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Top quark mass measurement [Elektronische Ressource] : prospects of commissioning studies for early LHC data in the ATLAS detector / Emanuel Rauter

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Top Quark Mass Measurement:
Prospects of Commissioning Studies for Early
LHC Data in theATLAS Detector
Emanuel Rauter
Dissertation¨ ¨TECHNISCHE UNIVERSITAT MUNCHEN
Max-Planck-Institut fur¨ Physik
(Werner-Heisenberg-Institut)
Top Quark Mass Measurement:
Prospects of Commissioning Studies for Early
LHC Data in theATLAS Detector
Emanuel Rauter
Vollst¨andiger Abdruck der von der Fakult¨at fu¨r Physik
der Technischen Universit¨at Mun¨ chen zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. A.J. Buras
Pruf¨ er der Dissertation:
1. Hon.-Prof. Dr. S. Bethke
2. Univ.-Prof. Dr. St. Paul
Die Dissertation wurde am 5. Juni 2009 bei der Technischen Universit¨at Mu¨nchen
eingereicht und durch die Fakult¨at fur¨ Physik am 1. Juli 2009 angenommen.Abstract
A study to prepare a top quark mass measurement with earlyLHC data was carried out.
The top quark reconstruction in the lepton plus jets decay channel using a simple and
robust cut-based method has been studied. In particular, no b-tagging information was
used. An in-situ jet calibration method was applied in order to minimise the systematic
error due to jet energy scale. The reconstruction method is explained and its application
−1on 145 pb of simulated data is presented. Systematic effects and limitations of the
method are discussed. Before applying the in-situ calibration, the dominant systematic
error is found to be the jet energy scale as well as the combinatorial background. After
applying the in-situ calibration, only the latter remains dominant. With a top quark
mass of 172.5 GeV as input to the simulation, the method e.g. in the electron channel
+4.07electronyields m = 167.22±(2.38) ( ) GeV. First real data - both from cosmicstat systt −3.16
muons and from LHC single beams - are used to study the in-situ performance of the
ATLAScalorimetersystem. Alsoincludedarecalibrationmeasurementsoftheelectronics
readout system. They yield a detector test of the expected calorimeter performance.Contents
Contents iii
Introduction 1
1 Top Quark Physics 3
1.1 A Reminder: The Standard Model of Particle Physics . . . . . . . . . . . . 3
1.1.1 Electroweak Interactions . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2 Strong Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Top Quark and its Role . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.1 Top Quark Production . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.2 Top Quark Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.3 Top Quark Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Monte Carlo Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 The ATLAS Detector - Design and Performance 15
2.1 The pp Collider Ring LHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Physics and Performance Goals for ATLAS . . . . . . . . . . . . . . . . . . 16
2.3 The ATLAS detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.1 ATLAS Coordinate System . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.2 Magnet System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.3 Inner Detector - Tracking System . . . . . . . . . . . . . . . . . . . . 20
2.3.4 Calorimeter System . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3.5 Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3.6 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
iii Contents
3 Detector Commissioning and Calibration 31
3.1 Understanding the Detector As-Built . . . . . . . . . . . . . . . . . . . . . . 31
3.2 Electronics Readout and Calibration System. . . . . . . . . . . . . . . . . . 31
3.3 Electronics Commissioning and Calibration . . . . . . . . . . . . . . . . . . 36
3.3.1 Pulse Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.3.2 Shape Prediction in the HEC . . . . . . . . . . . . . . . . . . . . . . 38
3.3.3 Jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3.4 Pedestal and Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.3.5 High Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.4 Detector Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.4.1 Reduced High Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.4.2 Distorted Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.4.3 Dead Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.4.4 Noisy Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.5 Commissioning with Cosmic Muons . . . . . . . . . . . . . . . . . . . . . . 49
3.5.1 Signal Studies with the First Real Physics Pulses . . . . . . . . . . . 51
3.5.2 Discussion of Residuals in Pulse Shape Prediction . . . . . . . . . . 53
3.5.3 Pathological Shapes in Cosmic Events . . . . . . . . . . . . . . . . . 56
3.6 First Beam Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.7 Influence of the Dead Power Supply in HEC C . . . . . . . . . . . . . . . . 61
4 Physics Objects Reconstruction and Calibration 65
4.1 Energy Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.1.1 Clustering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.1.2 Jet-Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.2 Detector Level Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.2.1 EM Scale Calibration in ATLAS . . . . . . . . . . . . . . . . . . . . 69
4.2.2 Method of the Hadronic Calibration in ATLAS . . . . . . . . . . . . 70
4.2.3 Global Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.2.4 Local Hadron Calibration . . . . . . . . . . . . . . . . . . . . . . . . 71
4.3 Jet Level Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.3.1 In-Situ Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
miss4.4 E Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
T
4.5 Electron Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.6 Muon Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76Contents iii
5 Top Quark Mass Reconstruction with Local Hadron Calibration 77
5.1 Monte Carlo Datasets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.1.1 Signal Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.1.2 Physics Background Simulation . . . . . . . . . . . . . . . . . . . . . 79
5.2 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2.1 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2.2 Lepton Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2.3 Jet Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
miss5.2.4 E Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83T
5.2.5 HEC Region Cut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2.6 Cut Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2.7 Event Selection for QCD Processes . . . . . . . . . . . . . . . . . . . 85
5.3 Top Quark Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.3.1 Method of Top Quark Reconstruction . . . . . . . . . . . . . . . . . 86
5.4 W Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.5 Iterative In-Situ Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.5.1 Top Quark and W Boson Mass Spectrum after In-Situ Calibration . 91
5.6 Purity and Efficiency of the Reconstruction Method . . . . . . . . . . . . . 92
5.7 Systematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.7.1 Jet Energy Scale and Calibration . . . . . . . . . . . . . . . . . . . . 92
5.7.2 Jet p Cut in Event Selection . . . . . . . . . . . . . . . . . . . . . . 95T
5.7.3 Fit and Combinatorics . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.7.4 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.7.5 Summary of Systematic Effects . . . . . . . . . . . . . . . . . . . . . 100
5.8 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6 Conclusion 107
A ATLAS acronyms 109
List of Figures 111
List of Tables 114
Bibliography 115