School of Physics - Theses
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ItemSearch for the Higgs Boson produced in association with a Vector Boson at ATLAS( 2015)On the 4th of July, 2012, the ATLAS and CMS experiments at CERN announced the discovery of a particle consistent with the Standard Model Higgs boson through its decay to bosons. Subsequent measurements have established that the particle’s spin, parity and coupling strengths are in line with Standard Model expectation, making this the strongest evidence of the Standard Model electroweak symmetry breaking mechanism. This thesis concerns the search for Higgs boson decays to two tau leptons, one of the leading Standard Model Higgs decay modes, with particular focus on events in which the Higgs is produced in association with a weak vector boson. Evidence for the existence of such events constitutes a crucial test of the Higgs’ coupling to leptons. The search is conducted using the 20.3 fb−1 of data collected by the ATLAS detector in 2012 from proton-proton collisions in the Large Hadron Collider at a centre-of-mass √ energy of s = 8 TeV. The search is optimised for events in which the associated vector boson decays to an electron or muon and one or both tau leptons from the Higgs decay subsequently decay to hadrons. The dominant background comes from events in which a jet is misidentified as the hadronic decay products of a tau decay, which is estimated from data samples in a signal-free control region using the rate of misidentification. The trigger and identification efficiencies are measured using dedicated calibration data samples with a tag-and-probe technique. No significant excess above the expected background yield is observed in any signal region. The signal strength of best fit, for a Higgs boson mass of 125 GeV, is 2.3 ± 1.6. Instead, limits are placed on the rate at which the event rate might exceed the Standard Model expectation. The event rate is found to be no greater than 5.6 times that expected by the Standard Model at a 95% confidence interval. This observed limit is less than the expected limit of 3.5 times the SM event rate, under the no signal hypothesis, or 3.7 times the SM event rate with signal injected, but is consistent within uncertainties.
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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.