Discovering dark matter at the LHC through its nuclear scattering in far-forward emulsion and liquid argon detectors

The LHC may produce light, weakly interacting particles that decay to dark matter, creating an intense and highly collimated beam of dark matter particles in the far-forward direction. We investigate the prospects for detecting this dark matter in two far-forward detectors proposed for a future forward physics facility: $\mathrm{FASER}\ensuremath{\nu}2$, a 10-ton emulsion detector, and FLArE, a 10- to 100-ton LArTPC. We focus here on nuclear scattering, including elastic scattering, resonant pion production, and deep inelastic scattering, and devise cuts that efficiently remove the neutrino-induced background. In the invisibly decaying dark photon scenario, DM-nuclear scattering probes new parameter space for dark matter masses $5\text{ }\text{ }\mathrm{MeV}\ensuremath{\lesssim}{m}_{\ensuremath{\chi}}\ensuremath{\lesssim}500\text{ }\text{ }\mathrm{MeV}$. When combined with the DM-electron scattering studied previously, $\mathrm{FASER}\ensuremath{\nu}2$ and FLArE will be able to discover dark matter in a large swath of the cosmologically favored parameter space with $\mathrm{MeV}\ensuremath{\lesssim}{m}_{\ensuremath{\chi}}\ensuremath{\lesssim}\mathrm{GeV}$.