Search for Dark Matter Coupled to the Higgs Boson at the Large Hadron Collider Jue Chen Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2020 c 2020 Jue Chen All Rights Reserved ABSTRACT Search for Dark Matter Coupled to the Higgs Boson at the Large Hadron Collider Jue Chen This work presents the search for Dark Matter particles associated with the Higgs Boson decaying into a b¯b quark pair. The dark matter search result is based on proton-proton collision data collected at a center-of-mass energy of 13 TeV by the ATLAS detector during Run II. The results are interpreted in the context of a simplified model (Z'-2HDM) which describes the interaction of dark matter and standard model particles via new heavy mediator particles. The new powerful Higgs tagging techniques, which exploit the jet substructure and heavy flavor information to a large extent, are developed to improve the search sensitivity of the search. The target physics signals are signature with an optimized search region and interpreted with background estimation result statistically. Table of Contents I Introduction1 1 Introduction 2 II The standard model and Dark Matter4 2 The Standard Model5 2.1 Introduction.........................................5 2.2 Particles...........................................6 2.3 Interactions.........................................7 2.4 Gauge theory........................................8 2.5 The formation of the Lagrangian of the Standard Model................8 2.6 Spontaneous Symmetry Breaking and the Higgs mechanism.............. 10 2.7 Challenges.......................................... 11 3 Dark matter 13 3.1 Dark matter: From astrophysics to particle physics................... 13 3.2 Weakly Interacting Massive Particles........................... 15 3.3 Searches for dark matter in collider experiments..................... 16 3.3.1 Effective field theory................................ 16 3.3.2 The simplified model................................ 18 3.3.3 Z0 and Two-Higgs-doublet models........................ 18 i III The LHC and ATLAS experiment 22 4 The LHC 23 4.1 The LHC performance................................... 24 4.1.1 Beam energy.................................... 24 4.1.2 Luminosity..................................... 24 4.2 The LHC operation..................................... 26 5 The ATLAS experiment 28 5.1 Inner detector........................................ 29 5.1.1 Pixel detector.................................... 32 5.1.2 Semiconductor Tracker (SCT)........................... 33 5.1.3 Transition Radiation Tracker (TRT)....................... 33 5.2 Calorimeter......................................... 33 5.2.1 Liquid Argon Calorimeter............................. 35 5.2.2 Tile Calorimeter.................................. 36 5.3 Muon Spectrometer (MS)................................. 36 5.4 Forward Detectors..................................... 39 5.5 Trigger and Data Acquisition............................... 41 IV ATLAS Physics Objects and Event Reconstruction 43 6 Physics Objects 44 6.1 Inner Detector Tracks and Primary Vertex........................ 44 6.2 Electrons.......................................... 45 6.3 Muons............................................ 46 6.4 Taus............................................. 47 6.5 Jets............................................. 48 6.5.1 Calorimeter jets.................................. 48 6.5.2 Track jets...................................... 51 6.5.3 Ghost Association................................. 52 ii 6.5.4 Flavor tagging................................... 52 miss 6.6 Missing Transverse Momentum (ET ) and Missing Transverse Momentum Significance 54 6.7 Higgs tagging........................................ 56 6.7.1 Higgs tagging with advanced subjets....................... 56 7 Double-b tagger 60 7.1 Introduction......................................... 60 7.2 Monte Carlo Simulation Samples............................. 61 7.3 Double-b tagger algorithm................................. 62 7.3.1 Input features.................................... 62 7.4 Neural network architecture................................ 63 7.5 Signal efficiency and Background mistag rate measurement.............. 64 V Search for Dark Matter Coupled to the Higgs Boson 67 8 Data and MC samples 68 8.1 Data samples........................................ 68 8.2 MC samples......................................... 68 8.2.1 2HDM+a and Z'-2HDM signal.......................... 68 8.2.2 V+jets........................................ 69 8.2.3 tt¯ ........................................... 70 8.2.4 Single top...................................... 70 8.2.5 Diboson....................................... 71 8.2.6 SM V h(b¯b)..................................... 71 8.2.7 tt¯+ Z=H ...................................... 71 8.3 Trigger............................................ 72 9 Signal selection 77 9.1 Physics object definition.................................. 77 9.1.1 Leptons....................................... 77 9.1.2 Jets......................................... 78 iii 9.1.3 Missing transverse momentum.......................... 79 9.2 Signal regions........................................ 79 9.2.1 Common event selection.............................. 80 9.2.2 Resolved region................................... 82 9.2.3 Merged region................................... 83 9.3 Control regions....................................... 84 9.3.1 1-lepton control region............................... 85 9.3.2 2-lepton control region............................... 85 10 Systematic uncertainties 88 10.1 Experimental systematic uncertainties.......................... 88 10.2 Theoretical systematic uncertainties........................... 88 10.3 Data-MC comparison.................................... 91 11 Statistical analysis 101 11.1 CLs method......................................... 101 11.1.1 Likelihood definition................................ 102 11.1.2 Nuisance parameters................................ 102 11.2 Pre-fit results........................................ 108 11.2.1 0-lepton (0L).................................... 108 11.2.2 1-lepton (1L).................................... 109 11.2.3 2-lepton (2L).................................... 110 11.3 Post-fit results....................................... 112 11.3.1 0-lepton (0L).................................... 113 11.3.2 1-lepton (1L).................................... 114 11.3.3 2-lepton (2L).................................... 115 11.4 Limit setting result..................................... 116 VI Conclusions 118 12 Conclusions 119 iv VII Appendices 120 A CombinedXbbScore Tagger implementation 121 A.1 Performance......................................... 121 A.2 Signal selection in Merged region using CombinedXbbScore.............. 122 A.2.1 Backgrounds breakdown of signal region and control regions.......... 123 A.2.2 Preliminary significance study........................... 127 VIII Bibliography 129 Bibliography 130 v List of Figures 2.1 Particles of the Standard Model of particle physics...................6 3.1 The rotation curve for spiral galaxy NGC 3198. The blue diamonds are the obser- vations and reveal the flat-like nature of the curve in the outer regions of the galaxy. The dashed green line is the curve for Newtonian gravity. It shows that the rota- tional velocity should decrease with distance from the galaxy center. The solid red line through the data points is the curve obtained by assuming a simple Gaussian energy scale variation and a simple Gaussian density distribution for the galaxy... 14 3.2 An illustration of effective field theory........................... 17 3.3 Feynman diagram of the interaction between Standard Model quarks and a Z0 boson. 19 3.4 The Feynman diagram of the interaction between the Standard Model quarks and Z0 boson........................................... 20 3.5 The Feynman diagram of b¯b decayed Higgs channel in Z0 + 2HDM.......... 20 4.1 The LHC full injection chain................................ 24 4.2 Cumulative luminosity versus time delivered to ATLAS (green), recorded by ATLAS (yellow) during stable beams for pp collisions at 13 TeV centre-of-mass energy in 2015-2018........................................... 25 5.1 Cut-away view of the ATLAS detector.......................... 29 5.2 An image of the ATLAS detector, showing the different layers and the interactions of different particle types through the layers....................... 30 5.3 Cut-away view of the ATLAS Inner Detector....................... 31 vi 5.4 A schematic view of the active region of the Pixel detector consisting of barrel and end-cap layers (5.4a) and the IBL detector before the insertion (5.4b)........ 32 5.5 Cut-away view of ATLAS calorimeter system...................... 34 5.6 Accordion structure of the barrel. The left figure is a view of a small sector of the barrel calorimeter in a plane transverse to the LHC beams. The right figure is a diagram of a sketch of barrel module showing both accordion structure and granularity in η φ of the cells on each of layer..................... 35 − 5.7 Illustration of a Tile calorimeter module.......................... 37 5.8 Geometric layout of muon sub-detectors in barrel (5.8a) and end-cap (5.8b) region.. 38 5.9 Cut-away view of the ATLAS Inner Detector....................... 39 5.10 b............................................... 40 5.11 Cross sections of physics