School of Physics (ETDs)

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Subject: search.filters.subject.Large Hadron Collider

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    Exploring the 95 GeV Excess with Extended Scalar Models
    (University of the Witwatersrand, Johannesburg, 2024-10) Mulaudzi, Anza-Tshilidzi; Mellado, Bruce; Kumar, Mukesh
    This thesis focuses on three interconnected studies investigating the presence of an additional scalar particle, S, of mass around mS ≈ 95 GeV. In the initial study, we explore the notion that an SU(2)L triplet scalar, characterised by a hypercharge Y = 0, could be the origin of the observed 95 GeV di-photon (γγ) excesses seen at ATLAS and CMS. By thoroughly examining its properties, particularly the neutral component, and considering a small mixing angle with the Standard Model Higgs boson, we uncover that this scalar naturally exhibits a substantial branching ratio to γγ. Additionally, we find that its Drell-Yan production via pp → W∗ → HH± adequately accounts for the observed excess. The second study examines how recent measurements of the W bosons’s mass by experiments such as ATLAS and CDF affect the theoretical predictions of the Two Higgs Doublet Model augmented with a Singlet Scalar (2HDM+S) model. It addresses how this model’s parameter space is further constrained by the inclusion of vector-like leptons, focusing on their impact on the muon g − 2 measurements. The third study involves exploring the potential discovery of the aforementioned scalar at future electron-positron colliders. Employing several methodologies, including the recoil mass method in e + e − collisions (e + e − → ZS, where Z → µ + µ − and S → b ¯ b), we leverage a Deep Neural Network to refine the differentiation between the Standard Model background and the targeted signal. The outcomes not only reinforce the potential for detecting the proposed scalar, but also enhance the scientific argument for the establishment of future electron-positron colliders like CEPC, FCC-ee or ILC. Together, these studies contribute valuable insights into the evolving landscape of particle physics.
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    Search for new resonances in the four-lepton channel and implementation of the LED integrator panel for the PROMETEO system in the ATLAS Tile Calorimeter
    (University of the Witwatersrand, Johannesburg, 2024) Mtintsilana, Onesimo; Kumar, Mukesh; Mellado, Bruce
    The Large Hadron Collider (LHC) has transformed our understanding of fundamental particles and forces, notably with the seminal discovery of the Higgs boson in 2012, which completed the Standard Model (SM) of particle physics. Despite its success, the SM leaves numerous unanswered questions, motivating a quest for new physics. This thesis explores three main avenues: Firstly, it investigates the possibility of an extended Higgs sector or alternative SM extensions, focusing on heavy ZZ resonances that decay into four leptons. Using a dataset of 139 fb−1 from proton-proton collisions at the LHC, this study explores both gluon-gluon fusion and vector-boson fusion production mechanisms. Although no significant signal for a new resonance is observed, upper limits on the production cross section of spin-0 or spin-2 particles are established. These limits provide constraints on specific theoretical models, such as Type-I and Type-II two-Higgs doublet models for spin-0 resonances, and the Randall-Sundrum model for spin-2 resonances. Intriguingly, the combined results of ATLAS and CMS for Run 2 and Run 3 data in the final state of 4 leptons exhibit an excess around 250 GeV, reaching a significance of 2.4σ which is in the region of interest of the multi-lepton anomalies.. In the second part, the analysis extends to heavy boson decays resulting in a final state with four leptons, specifically focusing on the R boson or the A boson decays into a combination of the SM Higgs boson and another boson, denoted S, which further decays into dark-matter candidates. No evidence contradicting SM predictions is found, yielding stringent upper limits on the production cross-sections of these hypothesised bosons and their branching ratios at a 95% confidence level. Lastly, the thesis highlights advancements in Higgs boson studies and new particle discovery potential in the upcoming High-Luminosity LHC era starting in 2029, emphasising improvements to the ATLAS detector electronics, particularly the integration of a new LED Integrator Panel within the Prometeo portable readout module system, enabling precise calibration and monitoring of individual detector components