Observation of top quark pair production and estimation of W+jets background with ATLAS at the LHC [Elektronische Ressource] / vorgelegt von B´alint Radics

Observation of top quark pair production and estimation of W+jets background with ATLAS at the LHC [Elektronische Ressource] / vorgelegt von B´alint Radics

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Observation of top quark pair production andestimation of W+jets background withATLAS at the LHCDissertationzurErlangung des Doktorgrades (Dr. rer. nat.)derMathematisch-Naturwissenschaftlichen Fakult atderRheinischen Friedrich-Wilhelms-Universit at Bonnvorgelegt vonB alint RadicsausDebrecenBonn 2010Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakult atder Universit at Bonn1. Gutachter Prof. Dr. Ian C. Brock2. Gutachter Prof. Dr. Norbert WermesTag der Promotion: 30.11.2010Erscheinungsjahr: 2010ContentsList of Figures VList of Tables XIAcknowledgements XIII1 Introduction 12 Top quark and the Standard Model 32.1 Introduction to the Standard Model . . . . . . . . . . . . . . . 32.2 Simulation of hard scattering processes at hadron colliders . . 172.3 Top quark pair production and decay . . . . . . . . . . . . . . 212.3.1 Top quark pair production cross section . . . . . . . . 212.3.2 Top quark decays . . . . . . . . . . . . . . . . . . . . . 232.4 Background processes for top quark pair signal . . . . . . . . . 262.5 Monte Carlo simulation of signal and background events . . . 272.5.1 Simulation of signal events . . . . . . . . . . . . . . . . 272.5.2 Simulation of W +jets and Z+jets events . . . . . . . . 292.5.3 Simulation of background events . . . . . . . . . . . . . 292.5.4 Monte Carlo cross sections . . . . . . . . . . . . . . . . 29I3 Overview of the ATLAS experiment 303.1 The Large Hadron Collider . . . .

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Observation of top quark pair production and
estimation of W+jets background with
ATLAS 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
B alint Radics
aus
Debrecen
Bonn 2010Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakult at
der Universit at Bonn
1. Gutachter Prof. Dr. Ian C. Brock
2. Gutachter Prof. Dr. Norbert Wermes
Tag der Promotion: 30.11.2010
Erscheinungsjahr: 2010Contents
List of Figures V
List of Tables XI
Acknowledgements XIII
1 Introduction 1
2 Top quark and the Standard Model 3
2.1 Introduction to the Standard Model . . . . . . . . . . . . . . . 3
2.2 Simulation of hard scattering processes at hadron colliders . . 17
2.3 Top quark pair production and decay . . . . . . . . . . . . . . 21
2.3.1 Top quark pair production cross section . . . . . . . . 21
2.3.2 Top quark decays . . . . . . . . . . . . . . . . . . . . . 23
2.4 Background processes for top quark pair signal . . . . . . . . . 26
2.5 Monte Carlo simulation of signal and background events . . . 27
2.5.1 Simulation of signal events . . . . . . . . . . . . . . . . 27
2.5.2 Simulation of W +jets and Z+jets events . . . . . . . . 29
2.5.3 Simulation of background events . . . . . . . . . . . . . 29
2.5.4 Monte Carlo cross sections . . . . . . . . . . . . . . . . 29
I3 Overview of the ATLAS experiment 30
3.1 The Large Hadron Collider . . . . . . . . . . . . . . . . . . . . 30
3.2 The ATLAS detector . . . . . . . . . . . . . . . . . . . . . . . 32
3.2.1 Inner detector . . . . . . . . . . . . . . . . . . . . . . . 34
3.2.2 Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2.3 Muon spectrometer . . . . . . . . . . . . . . . . . . . . 40
3.2.4 Trigger and data aquisition . . . . . . . . . . . . . . . 42
3.2.5 Luminosity . . . . . . . . . . . . . . . . . . . . . . . . 46
3.2.6 Detector simulation . . . . . . . . . . . . . . . . . . . . 48
4 Reconstruction of physics objects 49
4.1 Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2 Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2.1 Electron energy resolution . . . . . . . . . . . . . . . . 56
4.3 Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.3.1 Jet energy scale calibration . . . . . . . . . . . . . . . 58
4.3.2 Jet energy scale uncertainties . . . . . . . . . . . . . . 60
4.3.3 Jet energy resolution . . . . . . . . . . . . . . . . . . . 62
4.4 Missing transverse energy . . . . . . . . . . . . . . . . . . . . 63
miss4.4.1 Re ned E . . . . . . . . . . . . . . . . . . . . . . . 65T
miss4.4.2 E resolution . . . . . . . . . . . . . . . . . . . . . . 66T
4.5 Muons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.5.1 Non-prompt and fake muon sources . . . . . . . . . . . 67
4.5.2 Muon momentum resolution . . . . . . . . . . . . . . . 68
II5 Event selection 70
5.1 General outline of the analysis . . . . . . . . . . . . . . . . . . 70
5.2 Data sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.2.1 Data quality selection . . . . . . . . . . . . . . . . . . 74
5.2.2 Luminosity of data sample . . . . . . . . . . . . . . . . 75
5.2.3 Non-collision background rejection . . . . . . . . . . . 75
5.3 Selection of reconstructed objects . . . . . . . . . . . . . . . . 75
5.3.1 Muon trigger . . . . . . . . . . . . . . . . . . . . . . . 76
5.3.2 Muon selection . . . . . . . . . . . . . . . . . . . . . . 79
5.3.3 Jet selection . . . . . . . . . . . . . . . . . . . . . . . . 84
5.3.4 Missing transverse energy discrimination . . . . . . . . 87
5.3.5 Triangular cut . . . . . . . . . . . . . . . . . . . . . . . 89
5.3.6 Pile-up sensitivity . . . . . . . . . . . . . . . . . . . . . 95
5.4 Object and event selection summary . . . . . . . . . . . . . . 96
5.5 QCD background estimation . . . . . . . . . . . . . . . . . . . 98
5.5.1 Matrix method . . . . . . . . . . . . . . . . . . . . . . 100
5.5.2 Uncertainties of the QCD background estimation . . . 108
5.6 Data-driven estimation of W +jets background . . . . . . . . . 109
5.6.1 Brief summary of the W=Z ratio method . . . . . . . . 110
5.6.2 Berends scaling method . . . . . . . . . . . . . . . . . 111
5.6.3 Selection and composition of W ! candidates in
the control regions . . . . . . . . . . . . . . . . . . . . 113
5.6.4 W +jets background estimation in the signal region . . 114
5.6.5 Uncertainties on W +jets background . . . . . . . . . . 118
6 Observation of top-antitop events 121
III7 Summary 128
Bibliography 130
A Berends scaling 134
B Hypothesis test on the ratio of Poisson means 136
C Monte Carlo cross sections 141
IVList of Figures
2.1 The quark, antiquark and gluon momentum densities in the
proton as a function of the longitudinal proton momentum
2 2 2 2fraction x at = 10 GeV (left) and at = 10000 GeV
(right) from the MSTW2008 parametrization. . . . . . . . . . 22
2.2 The leading order Feynman diagrams contributing to top quark
pair production at LHC. . . . . . . . . . . . . . . . . . . . . . 23
2.3 QCD NLO and NLO+NNLL predictions for the top quark pair
production cross section at the TeV energies of proton-proton
colliders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4 Feynman diagrams illustrating at LO the three decay modes
of the top quark pair signal: left: all hadronic channel, middle:
dilepton channel, right: lepton+jets channel. . . . . . . . . . . 25
2.5 QCD predictions for hard scattering cross sections and event
rates at the Tevatron and the LHC at nominal luminosity of
p
33 2 1L = 10 cm s . The step at s = 4 TeV marks the
transition from the Tevatronpp collider to the LHCpp collider. 28
3.1 Schematic view of the CERN accelerator comlex. . . . . . . . 31
3.2 The ATLAS detector. . . . . . . . . . . . . . . . . . . . . . . . 34
3.3 The ATLAS inner detector. . . . . . . . . . . . . . . . . . . . 36
3.4 Plan view of a quarter section of the ATLAS inner detector
showing the major elements. . . . . . . . . . . . . . . . . . . . 36
V3.5 The ATLAS Calorimeters. . . . . . . . . . . . . . . . . . . . . 37
3.6 Sketch of an electromagnetic calorimeter barrel module. The
granularity in and of the cells of each of the layers and of
the trigger towers is shown. . . . . . . . . . . . . . . . . . . . 39
3.7 Schematic diagram showing the three FCal modules located
in the end-cap cryostat. . . . . . . . . . . . . . . . . . . . . . 39
3.8 The ATLAS Muon spectrometer. . . . . . . . . . . . . . . . . 43
3.9 Schematic view of the ATLAS trigger and data acquisition
system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.1 Expected distribution of jet multiplicity for candidate events
after W! +jets selection from Monte Carlo event gener-
ators and after full detector simulation. On the stacked jet
multiplicity plot the top quark pair signal expectation is in-
dicated with an arrow pointing to the red colored histogram,
the W ! +jet background contribution is indicated with
white color and the QCD multijet background is indicated
with brown color. . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.2 Trigger rates in run 160801 for L1 MU0 and L1 MU10 muon
triggers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.3 Trigger e ciency curves for the L1 MU10 and EF mu10 MSonly
muon triggers with respect to o ine reconstructed muons as
a function of the p and of the muon, using full simulatedT
Z! Monte Carlo events. . . . . . . . . . . . . . . . . . . 78
5.4 Distribution of the distance, R( jet), between reconstructed
muon and jet candidates. . . . . . . . . . . . . . . . . . . . . . 80
5.5 Control plot showing the distribution ofp of muon candidatesT
from Monte Carlo simulation and data after preselection cuts. 81
5.6 Control plot showing the distribution of pseudorapidity of
muon candidates from Monte Carlo simulation and data af-
ter preselection cuts except the cut on pseudorapidity. . . . . . 82
VI