School of Physics - Theses
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ItemSearching for signals of Dark Matter produced with top quark pairs using the ATLAS detector( 2019)Understanding the nature of Dark Matter is a key goal in modern physics. The observed gravita-tional interactions of galaxies and galactic clusters, along with theories of structure formation in the early universe, indicate the existence of Dark Matter. Evidence of the specific nature of Dark Matter remains elusive however. Particle collider experiments search for evidence of Dark Matter production within energetic proton collisions. One strategy employed in this field is to make minimal assumptions about new particles and couplings to Standard Model particles, in order to explore the range of possibilities without being overly constrained by narrow assumptions. This thesis focuses on the assumption that Dark Matter couples strongly to the heavier quarks, which motivates searching for processes where it is produced in association with pairs of top quarks. An analysis is presented on the 2015 and 2016 “Run 2" dataset taken with the ATLAS detector, consisting of 36.1 fb -1 of proton-proton collisions at the Large Hadron Collider. This analysis studies the hypothesis of Dark Matter production in conjunction with hadronically decaying top quarks. No excess above the estimated Standard Model backgrounds is observed, and constraints on the allowed cross-sections are presented. When making minimal assumptions about the nature of Dark Matter, scalar mediator masses below 20 GeV are excluded. These results are then translated to more specific and complete Two Higgs Doublet models that feature for example in Supersymmetry that also predict the same final states, and constraints on the parameter space of these models are presented.
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ItemBackground estimation studies for hadronically decaying tau leptons at the ATLAS experiment( 2018)This project aims to develop a data-driven technique for the estimation of the dominant background contribution in the inclusive search for new physics signals where equally charged lepton pairs are featured in the final state and where an hadronically decaying tau lepton can be found in a pair. The studies presented in this thesis were performed with data collected by the ATLAS experiment. A data driven technique has been developed for the abundant background of jets originated from the hadronisation of quarks or gluons which are mis-identified as hadronically decaying tau leptons. Mis-identification weighting factors have been measured for the extrapolation of this background into the signal region of the analysis and have been validated using a selection independent with respect to the the signal region. Systematic uncertainties have also been estimated. The work presented in this thesis will be incorporated in a general extrapolation technique within the ATLAS experiment aiming to be used by all ATLAS searches featuring hadronic tau decays in the final state.
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ItemProbing Higgs Boson physics in decays to tau leptons with the ATLAS experiment( 2019)The discovery of the Higgs boson in 2012 has opened up a new sector of particle physics to measurements of yet undiscovered couplings. Measurements of the numerous couplings of the Higgs boson - primarily through studies of bosonic decays - have consistently confirmed the Standard Model of Particle Physics. The measurement of fermionic Yukawa couplings, which is a free parameter of the Standard Model, is a critical test of the origin of mass in the Standard Model and serves as a probe for a wide array of new physics models. This thesis presents a measurement, search and feasibility study of couplings of the Higgs boson to the third generation lepton, the tau . A measurement is presented of the H -> tau tau cross-section which is performed using 36.1 inverse fb of sqrt(s) = 13 TeV collision data collected at the Large Hadron Collider by the ATLAS detector. A 6.4 sigma excess over the background-only hypothesis was observed in combination with sqrt(s) = 7, 8 TeV which constitutes a discovery of the final remaining coupling to third generation fermions. The total cross-section was measured as 3.70 +- 0.58(stat) +0.89-0.75(syst) pb, consistent with the expectation from the Standard Model. Several new physics models predict phenomena which can probed in supressed Higgs boson decays: one such phenomena is lepton flavour violation. A search for lepton flavour violating Higgs boson decays to e,mu and mu,e final states, using a new multivariate approach, is also detailed. This search used the same dataset as for the H -> tau tau cross-section. No excess is observed and upper limits on the branching ratio for H -> mu tau and H -> e tau of 0.47% and 0.28% are set, respectively. The limit on H -> e tau is now the new global limit. Finally a feasibility study for measuring the state of the Higgs boson using decays to two tau leptons is demonstrated. At tree-level, the H -> tau tau decays are sensitive to CP admixture couplings to a Higgs boson. One of the promising decay channels, where the tau lepton decays to three charged pions, is shown to be viable thanks to development of a robust neural network approach.
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ItemSearch for the Higgs boson in the WH production mode with H→WW* decay using the ATLAS detector( 2018)In order for fundamental particles to attain mass, the electroweak symmetry of the Standard Model (SM) of particle physics must be broken. The simplest way of breaking this symmetry is via the SM Higgs Mechanism, and it predicts the existence of a new particle called the SM Higgs boson. This particle should be experimentally accessible at the high-energy frontier, and so its discovery is considered as one of the most important goals in modern particle physics. This goal was partially achieved in July 2012, when the ATLAS and CMS experiments at the Large Hadron Collider (LHC) announced the discovery of a Higgs-like particle with a mass of around 125 GeV. The next step for both experiments is to scrutinize the properties of this new particle in order to determine whether it is, in fact, the SM Higgs boson. One high-priority objective of the ATLAS experiment is to confirm the SM predictions for Higgs boson production at the LHC. The main result presented in this thesis contributes to this objective by searching for the WH production mode with subsequent H→WW* decay. The H→WW* decay channel provides a sensitive probe of WH production due to its large branching ratio and clean detector signature. Moreover, this signal process provides important information on the Higgs boson couplings, since it only involves couplings to W bosons at both the production and decay vertices at tree-level. The search for this signal process was conducted using LHC proton-proton collision data collected by the ATLAS detector. This data was recorded at centre-of-mass energies of 7, 8 and 13 TeV corresponding to integrated luminosities of 4.5, 20.3 and 5.8 fb−1, respectively. The analysis strategy was first developed using the 7 and 8 TeV data samples and it was used to measure the relative signal strength with respect to the SM expectation. For a Higgs boson of mass 125 GeV, the observed value of the signal strength was determined to be 0.72 +1.2−1.1 (stat.) +0.4−0.3 (sys.). The analysis method was then extended to perform the first measurement of the signal strength at 13 TeV, with this quantity measured to be 3.2 +3.7−3.2 (stat) +2.3−2.7 (sys). An overall excess was observed at 0.66σ (0.77σ) significance with respect to the background-only hypothesis in the 7+8 TeV (13 TeV) data. All measurements are in agreement with the SM expectations.
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ItemMeasurements of the ATLAS tau trigger reconstruction and identification efficiencies using 2016 data from \(pp\) collisions at\(\sqrt s \) = 13 T\(e\)V( 2017)This thesis presents the performance of the tau trigger algorithm used by the ATLAS experiment to select hadronically decaying tau leptons in the LHC Run 2. Using the 33.3 \(f{b^{ - 1}}\) of \(pp\) collisions data recorded in 2016 at\(\sqrt s \) = 13 T\(e\)V, the performance of this algorithm is studied using a `tag-and-probe' based analysis in order to select Z boson decays to tau leptons, where one tau decays hadronically and the other leptonically. The reconstruction and identification efficiencies of the tau trigger algorithm are measured, and good performance is observed. The efficiency of the tau trigger in data is compared with that in simulation, and is parametrised as a function of the tau decay topology, its kinematics, and the average number of interactions per bunch crossing. The selection efficiency at each step of the high level trigger is measured, using dedicated intermediary triggers, and good agreement between data and simulation is observed. Using the comparison between reconstruction and identification efficiencies in data and simulation, correction factors for simulated events are measured, which are utilised by the entire ATLAS collaboration.
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ItemHiggs boson and new physics searches in events with same-sign lepton pairs at the LHC( 2017)The physics of elementary particles is well described by the Standard Model of particle physics. This theory predicts with great accuracy the electroweak and strong interactions of elementary particles. The introduction of the Brout-Englert-Higgs mechanism in Standard Model has also given justification to the non-null mass of the elementary particles. This mechanism has been validated by the discovery of the Higgs boson and the measurement of its properties. Despite the enormous and broad success of the Standard Model, a few observations cannot be explained by this theory. For instance, neutrino oscillations would require the neutrinos to have non-null mass in contrast with the Standard Model prediction. Several solutions to this problem have been proposed, but none has been validated yet. Some of these theories have additional modes to produce lepton pairs with same electric charge (same-sign lepton pairs). This production is also expected to be enhanced in other theories beyond the Standard Model. The focus of this thesis is the inclusive search for anomalous production of same-sign lepton pairs in proton-proton collisions. The specific case of resonant production, typical of the Type-II see-saw and Zee-Babu models of neutrino mass generation, is also considered. 20.3/fb of data collected by the ATLAS experiment at a centre-of-mass energy of 8 TeV is analysed. No significant production above the Standard Model prediction is found, thus the results are used to set upper limits on the cross-section of processes beyond the Standard Model. Limits are placed as a function of the di-lepton invariant mass within a fiducial region defined according to the event selection criteria. Exclusion limits are also derived for specific models of resonant production. The production of same-sign lepton pairs from proton-proton collisions occurs rarely in the Standard Model. One process which generates same-sign lepton pairs is the associated production of a Higgs boson and a top anti-top quark pair (ttH). The cross-section of this production mode is proportional to the Top Yukawa coupling squared, while other processes are only sensitive to the combination of this constant with several other couplings. The measurement of the ttH cross-section therefore constitutes an important test of the Standard Model and the Brout-Englert-Higgs mechanism. The ttH production has never been observed before, and is studied in this thesis. The analysis uses collisions at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment containing a same-sign lepton pair and jets. The measured production cross-section is 2.8+2.1−1.9 times the value expected in the Standard Model. This result has been combined with other analyses to obtain a more precise measurement of the ttH cross-section and the Top Yukawa coupling.
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ItemThe search for the Higgs boson in tauon pairs at the ATLAS experiment( 2013)The Higgs boson is a particle that’s predicted to exist by spontaneous electroweak symmetry breaking. Electroweak symmetry breaking is an essential part of the Standard Model of particle physics, as it generates masses for the electroweak gauge bosons. Finding the Higgs boson is integral to our understandings of the fundamental particles and their interactions. Searches for the Higgs boson are conducted by the ATLAS experiment using proton-proton collisions at the Large Hadron Collider. One of these searches is performed using the H→ττ decays, which has a clean detection signature and, with H →bb, is one of the only two viable fermonic search channels. Using the 4.7 f b−1 of data collected at √s = 7 TeV, the H→ττ analysis excludes the Higgs boson at approximately 3 times the expected cross section for 100 < mH < 120 GeV and 5 to 12 times the expected cross section for 130 < mH < 150 GeV. The H→ττ search results are combined with those from the other channels to achieve better sensitivities. The combined results have excluded most Higgs masses between 110 and 500 GeV. The only region that is not excluded is at mH = 126 GeV, where an excess above the background expectations is observed in multiple bosonic channels. This excess has a combined local significance of 5.9 σ. ATLAS claims this observed excess as a discovery of a new bosonic particle, whose properties have thus far been measured to be consistent with that of the Higgs boson.
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ItemSearching for the Light Supersymmetric Top Quark with the ATLAS experiment( 2011)The nature of dark matter and the source of the matter-antimatter asymmetry in the universe are two of the most important questions in particle physics and cosmology. The current Standard Model of particle physics, while being a very successful description of the observed fundamental particles and their interactions, cannot fully account for either of these phenomena. Theoretical extensions of the Standard Model, however, possibly can. One such extension is the Minimal Supersymmetric Standard Model (MSSM). This thesis begins by exploring the MSSM parameter space in which the matter- antimatter asymmetry of the universe is dynamically generated through electroweak baryogenesis. In this scenario, one of the supersymmetric partners to the top quark, the light stop quark, must be lighter than the top quark. It is found that this parameter space region is highly constrained by experimental limits on the electric dipole moment of the electron and the branching ratio of a bottom quark into a strange quark and a photon. If the additional requirement of matching the observed dark matter abundance by the relic density of the lightest supersymmetry particle is necessitated, the allowed MSSM parameter space is further constrained. The focus of the thesis then moves to the investigation of the collider phenomenology of supersymmetric electroweak baryogenesis, in particular, the evaluation of the discovery potential of light stop quark pair production at the LHC using the ATLAS experiment. This study assumes a light stop decay topology involving the lightest chargino and neutralino where the visible final state products mimic those from top quark pair production. Feasibility studies are performed for proton-proton collisions at centre of mass energies of 10 TeV and an integrated luminosity of 1/fb, concentrating on the dileptonic and semileptonic decay channels where there are two or one charged leptons in the final state. It is found that signal points with stop masses less then 120 GeV and stop-neutralino mass differences greater than 60 GeV have the greatest discovery potential in the dileptonic decay channel, while the semileptonic decay channel is swamped by backgrounds and requires detailed understanding of the detector and backgrounds in order to extract a signal. Finally, a preliminary study is conducted on 41.4/pb of data collected at collisions with centre of mass energies of 7 TeV in the dielectron decay channel, focusing on the understanding of selection variables and backgrounds.
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ItemDiscovery potential of neutral MSSM Higgs bosons decaying to tau-lepton pairs in the ATLAS experiment( 2011)The Large Hadron Collider (LHC) is the highest energy particle collider ever built. It recently began operation at CERN and will probe physics at unprecedented scales. ATLAS is a particle detector located at one of the collision points on the LHC ring and is designed to be sensitive to the wide range of physics that could be produced. The primary objective of the LHC experiments is to determine the mechanism of electroweak symmetry breaking, of which many theoretical models exist. In the Minimal Supersymmetric Standard Model (MSSM), electroweak symmetry breaking is achieved through the Higgs mechanism, however, the Higgs sector must be extended with respect to the Standard Model and contains five physical Higgs bosons. The discovery potential of the MSSM Higgs bosons in ATLAS has been evaluated in previous studies, demonstrating adequate sensitivity for discovery or exclusion over a large region of the parameter space. However, these studies were performed using now outdated software, without an estimation of the expected systematic uncertainties or the inclusion of data-driven background estimation procedures. In this thesis, the discovery potential of the neutral MSSM Higgs bosons when decaying to tau-lepton pairs in the ATLAS experiment is evaluated. One tau is required to decay leptonically while the other is required to decay hadronically. Higgs boson mass hypotheses in the range 150 GeV - 800 GeV are considered. The study assumes a proton-proton collision energy of 14 TeV and an integrated luminosity of 30/fb. The expected systematic uncertainty on the background measurement is evaluated and included in the calculation of the discovery potential. Data-driven estimation techniques are developed for the W+jets and QCD di-jet backgrounds. The contributions of all signal and background processes are estimated using Monte Carlo simulated event samples. The discovery potential is interpreted in the mh-max benchmark scenario, and is presented in the mA-tan(beta) plane. A small degradation in performance with respect to the previous studies is found for Higgs boson masses below 450 GeV due to the inclusion of systematic uncertainties. It is confirmed that a large fraction of the parameter space will be accessible to the ATLAS experiment, which will be able to probe far beyond the regions already excluded by the LEP and Tevatron experiments. Two separate studies are also included, describing contributions to the modelling of hadronic tau reconstruction in the ATLAS fast simulation packages ATLFAST-I and ATLFAST-II. Firstly, a complete parameterisation of the calorimeter-based hadronic tau reconstruction for use in ATLFAST-I is presented. Secondly, the validation of the track-based hadronic tau reconstruction in ATLFAST-II is presented, including the extraction of correction terms to match the performance in ATLFAST-II to the standard ATLAS simulation.