Abstract Utilizing Electrons in the Search for Associated Higgs Boson Production with the ATLAS Detector: Higgs decaying to a tau pair and vector boson decaying leptonically Savannah J. Thais 2019 The Higgsp boson was discovered by the ATLAS and CMS collaborations in 2012 using data from s=8TeV proton-proton collisions at the LHC. Since the initial discovery of the H ! 4l and H ! γγ decays, multiple other Higgs analyses of production modes and decay channels have reached discovery significance. This thesis describes the ongoing search for the still unobserved vector boson (V = W ;Z) associatedp Higgs production with the Higgs decaying to a tau lepton pair using 139 fb−1 of s=13 TeV proton-proton colli- sion data collected by the ATLAS detector during Run-2. This analysis requires the vector boson to decay leptonically in order to reduce background contributions from hadronic activity in the detector and this thesis focuses primarily on the usage of final state electrons in the analysis. The primary backgrounds in all analysis categories are misidentified (or ‘faked’) objects; these contributions are estimated using a data-driven technique which relies on Machine Learning (ML) for object identification and reconstruction. ML is used broadly in High Energy Physics analyses and this work is introduced, with a focus on techniques for improv- ing electron identification through image processing. CERN-THESIS-2019-105 14/06/2019 i Utilizing Electrons in the Search for Associated Higgs Boson Production with the ATLAS Detector: Higgs decaying to a tau pair and vector boson decaying leptonically a dissertation presented to the faculty of the graduate school of Yale University in candidacy for the degree of doctor of philosophy by Savannah J. Thais dissertation director: Professor Sarah Demers June 2019 iii ©2019 – Savannah J. Thais all rights reserved. Contents Listing of figures xiv Listing of tables xvi Acknowledgements xviii 0 Introduction 1 1 Theory Overview 3 1.1 The Standard Model ................................ 4 1.1.1 Particles .................................. 6 1.1.2 Forces and Interactions .......................... 7 1.2 Mathematical Formalism .............................. 8 1.2.1 Electroweak ................................ 9 1.2.1.1 QED ............................. 9 1.2.1.2 The Weak Interaction .................... 10 1.2.1.3 Electroweak Unification ................... 11 1.2.2 QCD ................................... 12 1.3 The Higgs Boson .................................. 13 1.3.1 Electroweak Symmetry Breaking ..................... 13 1.3.2 Yukawa Coupling ............................. 16 1.3.3 Higgs Production Methods ........................ 17 1.3.3.1 Gluon-Gluon Fusion ..................... 17 1.3.3.2 Vector Boson Fusion ..................... 18 1.3.3.3 Associated Production with a Vector Boson . 19 1.3.3.4 Top Fusion .......................... 19 1.3.4 Higgs Boson Decays ........................... 20 1.3.4.1 Massive Fermions ....................... 20 1.3.4.2 Massive Bosons ........................ 22 1.3.4.3 Massless Particles ....................... 22 2 The LHC and the ATLAS Detector 23 2.1 Particle Colliders .................................. 24 2.1.1 The Large Hadron Collider ........................ 26 2.1.2 The LHC Accelerator Complex ...................... 28 v 2.2 Particle Detection ................................. 30 2.2.1 The ATLAS Detector ........................... 31 2.2.1.1 Inner Detector ........................ 33 2.2.1.2 Calorimeters ......................... 35 2.2.1.3 Muon Spectrometer ..................... 40 2.2.1.4 Magnet System ........................ 42 3 ATLAS Computing and Software 44 3.1 Data Acquisition and Storage ........................... 45 3.1.1 Triggers and DAQ ............................ 46 3.1.1.1 Trigger Menus ........................ 48 3.1.2 Storage .................................. 50 3.2 Simulation ..................................... 51 3.2.1 Event Generation ............................. 52 3.2.2 Detector Simulation ........................... 53 3.3 Data Processing .................................. 55 3.3.1 Reconstruction .............................. 56 3.3.1.1 Tracking ........................... 56 3.3.1.2 Vertexing ........................... 58 3.3.1.3 Calorimeter Clustering .................... 60 3.3.2 Identification ............................... 62 3.3.2.1 Muons ............................ 62 3.3.2.2 Jets .............................. 63 3.3.2.3 Tau Leptons ......................... 67 3.3.2.4 Missing Transverse Energy . 68 4 Machine Learning 71 4.1 Central Concepts .................................. 72 4.2 Algorithms ..................................... 74 4.2.1 Boosted Decision Trees .......................... 74 4.2.2 Neural Networks ............................. 76 4.2.2.1 Convolutional NNs ..................... 79 4.2.2.2 Recurrent NNs ........................ 81 4.2.2.3 Adversarial Networks ..................... 83 4.3 Applications in LHC Physics ............................ 84 4.3.1 Reconstruction, Identification, and Calibration . 85 4.3.1.1 BDTs and Standard NNs ................... 85 4.3.1.2 Images and CNNs ...................... 87 4.3.1.3 RNNs ............................ 89 4.3.2 Analyses ................................. 90 4.3.2.1 ZZd ! llll Search ...................... 91 vi 4.3.3 Simulation ................................ 91 4.3.4 Systematics ................................ 93 5 Electrons in ATLAS 96 5.1 Electron Identification (ID) Software ........................ 97 5.1.1 The Likelihood Function ......................... 98 5.1.1.1 Electron Likelihood . 100 5.1.1.2 Operating Points . 103 5.1.2 Probability Density Functions (PDFs) . 105 5.1.2.1 Tag & Probe Method . 105 5.1.2.2 Smoothing PDFs . 106 5.1.2.3 Data vs Monte Carlo PDFs . 107 5.1.2.4 Correcting PDFs . 109 5.2 Low pT Electron ID ................................ 110 5.2.1 Motivation ................................ 110 5.2.2 Software Development . 112 5.2.3 Variable Optimization . 113 5.3 Machine Learning and Electron ID . 116 5.3.1 Future Studies .............................. 120 6 VH, H ! ττ Analysis 122 6.1 Motivation ..................................... 123 6.2 Definitions ..................................... 123 6.2.1 Object Criteria .............................. 125 6.2.1.1 Taus ............................. 125 6.2.1.2 Muons ............................ 125 6.2.1.3 Electrons . 125 6.2.1.4 Jets .............................. 127 6.2.1.5 MET ............................. 127 6.2.2 Overlap Removal ............................. 128 6.2.3 Mass Reconstruction . 128 6.2.3.1 Missing Mass Calculator . 128 6.2.3.2 Late Transverse Projected Mass . 130 6.3 Analysis Categories ................................. 131 6.3.1 W H; τlepτhad .............................. 133 6.3.2 W H; τhadτhad .............................. 134 6.3.3 ZH; τlepτhad ............................... 134 6.3.4 ZH; τhadτhad .............................. 135 6.4 Run-1 Results ................................... 135 vii 7 Background Estimation in VH, H ! ττ Analysis 138 7.1 Background Types ................................. 139 7.1.1 Irreducible Backgrounds . 140 7.1.2 Fake Backgrounds ............................. 140 7.2 The Fake Factor Method .............................. 141 7.2.1 Method Derivation ............................ 141 7.2.1.1 Two Object Case . 142 7.2.1.2 Three and Four Object Cases . 146 7.3 Fake Factor Measurements ............................. 147 7.3.1 Fake Region ............................... 147 7.3.2 Fake Candidate Selection . 148 7.3.3 Fake Electrons .............................. 148 7.3.3.1 Type and Origin Studies . 151 7.3.4 Fake Taus and Muons . 155 7.3.4.1 Fake Taus . 155 7.3.4.2 Fake Muons . 157 7.3.5 MC Corrections ............................. 157 7.4 Fake Factor Software ................................ 158 7.4.1 Closure Tests ............................... 159 7.4.1.1 WH hadhad Channel . 159 7.4.1.1.1 Pre-selection . 159 7.4.1.1.2 Z ! ττ Control Region . 161 8 Conclusions 164 8.1 V H; H ! ττ Analysis .............................. 165 8.1.1 Analysis Status .............................. 165 8.1.2 Expected Events .............................. 166 8.1.3 Machine Learning in the Analysis . 167 8.2 Final Remarks ................................... 168 Appendix A Author’s Individual Contributions 169 A.1 Electron ID ..................................... 169 A.1.1 Related Presentations . 170 A.2 V H; H ! ττ Analysis .............................. 171 A.2.1 Related Presentations . 171 A.3 Machine Learning ................................. 172 A.3.1 Related Presentations . 172 A.4 Outreach ...................................... 173 A.4.1 Related Presentations . 173 References 186 viii Listing of figures 1.1 Summary of several SM production cross section measurements, corrected for leptonic branching fractions, and compared to the corresponding theoretical predictions [6]. 4 1.2 Table formation of the Standard Model which includes the mass, charge, and spin of all known SM particles. ............................... 5 1.3 The SM interaction vertices. α describes the relative strength of the force as it is the dimensionless transformation of the coupling constant g [2]. ........... 8 1.4 An illustraion of the Higgs potential in the case that µ2 < 0 [20]. 14 1.5 Dominant SM Higgs production cross-sections as a function of Higgs mass [21]. 17 1.6 Feynman diagram of gluon-gluon fusion Higgs boson production with a top or bot- tom quark loop ..................................