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Study of top-quark pair cross-section measurement in the dilepton channel with the ATLAS detector at the LHC [Elektronische Ressource] / Duc Bao Ta. Mathematisch-Naturwissenschaftliche Fakultät

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204 pages
Study of top-quark paircross-section measurement in thedilepton channel with the ATLASdetector at the LHCDissertationzurErlangung des Doktorgrades (Dr. rer. nat.)derMathematisch-Naturwissenschaftlichen Fakult¨atderRheinischen Friedrich-Wilhelms-Universit¨at Bonnvorgelegt vonDuc Bao TaausHaiphongBonn, Oktober 2010Angefertigt mit Genehmigung derMathematisch-Naturwissenschaftlichen Fakult¨atder Rheinischen Friedrich-Wilhelms-Universit¨atBonn1. Referent: Prof. Dr. Norbert Wermes2. Referent: Prof. Dr. Ian BrockTag der Promotion: 28. Oktober 2010Erscheinungsjahr: 2011AbstractThe top quark is the heaviest of the known fundamental particles in the StandardModel(SM)with amassof173.3±1.1GeV. It isthe sixth quarkintheSMandtheweakisospinpartnerofthebottomquark. Propertiessuchasthemassandthecrosssection have been measured since its discovery in 1995 with great precision at theproton-antiproton Fermilab Tevatron Collider, Chicago at a centre-of-mass energyof 1.8GeV and 1.96GeV. With the start of the proton-protonCERN Large HadronCollider (LHC), Geneva, a new record in the centre-of-mass energy was achieved.TheLHC opens anewenergy frontier and probestheSMatenergies ofthe electro-weak symmetry-breaking scale andbeyond.
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Study of top-quark pair
cross-section measurement in the
dilepton channel with the ATLAS
detector at the LHC
Dissertation
zur
Erlangung des Doktorgrades (Dr. rer. nat.)
der
Mathematisch-Naturwissenschaftlichen Fakult¨at
der
Rheinischen Friedrich-Wilhelms-Universit¨at Bonn
vorgelegt von
Duc Bao Ta
aus
Haiphong
Bonn, Oktober 2010Angefertigt mit Genehmigung der
Mathematisch-Naturwissenschaftlichen Fakult¨at
der Rheinischen Friedrich-Wilhelms-Universit¨at
Bonn
1. Referent: Prof. Dr. Norbert Wermes
2. Referent: Prof. Dr. Ian Brock
Tag der Promotion: 28. Oktober 2010
Erscheinungsjahr: 2011Abstract
The top quark is the heaviest of the known fundamental particles in the Standard
Model(SM)with amassof173.3±1.1GeV. It isthe sixth quarkintheSMandthe
weakisospinpartnerofthebottomquark. Propertiessuchasthemassandthecross
section have been measured since its discovery in 1995 with great precision at the
proton-antiproton Fermilab Tevatron Collider, Chicago at a centre-of-mass energy
of 1.8GeV and 1.96GeV. With the start of the proton-protonCERN Large Hadron
Collider (LHC), Geneva, a new record in the centre-of-mass energy was achieved.
TheLHC opens anewenergy frontier and probestheSMatenergies ofthe electro-
weak symmetry-breaking scale andbeyond. The measurement ofthe top-quarkpair
cross-section at this new energy is an important step, since it will either re-establish
the SM at these energies by confirming the theoretical calculations or it can point
to new phenomena that help to understand the so far not explained experimental
findings in particle physics and cosmology.
This thesis presents a study of the production cross-section measurement of top-
quark pair in the dilepton channel at the LHC with the ATLAS experiment at√
−1a centre-of-mass energy of s =10TeV and for a data amount of L =200pb .int.
This study investigates the performance of the cross-section measurement with a
cut-and-count analysis on Monte Carlo simulated samples. Important background
processes are taken into account such as leptonically decaying Z-boson and single
top-quark events. Data-driven background methods for Z-boson and fake event
rates are discussed. A full list of systematic uncertainties is investigated and finally
combined with the statistical uncertainties in a likelihood to extract the combined
uncertainty. A “top mass”-like variable is presented that can help to distinguish
dileptonic top-quark pair decays from similar signatures in new physics models.
The combined sensitivity of the analysis in all dilepton decay-channels is a relative
uncertainty on the cross-section measurement of:
Δσcombined +3.1 +9.6 +26.2[%] = (stat) (syst) (lumi)−3.1 −8.7 −17.4σcombined
Finally the study is compared to the same analyses performed at different centre-
¯of-mass energies of 14GeV and 7GeV and two dilepton tt-pair event candidates in
−1280pb of first ATLAS data are presented.
vContents
1 Introduction 1
2 Theoretical Aspects 7
2.1 The Standard Model 7
2.2 The Physics of Top Quark in the Standard Model 10
2.2.1 Top-Antitop-Quark Pair Production Cross-Section and Decay 11
2.2.2 Other Top-Quark Properties 18
2.3 Top Quark and New Physics 29
2.3.1 Model-Independent Characterisation 30
2.3.2 Indirect and Direct Evidence of New Particles 31
2.3.3 Top-Quark Decay into New Particles 33
3 Experimental Environment 35
3.1 The Large Hadron Collider 35
3.2 The ATLAS Detector 38
3.2.1 Physics Requirements 39
3.2.2 Inner Tracking Detectors 41
3.2.3 Calorimeters 45
3.2.4 The Muon System 51
3.2.5 The Trigger System 55
4 Monte Carlo Simulation and Samples 57
4.1 Monte Carlo Generators 58
4.2 Monte Carlo Simulation 62
4.3 Experimental Signature and Background Events 63
4.4 Samples 66
5 Object Identification, Reconstruction and Selection 69
5.1 Electrons 69
5.2 Muons 73
5.3 Jets 77
viiContents
5.4 Missing Transverse Energy 82
5.5 Trigger Algorithms 85
5.6 Luminosity Determination 88√
5.7 Object Selection for the analysis at s=10TeV 90
5.7.1 Trigger 91
5.7.2 Electrons 92
5.7.3 Jets 92
5.7.4 Muons 93
5.7.5 missing transverse energy 95

6 Cut-and-count Analysis at s=10TeV 97
6.1 Event Selection 97
6.2 Expected Number of Events fromMC 100
6.3 Data-Driven Methods 104
+ −6.3.1 Estimation of Z →ℓ ℓ /Drell-Yan Events from Side Bands 105
6.3.2 Fake Rate Estimation using the Matrix Method 107
6.3.3 Background Normalisation in Control Regions 110
7 Systematic Uncertainties for the Analysis 113
7.1 Luminosity 113
7.2 Trigger and Lepton Identification Efficiencies 113
7.3 Missing Transverse Energy 114
7.4 Lepton Energy Scale 115
7.5 Jet Energy Scale 116
7.6 PDF Uncertainties 116
7.7 Initial and Final State Radiation 120
7.8 Monte-Carlo-Model 121
7.9 Theoretical Cross Section 122
7.10 Drell-Yan Background Estimation 123
7.11 Jets Misidentified as Leptons 123
7.12 Summary of Statistical and Systematic Uncertainties 123
7.13 Pile-up Effects 123
8 Expected Precision for the Analysis 127
8.1 Combination of Uncertainties 127
8.2 Cross-Check of Dilepton Events with the Stransverse Mass 133
viiiContents
9 Analyses at Different Centre-of-Mass Energies 143

9.1 Cross Section Analysis at s =14TeV 143
9.1.1 Differences Between the Analysis 144
9.1.2 Event Yields for the Event Selection 145
9.1.3 Systematic Uncertainties for the Analysis 145√
9.1.4 Expected Sensitivity for the Analysis at s =14TeV 146√
9.2 Cross Section Analysis at s =7TeV 147
9.2.1 Differences Between the Analysis 148
9.2.2 Event Yields for the Event Selection 152
9.2.3 Systematic Uncertainties for the Analysis 153√
9.2.4 Expected Sensitivity of the Analysis at s =7TeV 156
10 Conclusion and Outlook 159

A List of Samples Used for the Study at s =10TeV 163
B Coordinate System 169
Bibliography 171
Curriculum Vitae 193
ix