Search for the supersymmetric partner to the top quark in the all-hadronic final state with the ATLAS detector

The Large Hadron Collider and the experiments which rely on it constitute the largest research project for particle physics, both in terms of physical hardware and computing resources, to allow us to probe the frontiers of our understanding of nature, commonly referred to as the Standard Model (SM) of particle physics. A well motivated extension to the existing SM is that of supersymmetry (SUSY), posited to resolve some of the pressing limitations of the SM. SUSY predicts that for each given fermion spin state, there is an associated boson partner and vice-versa. This thesis will cover the decay where the scalar partner to the top quark ($\tilde{t}_1$) is pair produced and decays to a top quark and a stable supersymmetric particle called a neutralino ($\tilde{\chi}^0$). This search is undertaken using the ATLAS detector between LHC run periods 2015-2018 (LHC Run 2) corresponding to a centre-of-mass energy $\sqrt{s}=13\textrm{ TeV}$ and a total integrated luminosity of $139.0 \textrm{fb}^{-1}$, in particular considering events with no electrons or muons in the final state. Since the supersymmetric particles must be reconstructed from their final-state decay products, efforts were undertaken to constrain the SM backgrounds which produce the same final states, primarily the backgrounds arising from the $t\bar{t}+Z(\to\nu\nu)$ and $Z(\to\nu\nu)+\textrm{jets}$ processes. This thesis in addition also covers the workflows required from collision event (simulated using Monte Carlo or detected) to a final result. In the absence of any significant excesses, exclusion limits at the 95\% confidence level have been applied on the visible cross section for models with a branching ratio $BR(\tilde{t}_1\to t+\tilde{\chi}^0_1)=100\%$ up to $1.3\textrm{ TeV}$ in the case where $\Delta(m_{\tilde{t}_1},m_{\tilde{\chi}^0_1})>m_{\textrm{top}}$. Exclusion limits at the 95\% confidence level have been applied up to $\Delta(m_{\tilde{t}_1},m_{\tilde{\chi}^0_1})=(600,450)\textrm{ GeV}$ for signal models where $\Delta(m_{\tilde{t}_1},m_{\tilde{\chi}^0_1})\sim m_{\textrm{top}}$, similarly up to $(m_{\tilde{t}_1}\sim 660 \textrm{ GeV})$ for models where $ \Delta(m_{\tilde{t}_1},m_{\tilde{\chi}^0_1})<m_{\textrm{top}/W$ through the use of a new search strategy targeting four body decays ($\tilde{t}_1\to bqq'\tilde{\chi}^0$). \end{abstract}