measurements, which target Higgs-boson decays to heavy-flavour quarks, as well as their combinations are presented in this thesis. A novel analysis that measures Higgs-boson production in association with a heavy vector boson V (VH, with V=W,Z) at high energies is presented. Dedicated Higgs-boson reconstruction techniques are applied to reconstruct the highly Lorentz-boosted Higgs-boson decays into pairs of bottom quarks. The measurement is subsequently combined with a VH cross-section measurement at low and intermediate pT(V) to provide a differential cross-section measurement in kinematic fiducial volumes over the largest possible pT(V) range. All cross-section measurements agree with the SM predictions within relative uncertainties that range from 30% to 300%. The results are furthermore interpreted as limits on the parameters of a SM effective field theory. Finally, a combination of measurements of Higgs decays to heavy-flavour quarks is used to experimentally determine that the Higgs-boson coupling to charm quarks is weaker than to bottom quarks, as predicted by the SM. The target audience for the thesis are physicists and physics students, in particular those with a background in high energy physics. Supervisors’ Foreword Acknowledgements Summary It is All About Resolution Particle Physics and the Standard Model Deviations in the Tails Picturing the Higgs Boson Reconstructing Boosted Higgs Bosons Measuring VH,H to bbarb at High pTH A Combination of VH,H to bbarb Measurements Standard Model Effective Field Theories Reaching for the Second Generation Looking Ahead Contents 1 Introduction References 2 Theoretical Foundations 2.1 The Standard Model of Particle Physics 2.1.1 Particles and Forces 2.1.2 Quantum Electrodynamics 2.1.3 Quantum Chromodynamics 2.1.4 Electroweak Unification 2.1.5 Particle Masses and the Higgs Boson 2.1.6 The Full SM Lagrangian 2.1.7 Renormalisation and Constraints on the Particle Content 2.1.8 Free Parameters of the SM 2.2 Breaking the Standard Model 2.2.1 Theoretical Issues 2.2.2 Inexplicable Observations 2.3 Effective Field Theories 2.3.1 The Standard Model EFT 2.3.2 Operator Bases 2.3.3 Operator Selection 2.3.4 The Warsaw Basis 2.3.5 Advantages and Limitations of the SMEFT Approach 2.4 Phenomenology of Proton-Proton Collisions 2.4.1 Parton-Density Functions and Factorisation 2.4.2 Event Generation 2.4.3 Cross-Sections and Luminosity References 3 Higgs Boson Physics 3.1 The Importance of the Higgs Boson 3.2 Production Modes and Decay Channels at the LHC 3.3 Simplified Higgs Template Cross-Sections (STXS) 3.4 Effective Higgs Boson Coupling Modifiers 3.5 Experimental Status of Higgs Boson Physics References 4 The ATLAS Experiment at the Large Hadron Collider 4.1 The Large Hadron Collider 4.2 The ATLAS Experiment 4.2.1 The ATLAS Coordinate System 4.2.2 Particle Identification with the ATLAS Detector 4.2.3 The Inner Detector 4.2.4 The Calorimeters 4.2.5 The Muon Spectrometer 4.2.6 Luminosity Measurements 4.2.7 Triggering and Data Acquisition References 5 Reconstruction of Physics Objects 5.1 Tracks and Vertices 5.2 Topological Calorimeter Clusters 5.3 Electrons 5.4 Photons 5.5 Muons 5.6 Jets 5.6.1 Jet Algorithms 5.6.2 Small-R Jets 5.6.3 Large-R Jets 5.6.4 Variable-R Jets 5.7 τ Leptons 5.8 Missing Transverse Energy References 6 Flavour Tagging 6.1 Properties of Jets Originating from Heavy-Flavour Quarks 6.2 Flavour-Tagging Algorithms 6.3 Measurement of the b-Tagging Efficiency in Dense Jet-Environments 6.3.1 Simulated Samples 6.3.2 Object and Event Selection 6.3.3 Dense Jet-Environments 6.3.4 The Efficiency Measurement 6.3.5 Results and Outlook References 7 Measurement of Higgs Boson Production at High Transverse Momentum in the VH, Hrightarrowbbarb Channel 7.1 Motivation 7.2 Analysis Overview 7.2.1 The Higgs Boson Reconstruction 7.2.2 The Vector Boson Reconstruction 7.2.3 The Main Backgrounds 7.2.4 Goal of the Measurement 7.2.5 A Sketch of the Event Selection and Categorisation 7.2.6 The Signal Extraction 7.3 Data and Simulated Samples 7.3.1 The Data Sample 7.3.2 The Simulated Signal and Background Samples 7.4 Event Reconstruction and Selection 7.4.1 Object Definitions 7.4.2 Event Reconstruction 7.4.3 Event Selection and Categorisation 7.5 Background Composition and Estimation 7.5.1 Top Quark Pair Production 7.5.2 Single Top Production 7.5.3 V+jets Production 7.5.4 Diboson Production 7.5.5 QCD Multijet Production 7.5.6 Contamination from Other Higgs Boson Processes 7.6 Systematic Uncertainties 7.6.1 Experimental Uncertainties 7.6.2 Luminosity and Pile-Up Modelling 7.6.3 Electrons 7.6.4 Muons 7.6.5 Small-R Jets 7.6.6 Large-R Jets 7.6.7 Flavour Tagging 7.6.8 Missing Transverse Energy 7.6.9 τ Leptons 7.6.10 Principle Component Analysis of Uncertainties 7.6.11 Modelling Uncertainties 7.7 Statistical Treatment and Inference 7.7.1 The Likelihood Function Definition 7.7.2 The Fit Model 7.8 Results 7.8.1 Validation of the Maximum Likelihood Fit 7.9 Simplified Template Cross-Section Measurement References 8 (SM) EFT Interpretation 8.1 Simulation of the SMEFT Impact 8.1.1 Parameterising the STXS Cross-Sections 8.1.2 Selection of Effective Operators 8.1.3 Technical Setup 8.2 1D and 2D Limits 8.3 Effect of the STXS Split at 400 GeV 8.4 EFT Limits in the Analysis Eigenspace 8.5 Miscellaneous Considerations 8.5.1 SMEFT Effects on Experimental Acceptance 8.5.2 SMEFT Effects on Background Processes 8.5.3 Validity of the SMEFT References 9 Combination of Measurements of Higgs Boson Decays to Heavy-Flavour Quarks 9.1 Combination of Resolved and Boosted VH, Htobbarb 9.1.1 The Resolved VH, Htobbarb Analysis 9.1.2 The Modifications to the Input Analyses and the Combination Strategy 9.1.3 Statistical Treatment and Inference 9.1.4 VZ Cross-Check Measurement 9.1.5 Measurement Results 9.1.6 Constraints on Effective Interactions 9.1.7 Inclusion in the ATLAS-Wide Combination of Higgs Boson Measurements 9.2 Combination of VH, Htobbarb and VH, Htocbarc 9.2.1 The VH, Htocbarc Analysis 9.2.2 The Signal Cross-Contamination 9.2.3 The Combination Procedure 9.2.4 Results and Interpretation 9.2.5 Inclusion in the ATLAS-Wide Combination of Higgs Boson Measurements References 10 Conclusion and Outlook Reference Appendix A Supplementary Information to the SMEFT Interpretation of the Measurement of Higgs Boson Production at High Transverse Momentum in the VH, H tobbarb Channel Reference