Lubatti, HenryKvam, Audrey2022-04-192022-04-192022-04-192022Kvam_washington_0250E_23902.pdfhttp://hdl.handle.net/1773/48550Thesis (Ph.D.)--University of Washington, 2022This analysis is a search for events with two displaced vertices reconstructed from the decay of pair-produced neutral, weakly-interacting, long-lived particles using the muon spectrometer of the ATLAS detector. The search uses a model-independent strategy, and employs a custom trigger for reconstructing displaced hadronic jets in the muon spectrometer with 139 fb-1 of pp collision data at √s=13 TeV collected in Run 2 at the LHC. The results are interpreted in terms of a Higgs scalar portal model, which probes the existence of a hidden sector that weakly couples to the Standard Model. The search observed 0 events in the signal region, which is compatible with the expected background of 0.32 ± 0.05 events. Limits were set on long-lived particle lifetime for all simulated signal mass points. For the Higgs boson with a mass of 125 GeV, the analysis reports the first exclusion limits for branching fractions into neutral long-lived particles below 0.1%, while branching fractions above 10% are excluded at 95% confidence level for long-lived particle proper lifetimes ranging from 4 cm to 72.4 m. In addition, the analysis presents the first results for the decay of long-lived particles into tt ̅ in the ATLAS muon spectrometer.application/pdfen-USCC BYhadron colliderhigh-energy physicslong-lived particlesParticle physicsPhysicsPhysicsThesis