ALP production in lepton flavour violating meson, tau and gauge boson decays

A bstract In this paper we study axion-like particles (ALPs) with lepton-flavour-violating (LFV) couplings in the mass regime above the muon threshold, m a > m μ , where the strong bound from the exotic muon decay μ → ea no longer apply and the decay channel a → eμ becomes kinematically accessible. In this region, the ALP typically decays promptly, motivating new search strategies based on its production in decays involving virtual muons. We analyse charged-meson and W decays, neutral-current processes such as Z and quarkonium decays, and, when couplings to the third generation are present, LFV τ decays. The subsequent decay a → eμ leads to striking LFV signatures with negligible Standard Model backgrounds. Combining these production modes with current low-energy constraints, we assess the sensitivity of future high-energy e + e − colliders, flavour factories such as Belle II and STCF, fixed-target experiments such as NA62, and proton beam-dump facilities such as SHiP. Overall, our results identify LFV ALP production in meson, gauge-boson, quarkonium and τ decays (with displaced vertices) as a promising and largely unexplored avenue to test ALP interactions with charged leptons above the muon mass threshold.