ALPs at FASER: The LHC as a Photon Beam Dump

Abstract: The goal of FASER, ForwArd Search ExpeRiment at the LHC, is to discover light, weakly-interacting particles with a small and inexpensive detector placed in the far-forward region of ATLAS or CMS. A promising location in an unused service tunnel 480 m downstream of the ATLAS interaction point (IP) has been identified. Previous studies have found that FASER has significant discovery potential for new particles produced at the IP, including dark photons, dark Higgs bosons, and heavy neutral leptons. In this study, we explore a qualitatively different, `beam dump' capability of FASER, in which the new particles are produced not at the IP, but through collisions in detector elements further downstream. In particular, we consider the discovery prospects for axion-like particles (ALPs) that couple to the standard model through the $a \gamma \gamma$ interaction. TeV-scale photons produced at the IP collide with the TAN neutral particle absorber 130 m downstream, producing ALPs through the Primakoff process, and the ALPs then decay to two photons in FASER. We show that FASER can discover ALPs with masses $m_a \sim 30 - 400~\text{MeV}$ and couplings $g_{a\gamma\gamma} \sim 10^{-6} - 10^{-3}~\text{GeV}^{-1}$, and we discuss the ALP signal characteristics and detector requirements.