A Thesis Submitted for the Degree of PhD at the University of Warwick Permanent WRAP URL: http://wrap.warwick.ac.uk/134997 Copyright and reuse: This thesis is made available online and is protected by original copyright. Please scroll down to view the document itself. Please refer to the repository record for this item for information to help you to cite it. Our policy information is available from the repository home page. For more information, please contact the WRAP Team at: [email protected] warwick.ac.uk/lib-publications Search for heavy neutral MSSM Higgs bosons in the di-tau final state with the ATLAS detector by Adomas Jelinskas Thesis Submitted to the University of Warwick for the degree of Doctor of Philosophy Department of Physics January 2019 Contents List of Tables iv List of Figures vi Acknowledgments xii Declarations xiii Abstract xiv Chapter 1 Theoretical Description of Particle Physics 1 1.1 Standard Model Particles . .2 1.2 Standard Model Structure . .8 1.2.1 Local Gauge Symmetry . 10 1.2.2 Asymptotic Freedom . 13 1.3 Brout-Englert-Higgs Mechanism . 14 1.4 Fermion masses and the CKM matrix . 17 1.5 Proton-Proton Collisions . 19 1.6 Supersymmetry . 21 1.6.1 The Fine-Tuning Problem . 23 1.6.2 Supersymmetry . 25 1.7 Minimal Supersymmetric Standard Model . 28 1.7.1 Soft SUSY Breaking . 31 1.7.2 MSSM Higgs Sector . 31 1.8 MSSM Benchmark Scenarios . 32 1.9 Searches for MSSM Higgs Bosons . 36 i Chapter 2 ATLAS Experiment and the Large Hadron Collider 39 2.1 Large Hadron Collider . 39 2.2 ATLAS Detector . 42 2.2.1 ATLAS Coordinate System . 43 2.2.2 Inner Detector . 44 2.2.3 Calorimeter . 47 2.2.4 Muon Spectrometer . 50 2.3 ATLAS Trigger System . 50 2.4 Luminosity in ATLAS . 54 2.5 ATLAS Operation in Run 2 . 56 Chapter 3 Physics Objects 59 3.1 Tracks and Primary Vertices . 59 3.2 Electrons and Photons . 60 3.3 Muons . 68 3.4 Jets . 71 3.5 Missing Transverse Energy . 77 3.6 Taus . 79 Chapter 4 Search For Heavy Neutral MSSM Higgs Bosons 86 4.1 Event Selection . 86 4.1.1 Event Triggering . 87 4.1.2 Data Quality . 88 4.1.3 Event Pre-selection . 88 4.1.4 Event Selection and Categorization . 90 4.1.5 Di-tau Mass Reconstruction . 91 4.2 Monte Carlo Simulation . 93 4.2.1 Background Samples . 93 4.2.2 Signal Samples . 96 4.3 Background Model . 97 4.4 Fake Factor Method . 99 4.5 QCD Multi-jet Background . 103 4.5.1 Fake Lepton Control Region . 103 4.6 Background With a Correctly Identified Lepton and a Jet Mis-identified as τhad ................................... 106 4.6.1 W+jets Control Region . 107 ii 4.6.2 Same Sign Top Control Region . 112 4.7 Validation Region . 114 4.8 Signal and Top Control Regions . 120 4.9 Statistical Analysis . 125 4.9.1 Fitting procedure . 125 4.9.2 Statistical Interpretation . 126 4.9.3 Exclusion Limits . 128 4.10 Systematic Uncertainties . 129 4.10.1 Experimental Uncertainties . 130 4.10.2 Theoretical Uncertainties . 134 4.11 Results . 135 4.12 Conclusions . 142 Appendix A Lepton Fake Factors 145 Appendix B Nuisance Parameters 147 iii List of Tables 1.1 Representations of fermions under the SM gauge symmetry groups.9 1.2 List of chiral supermultiplets in the MSSM [31]. 30 1.3 List of vector supermultiplets in the MSSM [31]. 30 2.1 List of notable run conditions in ATLAS during Run 2. 58 3.1 List of variables used in the tau identification algorithm. Adapted from [96] . 84 4.1 List of object pre-selection requirements. 89 4.2 List of object selection requirements in the Signal Region. 92 4.3 A summary of Monte Carlo generators and PDF sets used in event simulation. The second label under the MC generator, if present, corresponds to the parton shower model. ME is the acronym of matrix element. 97 4.4 The amount of gluon-gluon fusion (ggH) signal events in the Signal and Top Control Regions for different mass points of the resonance. The signal is normalized to cross-section of 1 pb and only statistical uncertainties are shown. 123 4.5 The amount of b-associated production (bbH) signal events in the Signal and Top Control Regions for different mass points of the res- onance. The signal is normalized to cross-section of 1 pb and only statistical uncertainties are shown. 124 iv 4.6 Event yields in the Signal Region after the fitting procedure. Event yields are listed for the electron and muon channels b-veto and b-tag categories. The uncertainties include statistical and systematic un- certainties. The uncertainties of the individual background com- ponents do not add up to the total uncertainty because they are correlated. 139 4.7 Event yields in the Top Control Region after the fitting procedure. Event yields are listed for the electron and muon channels. The uncertainties include statistical and systematic uncertainties. The uncertainties of the individual background components do not add up to the total uncertainty because they are correlated. 140 v List of Figures 1.1 Particles of the Standard Model arranged by spin, mass, generation and electric charge [10]. .3 1.2 Interactions between particles in the SM [10]. .5 1.3 Parton distribution functions from the MSTW group [29]. The plots show the distributions for valence quarks, sea-quarks, and gluons, and the evolution of distributions from the interaction scale of (left) Q2 = 10 GeV2 to (right) Q2 = 104 GeV2................. 20 1.4 Cross sections of various processes in proton-(anti)proton collisions as a function of the center-of-mass energy [30]. Line discontinuities appear due to differences between proton{anti-proton (applicable at the Tevatron) and proton{proton (applicable at the LHC) collisions. 22 1.5 One-loop radiative corrections to the Higgs mass due to (left) a fermion and (right) a scalar. 24 1.6 Summary plot of ATLAS searches for additional Higgs bosons inter- preted in the hMSSM parameter space [44]. 36 1.7 Leading-order Feynman diagrams for the production of neutral Higgs bosons in pp collisions. The (left) gluon-gluon fusion dominates at low and moderate values of tan β, while b-associated production mode in (middle) four-flavor and (right) five-flavor schemes become significant at high values of tan β [2]. 37 1.8 Leading-order Feynman diagrams for the production of charged Higgs bosons in pp collisions. The (left) non-resonant and (middle) single- resonant top-quark productions dominate at large H+ masses, while (right) double-resonant top-quark production dominates at low H+ masses. The interference between these three main diagrams be- comes most relevant in the intermediate mass region [46]. 37 vi 2.1 CERN accelerator complex serving the LHC. The path of protons starts at LINAC 2, travel through PSB, PS and SPS, and finally reach the LHC [66]. 41 2.2 ATLAS detector in its entirety. The pixel detector, SCT tracker and TRT tracker make the inner detector which is surrounded by the solenoid magnet. Muon detectors and toroid magnets make the muon spectrometer. The entire calorimeter comprise tile and liquid argon calorimeters [59]. 43 2.3 Cut-away view of the ATLAS inner detector [59]. 45 2.4 Schematic view of the quarter-section of the ATLAS inner detector [59]. The Insertable B-layer is not shown but fits inside the pixel detector; the beam-pipe radius is reduced to 30 mm. 46 2.5 The structure of the ATLAS calorimeter [59]. 48 2.6 The ATLAS muon spectrometer and its components, located outside the ATLAS calorimeter [59]. 51 2.7 Schematic view of the trigger towers used as input to the L1Calo trigger algorithms [76]. 53 2.8 Cumulative distribution of the total integrated luminosity in AT- LAS during Run 2. Shown are the distributions of the LHC deliv- ered luminosity, ATLAS recorded luminosity, and luminosity of good quality data [83]. 57 2.9 Distributions of the average number of interactions per bunch cross- ing in ATLAS during Run 2 [83]. 57 3.1 (left) Electron and (right) background identification efficiencies for the three working points as a function on the transverse energy [84]. 63 3.2 Muon reconstruction and identification efficiency for the \medium" working point as a function of the (left) muon pT and (right) muon η [87]. 71 3.3 (left) The b-tagging efficiency of the 70% working point measured in data and simulation and (right) data-to-simulation scale factors [94]. 77 3.4 Tau lepton reconstruction performance presented as (left) the num- ber of associated core tracks in the reconstruction of hadronically decaying tau leptons for truth 1-prong and 3-prong tau decays, and (right) reconstruction efficiency as a function of the τhad-vis pT [96]. 81 vii 3.5 Tau energy resolution with the baseline and with the BRT calibra- tions applied [98]. 82 3.6 Distributions of the central energy fraction, (fcent), (left) for all tau candidates and (right) for tau candidates passing the \medium" tau identification working point. The distributions are obtained from a tag-and-probe analysis using 2015 data set [98]. 85 4.1 Leading order Feynman diagrams of the most relevant single-top pro- duction modes at the LHC. From left to right: t-channel, s-channel, W t-channel. 94 4.2 Leading order Feynman diagrams for (a) QCD multi-jet and (b) W+jets processes. 102 4.3 Electron pT distributions in the b-veto category of the FLCR.