Search for Long-Lived Particles Decaying into Displaced Hadronic Jets in the Muon Spectrometer in $\textit{pp}$ Collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

A search for the decay of neutral, weakly-interacting, long-lived particles (LLPs) leaving a single displaced hadronic jet in the Muon Spectrometer (MS) using data collected by the ATLAS detector is presented in this dissertation. The analysis uses $140.1\,fb^{-1}$ of proton-proton collision data at $\sqrt{s} = 13$ TeV collected during Run 2 of the LHC from 2015-2018. LLPs appear in many beyond the Standard Model (BSM) theories, motivating dedicated searches for their signatures. Two benchmark signal models utilizing a Higgs portal mechanism are considered. The primary benchmark model is a Hidden Sector extension with decays to pairs of scalar LLPs, while the second is a Higgs portal baryogenesis model. A specialized trigger algorithm that targets regions of high activity in the MS is discussed. A specialized software algorithm designed for reconstructing displaced vertices is also discussed. The background for the single-vertex signature is estimated using a data-driven ABCD method. The search sensitivity is extended to higher and lower LLP lifetimes using a toy MC extrapolation technique. The observed number of events is consistent with expected backgrounds. Limits on the benchmark signal models are presented. Combined limits with previous two-vertex results are also presented. For the SM Higgs boson Hidden Sector model, constraints are placed on the process cross-section at the 95% CL in the range from $0.5-50.5\,m$ for branching ratios above 10%, varying with LLP mass. For Higgs masses different than $125$ GeV, upper limits are placed on the LLP production cross section times branching ratio.