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

TL;DR

This paper investigates the potential of TeV-scale muon beams to detect lepton-flavor-violating processes mediated by axion-like particles, exploring experimental setups and implications for dark matter and muon g-2 anomalies.

Contribution

It introduces a novel experimental approach to probe LFV via ALPs using high-energy muon beams and discusses the potential reach of current and future detectors, including implications for dark matter and muon g-2.

Findings

Current experiments can probe LFV ALP interactions with muons.

Future muon colliders could significantly enhance detection sensitivity.

The model can accommodate dark matter and explain muon g-2 anomalies.

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 $\mu\tau$ LFV interactions and assume that the ALP is coupled to a dark state $\chi$, which can be either less or more massive than $a$. Such a setup is demonstrated to be consistent with $\chi$ being a candidate for dark matter, in the experimentally relevant regime of parameters. We consider the currently operating NA64-$\mu$ experiment and proposed FASER$\nu$2 detector as both the target and the detector for the process $\mu A \to \tau A\, a$, 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$\nu$2 data providing a pilot study towards that goal. The implications of the muon anomalous magnetic moment $(g-2)_\mu$ measurements for the underlying model, in case of a positive signal, are also examined, and a sample UV completion is outlined.