School of Physics - Theses

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    Probing Higgs Boson physics in decays to tau leptons with the ATLAS experiment
    Le, Brian ( 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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    The search for the Higgs boson in tauon pairs at the ATLAS experiment
    Shao, Qi Tao ( 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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    Discovery potential of neutral MSSM Higgs bosons decaying to tau-lepton pairs in the ATLAS experiment
    Davey, William Edwin ( 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.
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    A model of fermion masses without a Higgs mechanism
    STOKES, JAMES ( 2010)
    The purpose of this thesis is to understand the origin of fermion masses in a model where the electroweak symmetry is not a fundamental gauge symmetry. Electroweak symmetry is instead a global symmetry of a new strongly interacting sector, with the electromagnetic symmetry remaining as a gauged subgroup. The W and Z-bosons are thus composite vector mesons, which obtain their mass from strong dynamics rather than the traditional Higgs mechanism in spontaneously broken gauge theories. As a result, the hierarchy problem is evaded since the compositeness scale is exponentially small compared to the Planck scale MP. Fermions obtain mass by Yukawa coupling to a scalar vacuum expectation value, which breaks electroweak symmetry at the Planck scale. Interestingly, the natural scale of fermion masses is << M_P and the hierarchical pattern of masses and mixings can be solved by Planck-scale physics. After reviewing important background material on flavor physics, extra dimensions and branes, we describe the relationship between warping and compositeness using the holographic principle inspired by the anti de-Sitter/conformal field theory correspondence (AdS/CFT). Despite the non-perturbative nature of the physics underlying composite models, the technology of AdS/CFT is employed to construct a higher-dimensional, weakly-coupled description of fermions interacting with composite electroweak vector bosons in four spacetime dimensions. The techniques of electroweak precision analysis are explained and then applied to the model, using the higher-dimensional model as a calculational tool. It is demonstrated that despite the composite nature of the W/Z-bosons, the model is consistent with electroweak precision data for the first two generations, while the third generation leads to a tension with indirect bounds. Moreover, the model is shown to predict definite deviations from the Standard Model parameters at energies that will be explored by the LHC.