Searching for Supersymmetry using the Higgs Boson

The Large Hadron Collider and its experiments constitute the largest particle physics research programme to date, allowing for extensive research of the existing Standard Model and for potential evidence of physics beyond that of our current comprehension. This thesis documents a search for new physics and development towards future silicon pixel detectors for the envisioned upgrade to the ATLAS pixel detector in accordance with the High Luminosity-LHC project. Supersymmetry represents a well motivated extension to the existing Standard Model that could provide solutions to some of its most significant and unresolved problems. This thesis presents the search for the supersymmetric partner to the bottom-quark, the bottom-squark ($\tilde{b}_{1}$), using the full ATLAS Run 2 dataset. Following from its recent discovery, the Higgs boson is utilised as a tool in order to search for new physics in complex decay chains involving other supersymmetric particles known as neutralinos ($\tilde{\chi}^{0}_{2}$ , $\tilde{\chi}^{0}_{1}$). Given the dominant decay of the Higgs boson into a pair of $b$-quarks, the search targets final states with up to six hadronic deposits identified to have originated from $b$-quark fragmentation and large missing transverse momentum, posing an unlikely final state to have arisen from known Standard Model processes. In the absence of a significant deviation from the Standard Model expectation, limits on both generic models of new physics and on the signal models in question are derived. Exclusion limits at the 95% confidence level are placed on the visible cross-section and on the mass of the bottom-squark for various assumptions about the mass hierarchy of the $\tilde{b}_{1}$, $\tilde{\chi}^{0}_{2}$ and $\tilde{\chi}^{0}_{1}$. Bottom-squark masses up to 1.5 TeV are excluded for models with fixed m($\tilde{\chi}^{0}_{1}$) = 60 GeV and $\tilde{\chi}^{0}_{2}$ masses ∈ [0.5,1.1]TeV. In the case of the ∆m($\tilde{\chi}^{0}_{2}$,$\tilde{\chi}^{0}_{1}$) = 130 GeV signal scenario, bottom-squark masses up to 1.3 TeV are excluded for $\tilde{\chi}^{0}_{2}$ masses up to 750 GeV. As the first search for such scenarios carried out by ATLAS in Run 2, these results represent a considerable increase to the previous constraints on bottom-squark production. The comparatively lower fluence expected at the outer layers of the upgraded pixel detector during HL-LHC represents an opportunity to instrument the detector with a promising candidate technology. The HV-CMOS technology is a novel pixel detector concept that allows the pixel sensor and readout chip to be implemented in the same silicon bulk, allowing for significant reduction in cost of production. Prototype devices fabricated in different feature-sizes and resistivities were characterised after irradiation through extensive beam test measurements. The devices proved to be highly efficient at over 99% for all devices and with excellent timing properties. These promising results represent an initial step towards the development of fully monolithic HV-CMOS pixel detectors.