Glasgow Theses Service http://theses.gla.ac.uk/ theses@gla.ac.uk Ferreira de Lima, Danilo Enoque (2014) Top-antitop cross section measurement as a function of the jet multiplicity in the final state and beyond the Standard Model top-antitop resonances search at the ATLAS detector at CERN. PhD thesis. http://theses.gla.ac.uk/5015/ Copyright and moral rights for this thesis are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the Author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the Author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given Ph. D. Thesis Top-antitop cross section measurement as a function of the jet multipli c ity in the final state and beyond the Standard Model top-a ntitop resonances search at the ATLAS detector at C E RN c  Danilo Enoque Ferreira de Lima School of Physics and Astronomy College of Science and Engineering Submitted in fulfilment of the requirements for the degree of Doctor in Philosophy at the University of Glasgow February 2014 Abstract The top quark is the heaviest particle in the Standard Model, with a strong coupling to the Higgs boson. It is often seen as a window to new physics, there- fore understanding its production is a key ingredient for testing the Standard Model or physics Beyond t he Standard Model. In this document, the pro- duction cross section of top-antitop pairs in its semileptonic decay channel is measured as a function of the jet multiplicity in the ATLAS experiment, using proton-proton collisions at the center-of-mass energy o f √ s = 7 TeV. The top- antitop production with extra jets is the main background for many analyses, including the top-antitop-Higgs production studies. The analysis performed is extended in a search for Beyond the Standard Model physics which predicts a resonance decaying in a top-antitop pair, using ATLAS dat a at center-of-mass energy of √ s = 7 TeV. The latter analysis is repeated for ATLAS data col- lected with √ s = 8 TeV. Performance studies of b-tagging algorithms in the ATLAS Trigger System are also presented. Contents 1 Introduction 1 I Theoretical foundations 5 2 Theory overview 6 2.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.1 Matter fields and electroweak interactions . . . . . . . . 9 2.1.2 Quantum Chromodynamics . . . . . . . . . . . . . . . . 11 2.1.3 Electroweak symmetry breaking mechanism . . . . . . . 13 2.2 The Standard Model and the top quark . . . . . . . . . . . . . . 14 2.3 Top-antitop pair generation at the LHC . . . . . . . . . . . . . . 16 2.4 Monte Carlo event generato r s . . . . . . . . . . . . . . . . . . . 18 2.4.1 Factorisation theorem and perturbative treatment . . . . 20 2.4.2 Parton showers . . . . . . . . . . . . . . . . . . . . . . . 22 2.4.3 Next-to-leading order matrix element generators . . . . . 26 2.4.4 Hadronisation . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4.5 Underlying events . . . . . . . . . . . . . . . . . . . . . . 28 2.5 Beyond the Standard Model . . . . . . . . . . . . . . . . . . . . 29 2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 II The experimental setup 31 3 The ATLAS experiment 32 3.1 The ATLAS detector . . . . . . . . . . . . . . . . . . . . . . . . 32 3.1.1 Inner Detector . . . . . . . . . . . . . . . . . . . . . . . . 35 3.1.2 Calorimeters . . . . . . . . . . . . . . . . . . . . . . . . . 3 6 3.1.3 Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . 37 3.1.4 The ATLAS Trigger System . . . . . . . . . . . . . . . . 38 i 3.2 Multiple interactions in ATLAS . . . . . . . . . . . . . . . . . . 41 3.3 Electron reconstruction and identification . . . . . . . . . . . . . 41 3.4 Muon reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 47 3.5 Jet algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.6 b-tagging algorithms . . . . . . . . . . . . . . . . . . . . . . . . 54 3.7 Missing transverse energy reconstruction . . . . . . . . . . . . . 56 3.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4 b-jet trigger performance studies 59 4.1 b-jet trigger chains configuration . . . . . . . . . . . . . . . . . . 59 4.2 b-taggging algorithms . . . . . . . . . . . . . . . . . . . . . . . . 61 4.3 Data to Monte Carlo simulation comparison of with √ s = 7 TeV data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.4 Data to simulation comparison of the b-jet combined “physics” trigger using √ s = 8 TeV, 2012 data . . . . . . . . . . . . . . . 65 4.5 Data to simulation comparison in heavy flavour enriched sample with ATLAS 2012 √ s = 8 TeV data . . . . . . . . . . . . . . . . 68 4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 III Physics Analyses 73 5 Top-antitop differential cross section measurement as a func- tion of the jet multiplicity in the final state 74 5.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5.2 Top-antitop signal simulation and background estimates . . . . 75 5.3 Top-antitop event selection . . . . . . . . . . . . . . . . . . . . . 78 5.3.1 Tr igger and pile up-related selection . . . . . . . . . . . . 78 5.3.2 Lepton selection . . . . . . . . . . . . . . . . . . . . . . . 79 5.3.3 Jet selection . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.3.4 Missing energy requirements . . . . . . . . . . . . . . . . 80 5.4 Data-driven W+jets ba ckground estimate . . . . . . . . . . . . . 81 5.5 Data-driven QCD multi-jets background estimate . . . . . . . . 83 5.6 Corrections applied in simulation . . . . . . . . . . . . . . . . . 86 5.7 Data to signal and background comparison . . . . . . . . . . . . 89 5.8 Systematic uncertainties estimate at reconstruction level . . . . 92 5.9 Unfolding the effect of the detector . . . . . . . . . . . . . . . . 99 5.10 Propagation of systematic uncertainties through the unfolding procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 ii 5.11 Results at particle level and discussion . . . . . . . . . . . . . . 107 5.12 Correction factors and consistency checks for selections with jet cuts at 40 GeV, 60 GeV and 80 GeV . . . . . . . . . . . . . . . 1 31 6 Top-antitop resonances search at √ s = 7 TeV 147 6.1 Benchmark models and motivatio n . . . . . . . . . . . . . . . . 148 6.2 Search strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 48 6.3 Background modelling . . . . . . . . . . . . . . . . . . . . . . . 150 6.4 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 6.5 Corrections applied to simulation and data . . . . . . . . . . . . 156 6.6 Event reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 157 6.7 Systematic uncertainties . . . . . . . . . . . . . . . . . . . . . . 160 6.8 Data to expectation comparison . . . . . . . . . . . . . . . . . . 163 6.9 Limit setting and summary . . . . . . . . . . . . . . . . . . . . 171 7 Top-antitop resonances search at √ s = 8 TeV 177 7.1 Differences with respect to the √ s = 7 TeV analysis . . . . . . . 177 7.2 Multi-jet background modelling . . . . . . . . . . . . . . . . . . 178 7.3 Event reconstruction and results . . . . . . . . . . . . . . . . . . 191 7.4 Limit setting and summary . . . . . . . . . . . . . . . . . . . . 198 8 Summary 204 A Top-antitop + jets control plot distributions 206 iii List of Tables 2.1 Properties of the fundamental particles of the Standard Model. Information extracted from [7]. Particle’s masses were rounded to show their order of magnit ude. Latest measurements includ- ing errors can be found in [7]. . . . . . . . . . . . . . . . . . . . 7 5.1 Event yields for data and MC simulation in the electron and muon channels, selected with a 25 GeV jet p T threshold. The number of events passing all selection requirements are shown as a function of the reconstructed jet mulitplicity (n reco jets ). Alp- gen+Herwig is used for the t ¯ t simulation and MC expectations are normalised to an integrated luminosity of 4.7 fb −1 . The uncertainties on the expected values include systematic uncer- tainties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 5.2 Uncertainties on event yields at reconstruction level in the elec- tron channel, selected with a 25 GeV jet p T threshold. Alpgen is used for the t ¯ t simulation.The uncertainties are shown as a percentage of the expected t ¯ t signal. . . . . . . . . . . . . . . . 113 5.3 Uncertainties on event yields at reconstruction level in the muon channel, selected with a 25 GeV jet p T threshold. Alpgen is used fo r the t ¯ t simulation. The uncertainties are shown as a percentage of the expected t ¯ t signal. . . . . . . . . . . . . . . . 114 5.4 Signal reconstruction systematics and unfolding bias systemat- ics, in percentages, propagated through the unfolded distribu- tion in the electron channel. The p T cut on the jets is 25 GeV. 115 5.5 Signal reconstruction systematics and unfolding bias systemat- ics, in percentages, propagated through the unfolded distribu- tion in the muon channel. The p T cut on the jets is 25 GeV. . . 116 5.6 Signal reconstruction systematics and unfolding bias systemat- ics, in percentages, propagated through the unfolded distribu- tion in the electron channel. The p T cut on the jets is 40 GeV. 141 iv 5.7 Signal reconstruction systematics and unfolding bias systemat- ics, in percentages, propagated through the unfolded distribu- tion in the muon channel. The p T cut on the jets is 40 GeV. . . 142 5.8 Signal reconstruction systematics and unfolding bias systemat- ics, in percentages, propagated through the unfolded distribu- tion in the electron channel. The p T cut on the jets is 60 GeV. 143 5.9 Signal reconstruction systematics and unfolding bias systemat- ics, in percentages, propagated through the unfolded distribu- tion in the muon channel. The p T cut on the jets is 60 GeV. . . 144 5.10 Signal reconstruction systematics and unfolding bias systemat- ics, in percentages, propagated through the unfolded distribu- tion in the electron channel. The p T cut on the jets is 80 GeV. 145 5.11 Signal reconstruction systematics and unfolding bias systemat- ics, in percentages, propagated through the unfolded distribu- tion in the muon channel. The p T cut on the jets is 80 GeV. . . 146 6.1 Total contribution of each of the background samples in the t ¯ t resonances analysis at √ s = 7TeV in the resolved electron chan- nel with statistical uncertainties for the data and background samples, followed by the to tal systematic uncertainty for the backgrounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 6.2 Total contribution of each of the background samples in the t ¯ t resonances analysis at √ s = 7TeV in the resolved muon chan- nel with statistical uncertainties for the data and background samples, followed by the to tal systematic uncertainty for the backgrounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 6.3 Total contribution of each of the background samples in the t ¯ t resonances analysis at √ s = 7 TeV in the boosted electron chan- nel with statistical uncertainties fo r dat a and background sam- ples, followed by the systematic uncertainty for all background samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 6.4 Total contribution of each of the background samples in the t ¯ t resonances analysis at √ s = 7 TeV in the boosted muon channel with the statistical uncertainties for data and background sam- ples, followed by the systematic uncertainty for the background samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 v 6.5 Systematic uncertainties from all backgrounds in percentage variation of the t ¯ t sample, in the t ¯ t resonances analysis, in the resolved electron channel, using the maximum between the up and down variations. Total effect estimated in the yield of the background samples (no bin width weight applied). . . . . . 171 6.6 Systematic uncertainties from all backgrounds in percentage variation of the t ¯ t sample, in the t ¯ t resonances analysis, in the resolved muon channel, using the maximum between the up and down variations. Total effect estimated in the yield of the background samples (no bin width weight applied). . . . . . . . 174 6.7 Systematic uncertainties from all backgrounds in percentage variation of the t ¯ t sample, in the t ¯ t resonances analysis at √ s = 7TeV, in the boosted electron channel, using the max- imum between the up and down variations. Total effect esti- mated in the yield of the background samples (no bin width weight applied). . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 6.8 Systematic uncertainties from all backgrounds in percentage variation of the t ¯ t sample, in the t ¯ t resonances analysis, in the boosted muon channel, using the maximum between the up and down variations. Total effect estimated in the yield of the background samples (no bin width weight applied). . . . . . . . 176 7.1 Total contribution of each of the background samples in the t ¯ t resonances analysis at √ s = 8TeV in the resolved electron chan- nel with statistical uncertainties for the data and background samples, followed by the to tal systematic uncertainty for the backgrounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 7.2 Total contribution of each of the background samples in the t ¯ t resonances analysis at √ s = 8TeV in the resolved muon chan- nel with statistical uncertainties for the data and background samples, followed by the to tal systematic uncertainty for the backgrounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 7.3 Total contribution of each o f the background samples in the t ¯ t resonances analysis at √ s = 8TeV in the boosted electron channel with statistical uncertainties for data and background samples, followed by the to tal systematic uncertainty for the background samples. . . . . . . . . . . . . . . . . . . . . . . . . 192 vi 7.4 Total contribution of each of the background samples in the t ¯ t resonances analysis at √ s = 8TeV in the boosted muon chan- nel with the statistical uncertainties for data and background samples, followed by the to tal systematic uncertainty for the background samples. . . . . . . . . . . . . . . . . . . . . . . . . 192 7.5 Systematic uncertainties from all backgrounds in percentage variation of the t ¯ t sample, in the t ¯ t resonances analysis, in the resolved electron channel, using the maximum between the up and down variations. Total effect estimated in the yield of the background samples (no bin width weight applied). . . . . . 200 7.6 Systematic uncertainties from all backgrounds in percentage variation of the t ¯ t sample, in the t ¯ t resonances analysis, in the resolved muon channel, using the maximum between the up and down variations. Total effect estimated in the yield of the background samples (no bin width weight applied). . . . . . . . 201 7.7 Systematic uncertainties from all backgrounds in percentage variation of the t ¯ t sample, in the t ¯ t resonances analysis at √ s = 8TeV, in the boosted electron channel, using the max- imum between the up and down variations. Total effect esti- mated in the yield of the background samples (no bin width weight applied). . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 7.8 Systematic uncertainties from all backgrounds in percentage variation of the t ¯ t sample, in the t ¯ t resonances analysis, in the boosted muon channel, using the maximum between the up and down variations. Total effect estimated in the yield of the background samples (no bin width weight applied). . . . . . . . 203 vii [...]... which also deserves an introduction in Chapter 3 Chapter 4 shows a few performance studies in the selection of b-quark-enriched events in ATLAS The third part details the three physics analyses performed Chapter 5 explains the measurement of the top-antitop production cross section as a function of the jet multiplicity in the final state, using data of proton-proton collisions in ATLAS at a center -of- mass... that is able to predict a large amount of phenomena with excellent accuracy is called the Standard Model [4] It includes a myriad of fundamental elements with a complex interaction between them The “top” quark is one of the particles in the Standard Model and it interacts through all kinds of forces in the model: the strong interaction, the weak interaction and the electromagnetic interaction It was... was discovered at Fermilab, only in 1995 [5, 6], with interesting properties, including a large rest mass [7], compared to the other particles in the Standard Model It belongs to the classification of a “quark” in the Standard Model, of which there are six flavours An interesting effect of the fact that quarks interact through the strong force is that, in most cases, quarks cause showers of particles to... 7: The author contributed the data to simulation comparison estimates, including all systematic effects estimates and all Monte Carlo simulation background estimates (but not the W + jets data-driven parametrisation estimate) The author also contributed in the QCD multi-jets background parametrisation and estimation in this analysis xxiii Chapter 1 Introduction Particle physics is a recent topic in the. .. is the weak isospin operator 2 , whose components are the generators of the SU(2)L transformation and can be written 2 The symbol T in the superscript indicates that the transverse of the matrix is to be taken 2.1 The Standard Model 10 as T a = 1 σ a (where σ a are the Pauli matrices); α(x) is an arbitrary three2 component vector of functions of the space-time; β(x) is an arbitrary function of the. .. simulation; the propagation of systematic uncertainties through the unfolding procedure; the final unfolded data to particle-level simulaton comparison • Chapter 6: The author contributed the data to simulation comparison estimates, including all systematic effect estimates and all Monte Carlo simulation background estimates (but not the W + jets and QCD multijets backgrounds’ parametrisation estimate) • Chapter... text assumes that the Standard Model of particle physics, discussed in the next sections, is valid up to a good approximation, working as a reasonable effective theory The methods used in the physics analyses heavily rely on Monte Carlo simulation, which is also reviewed in Section 2.4 2.1 The Standard Model The best validated model of particle physics so far is the Standard Model (SM) [10, 13–26] It includes... the jet energy scale in data and simulation for Anti-kt R = 0.4 jets built using the EM scale (left) or using the LCW method (right) for 2011 ATLAS data Extracted from [87] 53 3.14 The efficiency in data and simulation (left) and their ratio (right) for the MV1 b-tagging algorithm in its 70% efficiency working point, calculated using ATLAS 2011 data and the prel method [90, T 92] Extracted from [92] ... Standard Model predicted and the observed spectra for the top-antitop invariant mass Comparisons are also done between data and alternate models for top-antitop pair production Chapter 7 extends the previous chapter, by performing a very similar analysis using data from proton-proton collisions at a center-ofmass energy of 8 TeV Chapter 8 summarises the targets proposed and the results obtained The appendix... which discusses the current understanding of the Standard Model in a brief overview, focusing on its relation with the relevant aspects of the top quark used in the studies in this thesis The second part focuses on the experimental setup used to perform the analyses The measurements and searches are done using the results of proton-proton collisions in the ATLAS [11] detector, at the Large Hadron Collider . multiplicity in the final state and beyond the Standard Model top-antitop resonances search at the ATLAS detector at CERN. PhD thesis. http://theses.gla.ac.uk/5015/ Copyright and moral. a function of the jet multipli c ity in the final state and beyond the Standard Model top -a ntitop resonances search at the ATLAS detector at C E RN c  Danilo Enoque Ferreira de Lima School of. a top-antitop pair, using ATLAS dat a at center -of- mass energy of √ s = 7 TeV. The latter analysis is repeated for ATLAS data col- lected with √ s = 8 TeV. Performance studies of b-tagging algorithms