Exploring muonphilic ALPs at muon colliders

Abstract

Axion-like particles (ALPs) are new particles that extend beyond the standard model (SM) and are extensively investigated. When considering ALPs within an effective field theory framework, their couplings with SM particles can be studied independently. It is a daunting task to search for GeV-scale ALPs coupled to muons in collider experiments because their coupling is proportional to the muon mass. However, a recent study by Altmannshofer, Dror, and Gori (2022) highlighted the importance of a four-point interaction, W-μ--a, as well as interactions from the chiral anomaly, whose couplings are not dependent on the muon mass. These interactions provide a new opportunity to explore muonphilic ALPs (μALPs) at the GeV scale. We explore various μALP production channels at muon colliders with μALPs decaying into a pair of muons. In particular, we find that a pair of neutrinos accompanied by a μALP is the most effective channel to search for μALPs in the electrowek violating (EWV) scenario. In contrast, a photon plus a μALP becomes a better channel to search for μALPs in the electroweak preserving (EWP) scenario because there is no W-μ--a interaction in this situation. Most importantly, we find that the future bounds for μALPs in the EWV scenario are considerably stronger than those in the EWP scenario and the existing bounds for exploring μALPs with GeV .