Torrence, EricFellers, Deion2024-08-072024-08-072024-08-07https://hdl.handle.net/1794/29717The FASER experiment at the LHC is designed to search for light, weakly-interacting particles produced in proton-proton collisions at the ATLAS interaction point that travel in the far-forward direction. FASER is sensitive to probing previously unconstrained dark photon models, which is a theoretical particle that could provide a portal between the standard model of particle physics and a dark sector that contains a dark matter particle. This dissertation presents the first results from a search for dark photons decaying to an electron-positron pair in FASER, using a dataset corresponding to an integrated luminosity of $27.0$\,$\mathrm{fb}^{-1}$ collected at center-of-mass energy $\sqrt{s} = 13.6$\,TeV in 2022 in LHC Run 3. No events are seen in an almost background-free analysis, yielding world-leading constraints on dark photons with couplings $\epsilon \sim 2 \times 10^{-5} - 1 \times 10^{-4}$ and masses $\sim 17~\mev - 70~\mev$. This dissertation contains previously published as well as unpublished co-authored materials.en-USAll Rights Reserved.Beyond the Standard ModelDark MatterDark PhotonElectronic Thesis or Dissertation