Search for Long-Lived Particles Produced in Proton-Proton Collisions at $\sqrt{s}$ =13 TeV That Decay to Displaced Hadronic Jets Detected With the Muon Spectrometer in ATLAS

The discovery of the Higgs boson in 2012 marked a great success of the Standard Model (SM) of particle physics. Up to now, the branching ratio of the Higgs boson to undetected particles beyond the Standard Model (BSM) is not well constrained (up to 22%). Long-lived particles (LLPs) show up in a variety of BSM theories, such as Higgs portal models. This thesis presents a search for displaced vertices from LLP decays in the ATLAS muon spectrometer (MS) using 139 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS detector at the LHC from 2015 to 2018. The signal benchmark model of interest is a Hidden Sector scenario, where a Higgs-like boson can decay into a pair of neutral long-lived scalars. A specialized trigger is used to collect events that produce high activity in the MS. A specialized software algorithm subsequently reconstructs vertices in the MS. Background to two LLP decays producing displaced vertices in the MS are determined using a data-driven method. To extend the sensitivity of the search to a wide range of LLP proper lifetime $c\tau_s$, we employ a toy Monte Carlo method. No events are found after event selection optimized for the LLP signal. The 95% confidence level limits on the branching ratio into LLPs for the SM-like Higgs boson with 125 GeV mass are subsequently constructed. The LLP $c\tau_s$ are excluded at 95% CL from about 4 cm to 71.3 m for the branching ratio above 10% depending on mass. If the Higgs-like boson has a mass different than 125 GeV, we set the 95% CL upper limit on the LLP production cross section.