ALP searches at the LHC: FASER as a light-shining-through-walls experiment

Abstract

We propose the use of FASER as a light-shining-through-walls experiment to search for axions and axionlike particles (ALPs). LHC collisions generate a high intensity and high energy photon flux in the forward direction which can oscillate into ALPs in the magnetic fields that are used to confine the beam. These ALPs then pass through about 100 m of rock before reaching the magnetic fields of FASER, where they can convert back into photons and be detected by an electromagnetic calorimeter. In the next years, FASER and its successor FASER2 at the Forward Physics Facility will be able to explore regions of the ALP parameter space inaccessible by other laboratory-based experiments.