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  • UNIVERSITÀ DEGLI STUDI DI PISA Facoltà di Scienze, Matematiche, Fisiche e Naturali

    Corso di Laurea in Fisica

    Investigation of the hadronic tau substructure and its application to the study of the CP

    properties of the Higgs boson with the ATLAS experiment at CERN LHC

    Advisor Prof. Vincenzo CAVASINNI

    Candidate Francesco LUCARELLI

    Academic year 2017/2018

  • Contents

    Nomenclature vii

    Introduction 1

    1 The Standard Model of Particle Physics 5 1.1 The Principle of Gauge Invariance . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Particles and Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    1.2.1 Fermions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.2 Bosons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    1.3 Quantum Electrodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4 Quantum Chromodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5 Weak Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.6 Electroweak Unification . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.7 The Brout–Englert–Higgs Mechanism . . . . . . . . . . . . . . . . . . . . 14

    1.7.1 The Fermion Masses . . . . . . . . . . . . . . . . . . . . . . . . . 16 1.8 Higgs Boson at the LHC . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

    1.8.1 Higgs Boson Production . . . . . . . . . . . . . . . . . . . . . . . 17 1.8.2 Higgs Boson Decay . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.8.3 Higgs Boson Discovery . . . . . . . . . . . . . . . . . . . . . . . 19

    1.9 Discrete Symmetries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1.9.1 CP Violation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    1.10 CP in the Higgs Sector . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 1.10.1 General Phenomenology of the 2HDMs . . . . . . . . . . . . . . . 23 1.10.2 Physical Higgs Fields and CP-Mixing . . . . . . . . . . . . . . . . 24 1.10.3 Yukawa Lagrangian in the Neutral Higgs Sector . . . . . . . . . . . 26

    2 The ATLAS Experiment at the Large Hadron Collider 27 2.1 The Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . . . . . 27

  • iv Contents

    2.2 General Layout of the ATLAS Experiment . . . . . . . . . . . . . . . . . . 30 2.2.1 Coordinate system . . . . . . . . . . . . . . . . . . . . . . . . . . 32

    2.3 Inner Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.3.1 Pixel Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.3.2 Semiconductor Tracker . . . . . . . . . . . . . . . . . . . . . . . . 34 2.3.3 Transition Radiation Tracker . . . . . . . . . . . . . . . . . . . . . 35

    2.4 Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.4.1 Electromagnetic Calorimeter . . . . . . . . . . . . . . . . . . . . . 35 2.4.2 Hadronic Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . 38

    2.5 Central Solenoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 2.6 Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 2.7 Trigger and Data Acquisition System . . . . . . . . . . . . . . . . . . . . . 43 2.8 Reconstruction of Taus with the ATLAS Detector . . . . . . . . . . . . . . 45

    3 Hadronic Tau Substructure 47 3.1 Tau Leptons at ATLAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.2 Event Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

    3.2.1 Event Selection Criteria . . . . . . . . . . . . . . . . . . . . . . . 49 3.2.2 Signal and Background Processes . . . . . . . . . . . . . . . . . . 50 3.2.3 Monte Carlo Simulations . . . . . . . . . . . . . . . . . . . . . . . 50

    3.3 OS-SS Background Estimation Method . . . . . . . . . . . . . . . . . . . 51 3.3.1 Signal and Control Regions . . . . . . . . . . . . . . . . . . . . . 53 3.3.2 Performance of the Method . . . . . . . . . . . . . . . . . . . . . 55

    3.4 Tau Particle Flow Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.4.1 Pion Reconstruction and Identification . . . . . . . . . . . . . . . . 58 3.4.2 Decay Mode Classification . . . . . . . . . . . . . . . . . . . . . . 59 3.4.3 Tau Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 62

    3.5 Tau Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 3.6 ρ Resonance in 1p1n Decay Mode . . . . . . . . . . . . . . . . . . . . . . 66 3.7 a1 Resonance in 3p0n Decay Mode . . . . . . . . . . . . . . . . . . . . . . 69

    4 CP Scenario in Higgs Decays to Tau Leptons 71 4.1 Event Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

    4.1.1 Monte Carlo Simulations . . . . . . . . . . . . . . . . . . . . . . . 72 4.1.2 Event Selection and Categorisation . . . . . . . . . . . . . . . . . 73

    4.2 Observable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 4.2.1 Signal and Background Distributions . . . . . . . . . . . . . . . . 78

  • Contents v

    4.3 Observable Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 78 4.3.1 Impact Parameter Method . . . . . . . . . . . . . . . . . . . . . . 79 4.3.2 ρ Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 4.3.3 Combined IP-ρ Method . . . . . . . . . . . . . . . . . . . . . . . 84

    4.4 Background Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 4.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

    4.5.1 Expected sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . 92

    Conclusions 97

    Appendix A Control Region Plots and Yields 99

    Appendix B Performance of the Background Estimation Method 109

    Appendix C Tau Identification Variables 117

    Bibliography 121

  • Nomenclature

    Abbreviations and acronyms

    2HDM Two-Higgs-Doublet Model

    ALICE A Large Ion Collider Experiment

    ATLAS A Toroidal LHC Apparatus

    BDT Boosted Decision Tree

    BEH Brout–Englert–Higgs

    BSM Beyond Standard Model

    CERN Conseil Européen pour la Recherche Nucléaire (European Organization for Nuclear Research)

    CKM Cabibbo-Kobayashi-Maskawa

    CMS Compact Muon Solenoid

    CR Control Region

    CS Central Solenoid

    CSCs Cathode Strip Chambers

    DAQ Data Acquisition System

    DM Decay Mode

    EF Event Filter

    EMCal Electromagnetic calorimeter

    EM Electromagnetic

  • viii Nomenclature

    FCal Forward calorimeter

    FCNCs Flavour-Changing Neutral Currents

    ggF gluon-gluon Fusion

    HadCal Hadronic calorimeter

    HLT High-Level Trigger

    IBL Insertable B-Layer

    ID Inner Detector

    IP Impact Parameter

    L1 Level 1 (Trigger)

    LAr Liquid Argon

    LHC Large Hadron Collider

    LS1 Long Shutdown 1

    MC Monte Carlo

    MS Muon Spectrometer

    MTDs Monitored Drift Tubes

    NLO Next to Leading Order

    NNLO Next to Next to Leading Order

    OS Anti-ID CR Opposite Sign Anti ID Control Region

    OS Opposite Sign

    PDF Parton Density Function

    PDG Particle Data Group

    PMT Photomultiplier

    PS Proton Synchrotron

    QCD Quantum Chromodynamics

  • Nomenclature ix

    QED Quantum Electrodynamics

    RF Radio Frequency

    ROS Readout System

    RPCs Resistive Plate Chambers

    SCT Semiconductor Tracker

    SM Standard Model

    SPS Super Proton Synchrotron

    SR Signal Region

    SS Anti-ID CR Same Sign Anti ID Control Region

    SS ID CR Same Sign ID Control Region

    SS Same Sign

    TDAQ Trigger and Data Acquisition System

    TGCs Thin Gap Chambers

    TileCal Tile Calorimeter

    TRT Transition Radiation Tracker

    tt̄H top-quark pair associated production

    VBF Vector Boson Fusion

    VEV Vacuum Expectation Value

    VH W /Z associated production

    ZMF Zero Momentum Frame

    Symbols

    C Charge conjugation

    η Pseudorapidity (ATLAS coordinate system)

    EmissT Missing transverse energy

  • x Nomenclature

    MT Transverse mass

    P Parity

    φ Azimuthal angle (ATLAS coordinate system)

    ϕCP Angle between the tau decay planes in h→ ττ

    ϕ∗CP Observable of the Higgs CP analysis

    φτ CP-mixing angle

    pT Transverse momentum

    √ s Centre of mass energy

    τhad Hadronically decaying tau

    τhad−vis Visible part of hadronic tau decays

    τlep Leptonically decaying tau

    τvis Visible part of tau decays

    θ Polar angle (ATLAS coordinate system)

  • Introduction

    The Higgs boson, discovered by the ATLAS and CMS collaborations at the Large Hadron Collider in 2012 [1, 2], is a fundamental ingredient in the Standard Model of particle physics: its existence, predicted in 1964 by Peter Higgs, François Englert and Robert Brout, is needed to explain the mass of the gauge bosons and to retain the principle of gauge invariance within the theoretical framework.

    After its discovery, much effort was made in measuring its properties. This thesis focuses on the CP quantum numbers of this particle. The Higgs boson is predicted by the Standard Model to be a CP-even scalar particle, i.e. to have quantum numbers JPC = 0++. Alternative hypotheses to the Standard Model concerning pure CP-eigenstate Higgs bosons have been tested in the bosonic sect