DEPARTMENT OF PHYSICS DISSERTATION DEFENSE: Rory O'Dwyer

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Title: 

A SEARCH FOR MESONIC DARK MATTER WITH 18 $pb^{-1}$ OF $E_{beam}=3.74$ GeV Data FROM THE HEAVY PHOTON SEARCH EXPERIMENT

Abstract: 

 In this thesis, I analyze data from the 2021 run of the Heavy Photon Search (HPS) experiment. Using an electron-positron trigger, the HPS collaboration collected 18 pb^{-1} of 3.74 GeV electron collisions against a thin tungsten target. The HPS detector has far forward acceptance that is especially designed to probe these electron-positron decays for MeV-scale dark matter mediators. I demonstrate how the 2021 data is calibrated, including how I comprehensively optimized the full HPS Silicon Vertex Tracker to maximize the acceptance of dark matter events. I then use the 2021 run data to search for one model of dark-photon-mediated matter, a Strongly Interacting Massive Particle (SIMP) with SU(3) internal symmetry. This thesis aims to detect these SIMPs through the decay of a dark vector meson (either a dark $\rho$ or $\phi$) into $e^{+}e^{-}$ pairs. Results are obtained as a function of the A' mass and coupling constant using both optimized cuts-based and machine-learning-based selections.