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

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TECHNISCHE UNIVERSITÄT MÜNCHENMax-Planck-Institut für Physik(Werner-Heisenberg-Institut)Search for Light Charged Higgs Bosons inHadronic τ Final States with the ATLAS DetectorThies EhrichVollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München zurErlangung des akademischen Grades einesDoktors der Naturwissenschaften (Dr. rer. nat.)genehmigten Dissertation.Vorsitzender: Univ.-Prof. Dr. A. IbarraPrüfer der Dissertation:1. Priv.-Doz. Dr. H. Kroha2. Univ.-Prof. Dr. L. OberauerDie Dissertation wurde am 17. Juni 2010 bei der Technischen Universität München eingereichtund durch die Fakultät für Physik am 7. Juli 2010 angenommen.AbstractCharged Higgs bosons are predicted in theories with a non-minimal Higgssector 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 channeland suggests the possibility of using the unique signature of hadronic τ finalstates 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 thedecay chain of the second top quark allow for efficient triggering.

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Published 01 January 2010
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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 . . . . . . . . . . . . . . . . . . . . . . 128
11 Summary 131
A Results of Data-Driven QCD Jet Rejection Measurements 133
B Performance of theτ-Jet andb-Jet Identification 141Chapter 1
Introduction
In the nineteen-sixties, the Standard Model of particle physics was developed to describe the ele-
mentary constituents of matter and their interactions. Three of the four known interactions, namely
the electromagnetic, the weak and the strong interactions, are described by gauge theories requir-
ing invariance under transformations of the gauge symmetry groupU(1)⊗SU(2)⊗SU(3). To
date, the predictions of the Standard Model are in excellent agreement with experimental data.
However, one basic ingredient of the Standard Model has not been observed so far. The Higgs bo-
son, associated with the generation of particle masses by spontaneous gauge symmetry breaking,
remains elusive. Electroweak precision measurements indicate a rather light Higgs boson with a
mass below 186 GeV, which allows for either its discovery or its exclusion at the LHC.
Even if the Higgs boson is found, there are doubts that the Standard Model fully describes nature
up to the highest energies far beyond the electroweak scale of about 1 TeV since it cannot explain
why the Higgs boson should be light. Another argument for physics beyond the Standard Model is
the unification of the couplings of the three gauge interactions at high energies. Finally, the matter
in the universe is dominated by dark matter, not described by the Standard Model.
These problems are solved by extending the Standard Model with Supersymmetry, a symmetry
relating fermions and bosons. It postulates superpartners for each Standard Model particle and at
least five Higgs bosons, three of them neutral and two charged. One neutral Higgs boson is pre-
dicted to be naturally light and the three gauge couplings can unify at high energies. Depending
on the choice of parameters, one of the new particles is a candidate for the observed dark matter
in the universe.
The Minimal Supersymmetric Extension of the Standard Model (MSSM) is the most simple and
± 1best studied supersymmetric theory of elementary particles. Charged Higgs bosons (H ) are
+produced in decays of the top quark, t → H b, if they are light enough. Due to the high pro-
¯duction cross section of tt quark pairs in proton-proton collisions at a center of mass energy of
14 TeV, light charged Higgs bosons are copiously produced at the LHC, if they exist. In most
+of the MSSM parameter space, the decayH → τν acquires a branching ratio of close to one,
¯allowing for searches in tt events with final states including τ leptons. In this thesis, a strat-
egy is developed to search for light charged Higgs bosons in the semi-leptonic decay channel
1 +In the following, only one of the two charged Higgs boson statesH is mentioned implying the corresponding
−relation for the charged conjugated stateH .
12 Chapter 1 – Introduction 1.0

+ ¯ ¯¯tt → (H b) Wb → (τ νb) ℓν¯b with the ATLAS detector. Emphasis is put on the re-had
duction of detector related systematic uncertainties by estimating the dominant Standard Model
¯background oftt production without decays in charged Higgs bosons from data. This background
consists of two contributions, one with a correctly identifiedτ-jet and one where a light parton jet
is wrongly reconstructed as aτ-jet. In each case, the accuracies of the proposed methods are in-
vestigated using differentτ-jet identification algorithms. Signal significances and exclusion limits
are calculated for the MSSMm -max scenario.h
This thesis is organized as follows: In Chapters 2 and 3 the Standard Model and its supersymmet-
ric extension are outlined. The phenomenology of charged Higgs boson production and decay at
the LHC is presented in Chapter 4, while the ATLAS experiment is described in Chapter 5. Chap-
ter 6 is dedicated to the investigation of the particle reconstruction and identification performance
of the ATLAS detector using Monte Carlo simulation. The search strategy for light charged Higgs
¯bosons is presented in Chapter 7, while in Chapters 8 and 9 the methods for estimating the tt
background from the data are described and their accuracies are estimated. The resulting expec-
tations for the achievable signal significances and exclusion limits for light charged Higgs bosons
are summarized in Chapter 10.