UC Irvine

The ForwArd Search ExpeRiment (FASER) is collecting far-forward proton-proton collision data during the Large Hadron Collider's third running period (2022-2025). This thesis describes two of the first-proposed searches: for dark photon ($A'$) and the $B-L$ gauge boson ($A_{B-L}$). These searches are motivated by the proposed dark sector --- an extension to the Standard Model of particle physics that includes dark matter. These long-lived particles would mediate interactions between the Standard Model and the dark sector.

Although larger experiments like ATLAS and CMS are well-suited to detect new, heavy particles, there may be low-mass particles produced in abundance along the axis of collision, in the far forward region. FASER, a 5m-long detector comprised of a magnetic spectrometer, scintillators, and calorimeters, is designed to study high-energy charged particles in this region. The experiment is installed in the repurposed SPS maintenance tunnel, TI12, 480m from the ATLAS interaction point and oriented along the line-of-sight. FASER's highly efficient veto system rejects all incoming muon events. Tracker readout is triggered by a timing scintillator or calorimeter signal.

Dark sector models predict $A'$ and $A_{B-L}$ from the decay of SM mesons, which are copiously produced in the far forward direction at ATLAS. They share a striking experimental signature, decaying into energetic electron-positron pairs. Event selection requires no entering (veto) signal, two good reconstructed tracks, and a calorimeter deposit of at least 500 GeV. The expected background for the search is $(2.3\pm2.3\times10^{-3})$ events for 27 fb$^{-1}$ luminosity and is dominated by neutrino interactions with the detector materials.

FASER collected its first year of data in 2022, with results first presented at the March 2023 Moriond conferences. No candidates were observed. FASER excluded new regions of parameter space for $A'$ and $A_{B-L}$ at 90\% confidence level.