A Search for Dark Photons with the FASER Detector at the LHC

The 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 fb$^{-1}$ collected at center- of-mass energy $\sqrt{s}$ = 13.6TeV 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 ε ~ 2×10$^{−5}$ − 1×10$^{−4}$ and masses ~ 17 MeV − 70 MeV. This dissertation contains previously published as well as unpublished co-authored materials.