Sensitivity of the ATLAS Experiment to discover the Decay H → ττ → ll + 4ν of the Standard Model Higgs Boson produced in Vector Boson Fusion [Elektronische Ressource] / Martin Schmitz

Martin Schmitz

Sujets

• Physik

Informations

• Publié par: rheinische_friedrich-wilhelms-universitat_bonn
• Publié le: 01 janvier 2011
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Sensitivity of the ATLAS Experiment to discover
the Decay H! ! ‘‘+4
of the Standard Model Higgs Boson produced in
Vector Boson Fusion
Dissertation
zur
Erlangung des Doktorgrades (Dr. rer. nat.)
der
Mathematisch-Naturwissenschaftlichen Fakult at
der
Rheinischen Friedrich-Wilhelms-Universit at Bonn
vorgelegt von
Martin Schmitz
aus
Bonn
Bonn 2011Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakult at der
Rheinischen Friedrich-Wilhelms-Universit at Bonn
1. Gutachter: Prof. Dr. Markus Schumacher
2.hter: Prof. Dr. Norbert Wermes
Tag der Promotion: 17.05.2011
Erscheinungsjahr: 2011Abstract
A study of the expected sensitivity of the ATLAS experiment to discover the Standard
Model Higgs boson produced via vector boson fusion (VBF) and its decay to H ! !
‘‘+4 is presented. The study is based on simulated proton-proton collisions at a centre-
of-mass energy of 14TeV. For the rst time the discovery potential is evaluated in the
presence of additional proton-proton interactions (pile-up) to the process of interest in a
completeandconsistentway. Specialemphasisisplacedonthedevelopmentofbackground
estimation techniques to extract the main background processesZ! andtt production
using data. The tt background is estimated using a control sample selected with the VBF
analysis cuts and the inverted b-jet veto. The dominant background process Z !
is estimated using Z ! events. Replacing the muons of the Z ! event with
simulated -leptons, Z ! events are modelled to high precision. For the replacement
of the Z boson decay products a dedicated method based on tracks and calorimeter cells is
developed. Withoutpile-upadiscoverypotentialof3to3:4inthemassrange115GeV <
1M <130GeVisobtainedassuminganintegratedluminosityof30fb . InthepresenceofH
pile-upthesignalsensitivitydecreasesto1:7 to1:9 mainlycausedbytheworseresolution
of the reconstructed missing transverse energy.Contents
1 Introduction 1
2 Theoretical Overview 3
2.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Spontaneous Symmetry Breaking and the Higgs Mechanism . . . . . . . . . 5
2.3 Bounds on the Higgs Boson Mass . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.1 Theoretical Bounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.2 Experimental Bounds . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Higgs Boson Production at the Large Hadron Collider . . . . . . . . . . . . 9
2.5 Decay of the SM Higgs Boson . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 The LHC and the ATLAS Experiment 13
3.1 Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 The ATLAS-Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.1 Coordinate System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.2 Inner Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.3 Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.4 Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.5 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4 Signal and Background Processes 21
4.1 Signal process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2 Background processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2.1 Z(! =ee= )+jets . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2.2 Top Quark Pair Production . . . . . . . . . . . . . . . . . . . . . . . 26
4.2.3 W+jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.2.4 Boson Pair Production . . . . . . . . . . . . . . . . . . . . . . . . . . 27ii CONTENTS
4.3 Detector Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.4 Pile{Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5 Event Reconstruction 33
5.1 Electron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.2 Muon Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.3 -Lepton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.4 Jet Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.5 Missing Transverse Energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.6 b-Tagging Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6 Event Selection 39
6.1 Mass Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
6.2 Event Selection Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.2.1 Pre-Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6.2.2 Object Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.2.3 Event Topology Criteria . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.3 Cut Factorisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6.4 Shape Parametrisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
6.5 Results of the Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
7 Background Estimation using Data 67
7.1 Estimation of tt background . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
7.1.1 Validation using Monte Carlo Simulation . . . . . . . . . . . . . . . 68
7.1.2 Control Sample Selection . . . . . . . . . . . . . . . . . . . . . . . . 69
7.1.3 Validation using Data . . . . . . . . . . . . . . . . . . . . . . . . . . 71
7.2 Estimation of Z! background . . . . . . . . . . . . . . . . . . . . . . . 76
7.2.1 Conversion of Z! events into Z! events . . . . . . . . . . . 76
7.2.2 Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.2.3 Z! Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . 82
7.2.4 Validation using Data . . . . . . . . . . . . . . . . . . . . . . . . . . 86
7.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90CONTENTS iii
8 Discovery Potential 91
8.1 Statistical Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
8.1.1 Establishing Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 91
8.1.2 Hypothesis Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
8.1.3 Sampling Distribution of Likelihood Ratio . . . . . . . . . . . . . . . 94
8.2 Systematic Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
8.2.1 Z! Background Estimation . . . . . . . . . . . . . . . . . . . . 95
8.2.2 Background Estimation of the Top Quark Pair Production . . . . . 96
8.2.3 Treatment of Systematic Uncertainties in the Likelihood . . . . . . . 97
8.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
9 Conclusion 107
A Likelihood Fits for M =115;125;130GeV 109H
Bibliography 120iv CONTENTS