Supersymmetry searches in the single lepton final state with the ATLAS detector [Elektronische Ressource] / Keith Morgan Edmonds

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Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2012
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 11 Mo

Extrait

Supersymmetry Searches in the Single Lepton Final State with
the ATLAS Detector
by
Keith Morgan Edmonds
A Thesis Submitted in Partial Fullfillment of the
Requirements for the Degree of
DOCTOR RERUM NATURALIUM
in the Department of Physics
Keith Morgan Edmonds, 2011
Johannes Gutenberg - Universit¨at Mainziiiii
Abstract
This thesis presents an analysis for the search of Supersymmetry with the ATLAS
detector at the LHC. The final state with one lepton, several coloured particles and large
missingtransverseenergywaschosen. Particularemphasiswasplacedontheoptimization
of the requirements for lepton identification. This optimization showed to be particularly
useful when combining with multi-lepton selections. The systematic error associated
withthehigherorderQCDdiagramsinMonteCarloproductionisgivenparticularfocus.
Methodstoverifyandcorrecttheenergymeasurementofhadronicshowersaredeveloped.
Methods for the identification and removal of mismeasurements caused by the detector
environment are applied. A new detector simulation system is shown to provide good
prospects for future fast Monte Carlo production. The analysis was performed for ≈
−135 pb and no significant deviation from the Standard Model is seen. Exclusion limits
arefoundinthesinglemuonandfourjetssubchannelforminimalSupergravity. Previous
limits set by Tevatron and LEP are extended.ivTable of Contents
Supervisory Committee i
Abstract iii
Table of Contents v
1 Introduction 1
2 Particle Physics 3
2.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Symmetries and Interactions . . . . . . . . . . . . . . . . . . . . . 6
2.1.2 The Higgs Mechanism . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Supersymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.1 The MSSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3 The LHC and ATLAS 25
3.1 LHC at CERN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 ATLAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.1 The ATLAS Calorimeters . . . . . . . . . . . . . . . . . . . . . . 29
3.2.2 The ATLAS Inner Detector . . . . . . . . . . . . . . . . . . . . . 32
3.2.3 The ATLAS Muon System . . . . . . . . . . . . . . . . . . . . . . 35
3.2.4 The Trigger System . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4 Searches for SUSY 41
4.1 SUSY Phenomenology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.2 SUSY Backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
¯4.2.1 tt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.2.2 W+Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.2.3 Minor Backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.3 Current Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.4 Signal Significance and Limit Calculation . . . . . . . . . . . . . . . . . . 53
5 Monte Carlo Production and Reconstruction 55
5.1 Monte Carlo Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.1.1 Monte Carlo Truth . . . . . . . . . . . . . . . . . . . . . . . . . . 59
vvi
5.2 Detector Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.1 Fast Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.3 Digitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.4 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.4.1 Jet Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.4.2 Inner Detector Track Reconstruction . . . . . . . . . . . . . . . . 65
5.4.3 Electron Reconstruction . . . . . . . . . . . . . . . . . . . . . . . 67
5.4.4 Muon Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.4.5 Missing Transverse Energy Reconstruction . . . . . . . . . . . . . 72
6 Event Cleaning 73
6.1 Detector Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.2 Cosmic Muons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
6.2.1 Cosmic Muon Removal . . . . . . . . . . . . . . . . . . . . . . . . 76
6.3 Cleaning Cuts Used for Data . . . . . . . . . . . . . . . . . . . . . . . . . 79
7 Single Lepton SUSY Analyses 81
7.1 Overview of the 1 Lepton and 4 Jets Analysis . . . . . . . . . . . . . . . 81
7.2 ATLFAST-II Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.2.1 ATLFAST-II for SUSY . . . . . . . . . . . . . . . . . . . . . . . . 87
7.3 Lepton Definition Optimization . . . . . . . . . . . . . . . . . . . . . . . 90
7.3.1 Isolation Optimization . . . . . . . . . . . . . . . . . . . . . . . . 93
7.3.2 Lepton Quality Investigation . . . . . . . . . . . . . . . . . . . . . 95
7.3.3 Lepton Quality Optimization . . . . . . . . . . . . . . . . . . . . 98
7.3.4 Verification of Optimization . . . . . . . . . . . . . . . . . . . . . 103
7.3.5 Summary of Proposed Selection . . . . . . . . . . . . . . . . . . . 106
7.3.6 Object Definition used in Data . . . . . . . . . . . . . . . . . . . 107
8 Determination of Jet Energy Scale 109
8.1 Sources of JES Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . 110
¯8.2 Hadronic W Decays in tt Events . . . . . . . . . . . . . . . . . . . . . . . 111
8.2.1 High Luminosity Method . . . . . . . . . . . . . . . . . . . . . . . 113
8.2.2 Systematic Error . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
8.2.3 Non-global Determination . . . . . . . . . . . . . . . . . . . . . . 118
8.2.4 Low Luminosity Method . . . . . . . . . . . . . . . . . . . . . . . 119
8.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
8.3 Official JES Uncertainty Measurement . . . . . . . . . . . . . . . . . . . 122
9 W+Jets Production and Uncertainties 125
9.1 ALPGEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
9.2 Jet-Parton Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
9.2.1 MLM Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
9.2.2 Systematic Variations. . . . . . . . . . . . . . . . . . . . . . . . . 133
9.3 Hadronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
9.3.1 PYTHIA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136vii
9.3.2 HERWIG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
9.3.3 Systematic Variations. . . . . . . . . . . . . . . . . . . . . . . . . 137
9.4 Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
9.4.1 Systematic Variations. . . . . . . . . . . . . . . . . . . . . . . . . 142
9.4.2 Unscaling Systematic Variations . . . . . . . . . . . . . . . . . . . 145
9.5 Normalization in Control regions . . . . . . . . . . . . . . . . . . . . . . 147
9.6 Sherpa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
10 Analysis on Data 153
10.1 Samples Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
10.2 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
10.3 Pile-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
10.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
10.4.1 Systematic Uncertainties . . . . . . . . . . . . . . . . . . . . . . . 161
10.4.2 Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
11 Conclusion and Outlook 173
Acknowledgements 177
List of Figures 178
List of Tables 182
Bibliography 184viiiChapter 1
Introduction
The Standard Model of elementary particle physics is very successful in describing the
fundamental constituents of matter and their interactions. The vast majority of experi-
mental measurements are consistent with the Standard Model. It represents the current
bestrepresentationofthebuildingblocks ofreality. Nevertheless, therearetheoreticalas
well as experimental difficulties that motivate an extension of the Standard Model to a
more general theory. Supersymmetry is an interesting possibility for naturally extending
the Standard Model.
Searches for Supersymmetry are among the main physics goals of the ATLAS exper-
iment at the CERN Large Hadron Collider (LHC). If they exist, the LHC will be able
to copiously produce Supersymmetric particles with masses up to several TeV so there
are very strong possibilities for discovery. If new particles are found, it is important to
investigate the properties of these particles, such as masses, couplings and lifetimes, to
determinewhatpossiblemodelsareconsistentwiththosemeasurements. Manymodelsof
Supersymmetryarebeingexploredandtheyspanalargephenomenologicalspace. Tore-
ducethenumberofparametersandthereforefacilitatethestudyofthephenomenologyat
colliders, some simplified models are defined. These are defined by imposing conditions
on the breaking of Supersymmetry. Among the most appealing for study is minimal
12 CHAPTER 1. INTRODUCTION
Supergravity (mSUGRA), with the additional requirement that the lightest particle is
stable and only weakly interacting. This leads to final states characterized by multiple
high energy jets and large missing transverse energy.