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Strong supersymmetry: A search for squarks and gluinos in hadronic channels
using the ATLAS detector
van der Leeuw, R.H.L.
Publication date
2014
Link to publication
Citation for published version (APA):
van der Leeuw, R. H. L. (2014). Strong supersymmetry: A search for squarks and gluinos in
hadronic channels using the ATLAS detector. Boxpress.
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Contents
Introduction 1
1 Theory of the Standard Model and Supersymmetry 5
1.1 The Standard Model . . . 5
1.1.1 The Standard Model Lagrangian . . . 8
1.1.2 Quantum Chromodynamics . . . 13
1.2 Shortcomings of the Standard Model: motivations for SUSY . . . 20
1.3 Supersymmetry . . . 24
1.3.1 SUSY fundamentals . . . 26
1.3.2 Particle content of the MSSM . . . 27
1.3.3 Supersymmetry breaking . . . 29
1.3.4 SUSY Phenomenology . . . 30
1.3.5 Constraints on SUSY . . . 35
2 The LHC and the ATLAS detector 39 2.1 The LHC accelerator . . . 39
2.1.1 Short description of LHC operations . . . 41
2.2 Overview of ATLAS . . . 42
2.3 Magnet system . . . 46
2.4 The Inner Detector . . . 47
2.4.1 General layout . . . 47
2.4.2 Pixel detector . . . 49
2.4.3 Semiconductor Tracker . . . 49
2.4.4 Transition Radiation Tracker . . . 53
2.5 Calorimeters . . . 54
2.6 The Muon Spectrometer . . . 57
2.7 Trigger system . . . 60
2.7.1 Three trigger levels . . . 61
2.7.2 Trigger menu and streams . . . 62
2.8 Operational performance of the ATLAS detector . . . 62
2.8.1 Operational status and hit efficiency of the SCT . . . 62
2.8.2 Operational status of the other sub-detectors . . . 69
2.8.3 Pile-up and vertexing in ATLAS . . . 71
ii Contents
3 SUSY Cross Sections 73
3.1 Theoretical introduction . . . 73
3.1.1 Next-to-leading order and beyond . . . 74
3.1.2 Prospino . . . . 75
3.1.3 NLL-Fast . . . 76
3.2 NLO SUSY cross sections . . . 76
3.3 Contributions to the cross section uncertainties . . . 78
3.3.1 PDF variations . . . 79
3.3.2 Variations in the strong coupling . . . 80
3.3.3 Scale variations . . . 81
3.4 Combining the contributions: two methods . . . 85
3.4.1 Method 1: Straightforward usage . . . 85
3.4.2 Method 2: Final official ATLAS and CMS agreement . . . . 86
3.4.3 Comparison of methods . . . 89
3.4.4 Method for specific models . . . 93
3.4.5 Discussion . . . 97
4 Event reconstruction and simulation in ATLAS 99 4.1 Track and vertex reconstruction . . . 99
4.2 Jets . . . 103 4.2.1 Energy scale . . . 104 4.2.2 b-jets . . . 105 4.2.3 Jet selection . . . 106 4.3 Leptons . . . 106 4.3.1 Muons . . . 107 4.3.2 Electrons . . . 110 4.4 Photons . . . 113
4.5 Missing transverse energy . . . 114
4.6 Overlapping objects . . . 117
4.7 Monte Carlo simulation . . . 117
4.7.1 Event generation . . . 118
4.7.2 Generators . . . 119
4.7.3 Simulation of the ATLAS detector . . . 120
5 Search for SUSY in events with jets and6ET 121 5.1 Overview of the 0-lepton analysis strategy . . . 122
5.1.1 Standard Model backgrounds to hadronically decaying SUSY . 126 5.2 Dataset and Monte Carlo samples . . . 128
5.2.1 Monte Carlo Samples . . . 128
5.3 Event selection of the 0-lepton analysis . . . 132
5.3.1 Trigger and event selection . . . 132
5.3.2 Optimisation for compressed spectra . . . 138
5.3.3 High mass optimisation . . . 145
Contents iii
5.4 Background estimation . . . 146
5.4.1 Control regions . . . 149
5.4.2 Validation regions . . . 152
5.4.3 Agreement of data and MC in CRs . . . 152
5.4.4 Sherpa heavy-flavour issue . . . . 155
5.4.5 Final CR distributions . . . 155
5.4.6 Sources of systematic uncertainties on SM background . . . . 155
5.4.7 Uncertainties on signal samples . . . 161
5.5 Statistical procedure for discovery or exclusion . . . 163
5.5.1 Estimated number of background events . . . 164
5.5.2 The likelihood function . . . 165
5.5.3 Profile log likelihood ratio . . . 165
5.6 Results . . . 167
5.6.1 Interpretation of the results . . . 173
6 Discussion and implications 181 6.1 Discussion . . . 181
6.1.1 Update with full 2012 dataset . . . 182
6.2 Implication of the results . . . 183
6.2.1 Results from other search channels . . . 184
6.2.2 Implications of the analysis on a phenomenological MSSM . . 185
6.2.3 General SUSY fits . . . 187
6.3 Outlook for√s = 13 TeV . . . 187
Bibliography 189
Summary 207
Samenvatting 217
