A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. August 11, 2014. / The Z' gauge boson is a hypothetical neutral particle of spin-1 appearing in many theories beyond the Standard Model. In the context of the Z' boson of the Sequential Standard Model in the hadronic decay channel Z' ThadThad, this project investigates improving the sensitivity of the analysis. It is important in the analysis to have high signal e ciency and to be able to distinguish against background processes, thus this project begins with an investigation into new triggers and their combinations that aim to improve the signal e ciency, followed by a study to improve the tau identi cation using tracking. Monte Carlo samples were compared with the data sample for various cut-based variables to investigate which of the variables o er a good discrimination power against the multijet background. The signi cance of each trigger and trigger combinations are shown, as well as the performance of di erent techniques and ideas used to improve the tau identi cation using tracking. Since it is important in this project to be able to reconstruct a tau lepton from its hadronic decay products, this dissertation takes the opportunity to present preliminary results on the study of the light collection uniformity and response in the crack scintillation counters of the ATLAS hadronic tile calorimeter as part of the contribution towards the detector upgrade.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 126-132).

Thesis (Ph. D.)--University of Oregon, 2000. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 144-147). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (Ph. D.)--University of Chicago, Dept. of Physics, December 2003. / Includes bibliographical references. Also available on the Internet.

No description available.

Following the announcement of the discovery of a new particle on the 4th of July 2012 at the ATLAS and CMS experiments at the LHC, many efforts were needed for the understanding of its properties and to discern whether it is the Standard Model Higgs boson. The research presented in this thesis is based on the H → ZZ(*) → 4l decay channel. Three main contributions are discussed: the Standard Model Higgs boson mass measurement, the search for a heavy Higgs boson, and lastly, the implementation of a kinematic likelihood fitter as a new approach to improve the invariant mass resolution of the final states. The Standard Model Higgs boson mass measurement is presented. The measured mass is 124:51± 0:52(stat)± 0:06(syst) GeV for the combined data taken during 2011 and 2012 (4:6 fb-¹ at 7 TeV and 20:7 fb-¹ at 8 TeV). Contributing to the mass measurement, a tool was developed to validate the model used by generating several pseudo datasets from Monte Carlo samples and fitting them with the profile likelihood. The results show that the model is correct and only small deviations are seen in the parameters of interest, mH, and the signal strength, μ. Studies in the asymptotic limit show that these deviations are a symptom of low statistics in some of the final states. The search for a heavy Higgs boson is presented as well. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the profile likelihood gives 95% confidence level upper limits on the production cross-section of a heavy Higgs times the branching ratio to Z boson pairs in the mass range from 140 GeV to 1 TeV. Contributing to this search, a pseudo dataset, called Asimov dataset, is created from the Monte Carlo samples to test the profile likelihood fits and validate the model used. The results show that fit the model is correct. In addition, the limits are also interpreted in the context of Type I and Type II Two Higgs Doublet Models (2HDM). Finally, a Kinematic Likelihood Fitter (KLFitter) is studied and used to constrain the Z boson mass as an alternative to the standard tool used for the 2011 and 2012 measurement. This affects the distribution of the invariant mass, m4l, from which the Higgs boson mass is inferred. Small improvements are seen in the invariant mass resolution when higher hypothetical Higgs boson masses are considered.

This thesis presents a search for R-parity violating supersymmetry at sqrt(s)=13 TeV, using approximately 13.3 fb-1 of data collected by ATLAS in 2015 and the first half of 2016. Events are required to contain at least four leptons (electrons or muons only) that are not the product of a Z boson decay, and this requirement results in a low Standard Model background and a high sensitivity to various physics models beyond the Standard Model. No significant deviations from the Standard Model are observed in data, and results are used to set upper limits on the event yields from processes beyond the Standard Model. In a simplified model of chargino production with indirect R-parity violating decays, limits are extended by approximately 400 GeV relative to the Run 1 search, excluding chargino masses below 1.1 TeV.

This thesis describes an analysis of the process e⁺e^— → l⁺l^- (where l = e, μ,, τ) at centre-of-mass energies between 88 GeV and 94 GeV, using the data collected by the DELPHI detector between the years 1991 and 1993. The leptonic decays of the Z° boson are selected without attempting to separate the three lepton types, thus making it an inclusive lepton analysis. The theory behind lepton pair production is introduced and the extraction of various electroweak parameters from the experimental observables is discussed. The LEP collider and the DELPHI detector are described, with special emphasis being given to the sub-detectors used in the analysis. The criteria used to select a high purity leptonic sample are described along with calculations of various backgrounds and efficiencies. The sample of selected leptonic events is then used to measure the cross-sections and forward-backward asymmetries. Finally, a fit to these cross-sections and asymmetries, together with the hadronic (e⁺e^- → qq̄) cross-sections, is carried out. Various Z° parameters are obtained: the mass and total width Mz-91.1876 ± 0.0052 GeV/c², Γ_z = 2.4971 ± 0.0061 GeV, the ratio of the hadronic to leptonic partial widths R_l = 20.73 ± 0.09, and the pole leptonic asymmetry (A°_FB)¹ = 0.0195 ± 0.0042. Using these results and the value of the strong coupling constant (α_s), determined by the DELPHI collaboration, the number of light neutrino species is determined to be N_ν = 3.045 ± 0.035. The leptonic partial width is found to be: Γ_l = 83.82 ± 0.29 MeV. Using the measured leptonic forward-backward asymmetries, the squared vector and axial-vector couplings of the Z° to charged leptons are found to be (ĝ_v¹)^{2</sup = (1.65 ± 0.36) x 10^-3 and (ĝ_a¹)}2</sup> = 0.2505 ± 0.0009. These values can be used to determine the effective rho parameter and the effective weak mixing angle: p̂ = 1.0020 ± 0.0036, and sin² θ^eff_lept = 0.2297 ± 0.0024. A full Standard Model fit to the data gives the values of the strong coupling constant, α_s, and the mass of the top quark, m_top, as being: α_s = 0.123 ± 0.010, m_top = 178⁺²²_-25 (expt)<sup>+18<sup>_-16(Higg s)GeV/c², where 60 < m_Higgs (GeV/c²) < 1000 with a central value of 300 GeV/c². All the results obtained agree with the results from the lepton-identified analyses (analyses in which leptonic events are selected on the basis of their individual flavour) and with the predictions of the Standard Model.

The Standard Model of particle physics constitutes the most accurate and comprehensive known description of the fundamental building blocks of the universe. However, overwhelming evidence suggests that the theory is incomplete and that new physics may be hiding at the TeV-scale. The Large Hadron Collider (LHC) at CERN probes these high-energy scales, opening a potential gateway to access physics beyond the Standard Model (BSM). Long-lived particles (LLPs) arise in many promising BSM theories, but they remain weakly constrained at the LHC. This thesis presents a novel search for displaced and delayed diphoton and dielectron vertices originating from the decay of a neutral LLP. The analysis uses the full LHC Run 2 dataset of pp collisions at a center-of-mass energy of √s = 13 TeV recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb-1. The search harnesses the capabilities of the ATLAS Liquid Argon calorimeter to precisely measure the displacement and delay of the final state electromagnetic objects. The results are interpreted in a gauge-mediated supersymmetry breaking model that features the pair-production of LLPs, with each LLP subsequently decaying into either a Higgs boson or a Z boson. Since no significant excess is observed above the background expectation, the results are used to set upper limits on the cross section of LLP pair-production for signals with an LLP mass between 100 and 725 GeV and lifetime between 0.25 ns and 1 μs. A model-independent limit is also set on the production of pairs of photons or electrons with a significantly delayed arrival at the calorimeter.

A search for the production of three massive vector bosons with at least one Z boson is presented. 140 fb⁻¹ of pp collisions at center-of-mass energy sqrt(s) = 13 TeV collected with the ATLAS detector at the LHC from 2015 to 2018 are analyzed. Events with three leptons and at least one reconstructed jet are discussed, targeting the WWZ → lνqqll and WZZ → lνqqll decay channels. Evidence for the searched-for production process is observed. Additional results demonstrating the combination of the three lepton search with searches including four or more leptons are shown. Pending the unblinding and combination of these results, there is potential for the discovery of the searched-for production process.