Search for light charged Higgs bosons in hadronic _t63 [tau] final states with the ATLAS detector [Elektronische Ressource] / Thies Ehrich

Thies Ehrich

Sujets

• Physik

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• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2010
• Nombre de lectures: 16
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

TECHNISCHE UNIVERSITÄT MÜNCHEN
Max-Planck-Institut für Physik
(Werner-Heisenberg-Institut)
Search for Light Charged Higgs Bosons in
Hadronic τ Final States with the ATLAS Detector
Thies Ehrich
Vollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München zur
Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. A. Ibarra
Prüfer der Dissertation:
1. Priv.-Doz. Dr. H. Kroha
2. Univ.-Prof. Dr. L. Oberauer
Die Dissertation wurde am 17. Juni 2010 bei der Technischen Universität München eingereicht
und durch die Fakultät für Physik am 7. Juli 2010 angenommen.Abstract
Charged Higgs bosons are predicted in theories with a non-minimal Higgs
sector like the Minimal Supersymmetric Extension of the Standard Model
(MSSM). At the LHC, light charged Higgs Bosons might be produced in
+on-shell top quark decays t → H b, if m ± < m −m . In most of thet bH
+MSSM parameter space, the decayH → τν is the dominant decay channel
and suggests the possibility of using the unique signature of hadronic τ final
states to suppress the backgrounds.
The subject of this study is the estimation of the sensitivity of the ATLAS
¯detector for charged Higgs boson searches in tt events. Leptons from the
decay chain of the second top quark allow for efficient triggering. A search
strategy is developed and estimates of signal significances and exclusion
limits in the MSSM m -max scenario are presented based on Monte Carloh
−1
simulations. For an integrated luminosity of 10 fb , the discovery of charged
Higgs bosons is possible for tanβ > 32. Exclusion limits are given for
values of tanβ > 17, significantly improving the current best limits from the
Tevatron.
The most important systematic uncertainties were found to be the errors
on the jet energy scale and the missing transverse energy, resulting in a
total systematic uncertainty of 40% on the signal. To reduce the systematic
¯uncertainty for the most important Standard Model background,tt production,
emphasis is put on estimating this background using data instead of Monte
¯Carlo simulations. Thett background consists of two contributions, one with
a correctly identified τ-jet in the final state, which is irreducible, and one
where the hadronicτ decay is faked by a light parton jet. For each background
a method has been developed to estimate its contribution with minimal use
of Monte Carlo simulations. In this way, the systematic uncertainty on the
background can be significantly reduced.Contents
1 Introduction 1
2 The Standard Model of Particle Physics 3
2.1 Lagrange Formalism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Quantum Electrodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 The Electroweak Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4 Quantum Chromodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5 Spontaneous Electroweak Symmetry Breaking – The Higgs Mechanism . . . . . 8
2.6 Higgs Mass Bounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.6.1 Theoretical Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.6.2 Experimental Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.7 Limitations of the Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . 13
3 Supersymmetric Extensions of the Standard Model 15
3.1 General Concept of Supersymmetry . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 The Minimal Supersymmetric Extension of the Standard Model . . . . . . . . . 16
3.2.1 The Superpotential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.2 R parity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.3 Supersymmetry Breaking in the MSSM . . . . . . . . . . . . . . . . . . 19
3.2.4 The MSSM Higgs Sector and Gauge Symmetry Breaking . . . . . . . . 20
4 Charged Higgs Bosons 23
4.1 Luminosity and Cross Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2 Charged Higgs Boson Production and Decay at the LHC . . . . . . . . . . . . . 26
4.2.1 Models with Charged Higgs Bosons . . . . . . . . . . . . . . . . . . . . 26
4.2.2 Mass Relations in them -max Scenario . . . . . . . . . . . . . . . . . . 26h
4.2.3 Production of Charged Higgs Bosons . . . . . . . . . . . . . . . . . . . 28
4.2.4 Decays of Charged Higgs Bosons . . . . . . . . . . . . . . . . . . . . . 29
4.2.5 τ Final States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.3 Experimental Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.3.1 Direct Searches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.3.2 Indirect Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
VVI Contents
5 The ATLAS Experiment at the Large Hadron Collider 37
5.1 The Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.2 The ATLAS Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.2.1 Inner Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.2.2 The Calorimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.2.3 The Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.2.4 Trigger System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
6 ATLAS Detector Performance 51
6.1 Monte Carlo Event Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
6.2 Particle Reconstruction and Identification . . . . . . . . . . . . . . . . . . . . . 52
6.2.1 Muon Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.2.2 Electron Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6.2.3 Jet Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
miss6.2.4 E Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59T
6.2.5 Reconstruction and Identification of Hadronicτ Lepton Decays . . . . . 59
6.2.6 b-Jet Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
7 The Search for Light Charged Higgs Bosons 75
7.1 Signal and Background Simulation . . . . . . . . . . . . . . . . . . . . . . . . . 75
7.2 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.2.1 Selection Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.2.2 Cut Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
7.2.3 Cut Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
¯7.2.4 Composition of thett Background . . . . . . . . . . . . . . . . . . . . . 86
7.3 Systematic Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
7.3.1 Experimental Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . 89
7.3.2 Theoretical Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . 91
7.3.3 Effect of Systematic Uncertainties . . . . . . . . . . . . . . . . . . . . . 91
¯8 Estimation of the Irreduciblett Background from Data 93
8.1 Description of the Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
8.2 Validation of the Embedding Method . . . . . . . . . . . . . . . . . . . . . . . . 96
±8.2.1 Distributions of Variables forH Searches . . . . . . . . . . . . . . . . 96
8.2.2 Cut Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
8.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
¯9 Estimation of thett Background Containing Misidentifiedτ-Jets 103
9.1 Monte Carlo Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
9.2 Data-Driven Estimation of the Light Parton Jet Rejection . . . . . . . . . . . . . 105
9.2.1 Selection of QCD Dijet Events . . . . . . . . . . . . . . . . . . . . . . . 106
9.2.2 Selection ofZ+Jets Events . . . . . . . . . . . . . . . . . . . . . . . . . 107
9.2.3 Results of the Data-Driven Rejection Measurement inp Bins . . . . . . 112T
9.2.4 Jet Shapes inZ+Jets and QCD Dijet Events . . . . . . . . . . . . . . . . 112Contents VII
9.2.5 Jet Shape Dependence of the Rejection . . . . . . . . . . . . . . . . . . 113
9.3 Background Estimation for Light Charged Higgs Searches . . . . . . . . . . . . 113
9.3.1 Description of the Method . . . . . . . . . . . . . . . . . . . . . . . . . 113
9.3.2 Background Estimation withp Dependent Rejection . . . . . . . . . . . 116T
9.3.3 Background Estimation with[p ,R ] Dependent Rejection . . . . . . . 118T em
tracks9.3.4 Background Estimation with p ,R ,p /E Dependent Rejection . 118T em TT
9.4 Background Estimation with the “loose” Identification Flag . . . . . . . . . . . . 121
9.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
10 Discovery Potential and Exclusion Limits 125
10.1 The Profile Likelihood Method . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
10.1.1 Signal Significance and Exclusion Limits . . . . . . . . . . . . . . . . . 127
10.1.2 The Likelihood Function . . . . . . . . . . . . . . . . . . . . . . . . . . 127
10.2 Charged Higgs Discovery and Exclusion . . . . . . . . . . .