By Tau's Track Through the Veil of Strong Supersymmetry - and the Forging of the ATLAS 3D Pixels for Their High-Luminosity Vigil

Supersymmetry extends the Standard Model by introducing a symmetry between fermions and bosons that stabilises the electroweak scale against quadratic divergences, improves gauge coupling unification, and, with a conserved 𝑅-parity, predicts a stable lightest supersymmetric particle (LSP) as a dark matter candidate. Motivated by long-standing theoretical and phenomenological arguments for TeV-scale supersymmetry, this thesis presents a search for 𝑅-parity conserving supersymmetry in final states with jets, large $E_{T}^{miss}$ , and hadronically decaying tau leptons, interpreted within the context of simplified models that specify the cascade decays of pair-produced gluinos and squarks to the lightest neutralino LSP. The analysis uses ATLAS 𝑝𝑝 collision data of 140 fb^{−1} at $/sqrt{𝑠}$ = 13 TeV (2015–2018) and 51.8 $fb^{−1}$ at $/sqrt{𝑠}$ = 13.6 TeV (2022–2023).

No significant deviation from the Standard Model prediction is observed. Within simplified model interpretations used in this analysis, the following exclusions are obtained at 95% confidence level: gluino masses are excluded up to 2.25 TeV and the LSP is excluded up to $m_{\tilde{X}_{1}^{0}}$= 1.35 TeV (for $m_{\tilde{g}}$ ≈ 2 TeV), squark masses are excluded up to 1.7 TeV, with corresponding LSP exclusions reaching up to $m_{\tilde{X}_{1}^{0}}$= 0.85 TeV, subject to the assumed decay chains and mass splittings. These constitute the most stringent constraints in the 𝜏-enriched channels studied. Model-independent limits are set separately for Run 2 and Run 3 using simultaneous signal-plus-background fits to the most sensitive bins of the signal regions; the resulting 95% CL upper limits on the visible cross section for new-physics processes range from 0.02 to 0.31 fb across the probed regions. 

Looking ahead, the high-Luminosity LHC is expected to deliver about 3 $ab^{−1}$ at $/sqrt{s}$ ∼ 14 TeV, approximately sixteen times the dataset used in this analysis, enabling more precise Standard Model measurements and enhanced sensitivity to rare phenomena. To operate in the associated high pile-up conditions, ATLAS will deploy an all-silicon Inner Tracker with increased granularity, radiation tolerance, and coverage. As a contribution to the HL-LHC upgrade, this thesis documents the development, qualification, and utilisation of a quality-control setup at the University of Bergen (UiB) for ATLAS ITk 3D silicon pixel modules and their main assembly components, namely bare modules (3D sensor and ASIC assemblies) and flexible printed circuit boards (flex PCBs). The testing campaign that led to the qualification of UiB as an ITk reception site for bare modules, flex PCBs, and full modules is reported, and selected results obtained with this setup are also presented.