Reconstructing the Top Quark in a Search for a Pair-Produced Supersymmetric Partner in the All-Hadronic plus Missing Energy Final State Using $139$ $\text{fb}^{-1}$ of $\sqrt{s}=13$ TeV Proton-Proton Collisions Delivered by the Large Hadron Collider and Collected by the ATLAS Detector

The Large Hadron Collider (LHC), the world's most powerful particle accelerator, is operated by the CERN laboratory near Geneva, Switzerland and was built to probe the tera-electron-volt energy scale in search of New Physics. ATLAS is one of several international collaborations at CERN and uses a 7,000-ton general purpose detector to collect collision data from the LHC. The top quark is the most massive particle under the Standard Model and carries the largest Yukawa coupling to the recently discovered Higgs Boson, making it sensitive to effects of heavy new physics. Supersymmetry offers a diverse class of theoretical models providing potential solutions to the most salient phenomenological inconsistencies of modern particle physics, namely it provides a mechanism for stabilizing the Higgs boson mass while predicting the existence of several new particles at the tera-scale. This dissertation presents a search for the pair production of a supersymmetric partner to the top quark, the stop ($\tilde{t}$), using $139$ $\text{fb}^{-1}$ of proton-proton collision data collected by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC) at a center of mass energy of $\sqrt{s}=13$ TeV. This dissertation focuses on understanding the hadronic top decay and its reconstruction with the ATLAS trackers and calorimeters, as well as estimating the Standard Model $t\bar{t}$ background to the search in the signal regions design to be sensitive to boosted and semi-resolved top decays. The experimental signature of the search presented is: at least four jets originating from two hadronically-decaying top quarks and large missing transverse energy from the pair of stable, light, and neutral supersymmetric neutralinos ($\chi^{0}_{1}$). No excesses over the expected Standard Model predictions were observed and exclusion limits can be placed to stop masses up to 1.25 TeV, assuming a $100\%$ branching fraction of $\tilde{t}\rightarrow t\chi^{0}_{1}$. This dissertation includes previously published and unpublished co-authored material.