Axion-like Dark Matter Mediators

Abstract: During the last decades, experimental advances have significantly constrained the standard electroweak-scale WIMP produced via thermal freeze-out, leading to a shift away from this standard paradigm. Here we explore the possibility of an axion-like particle (ALP), the pseudo-Goldstone boson of an approximate U(1) global symmetry spontaneously broken at a high scale $f_a$, acting as a mediator between the Standard Model (SM) particles and the dark matter (DM) particles. We focus on the case where the couplings are too small to allow for DM generation via freeze-out and the DM is thermally decoupled from the SM particles. However, alternative mechanisms like freeze-in and freeze-out from a decoupled dark sector can still reproduce the observed DM relic density. Having determined the region of parameter space for these scenarios, we then revisit experimental constraints on ALPs from electron beam dump experiments, astrophysics and rare B and K decays.