UC Santa Barbara

Long-lived particles are a class of particles that are predicted by many different theories of physics beyond the Standard Model. They are a promising avenue for discovering new physics because their signatures can be quite different from most Standard Model backgrounds.

This dissertation presents a search for long-lived particles. The search utilizes the full Run 2 dataset totaling 138 $\textrm{fb}^{-1}$ of proton-proton collisions recorded by the CMS experiment at $\sqrt{s} = 13$ TeV. Hits in the barrel muon system drift tubes are clustered together and properties of the cluster are used to reject backgrounds. The results of the search are interpreted using a Twin Higgs model, showing sensitivity to Higgs-mediated long-lived particle production with proper lifetimes up to 100m and long-lived particles masses as low as 0.4 GeV. Limits are also set on a previously untested model where a dark quantum chromodynamic sector couples to the Higgs boson through various possible portals. Limits as low as $10^{-3}$ are set on the Higgs branching fraction to long-lived particles for both interpretations.