Lepton-Flavor-Violating ALP Signals with TeV-Scale Muon Beams

Abstract

We explore the feasibility of using TeV-energy muons to probe lepton-flavor-violating (LFV) processes mediated by an axion-like particle (ALP) $a$ with mass $\mathcal{O}(10~\textrm{GeV})$. We focus on $μτ$ LFV interactions and assume that the ALP is coupled to a dark state $χ$, which can be either less or more massive than $a$. Such a setup is demonstrated to be consistent with $χ$ being a candidate for dark matter, in the experimentally relevant regime of parameters. We consider the currently operating NA64-$μ$ experiment and proposed FASER$ν$2 detector as both the target and the detector for the process $μA \to τA\, a$, where $A$ is the target nucleus. We also show that a possible future active muon fixed-target experiment operating at a 3 TeV muon collider or in its preparatory phase can provide an impressive reach for the LFV process considered, with future FASER$ν$2 data providing a pilot study towards that goal. The implications of the muon anomalous magnetic moment $(g-2)_μ$ measurements for the underlying model, in case of a positive signal, are also examined, and a sample UV completion is outlined.