Reconstruction efficiency, primary vertex assignment, and muon isolation for long-lived particles decaying into muon pairs with the ATLAS detector

The Standard Model (SM) of particle physics successfully describes the fundamental constituents of matter and their interactions. Despite its extensive experimental success, however, it cannot explain several observed phenomena. One outstanding question is the nature of dark matter, whose existence is supported by astrophysical and cosmological evidence but whose particle constituents have not yet been observed. Although numerous theories propose possible dark matter particle candidates, another approach is to search for particles that mediate interactions between dark matter and Standard Model particles. One such mediator is the dark photon, which can interact with Standard Model particles through kinetic mixing. In the Hidden Abelian Higgs Model, the Standard Model Higgs boson may decay into a pair of dark photons. Provided being sufficiently long-lived, a dark photon can travel a measurable distance through the detector before decaying into fermions. Decays into collimated muon pairs produce a distinctive displaced signature that can be investigated with the ATLAS detector at the Large Hadron Collider (LHC). This thesis presents studies of custom techniques designed to improve displaced vertex reconstruction in searches for long-lived dark photons using Run 3 LHC data. These include a dedicated procedure for recovering displaced muons and secondary vertices (SV), as well as a primary vertex (PV) reassignment method based on the reconstructed muons. The effect of this reassignment on muon isolation is investigated using simulated Monte Carlo (MC) signal samples and collision data. The studies demonstrate improved reconstruction efficiency for displaced muons and show that the reassignment procedure more frequently identifies the true primary vertex in simulation. The reassigned PV modifies the track-based isolation for a subset of muons, providing an isolation calculation better suited to displaced particle signatures. Together, these techniques can improve signal acceptance and support the sensitivity of future ATLAS searches for long-lived particles decaying into muons.