Flavour-violating decays of leptons into axion-like particles

Axion-like particles (ALPs) with flavour-violating couplings to the Standard Model (SM) leptons are a consequence of spontaneously-broken global U(1) symmetries within a wide class of models. In this seminar, I will present some explicit examples where such lepton-flavour-violating (LFV) ALP scenario is realised. A general prediction of it is the occurrence of flavour-violating decays of SM leptons into a light axion-like particle, which would typically escape detection at experiments searching for rare muon and tau decays. I will discuss the bounds set on this exotic signature by past experiments, as well as the prospects at upcoming searches for LFV muon and tau decays. I will focus in particular on a new proposal for a modified setup of the MEG II experiment that would be maximally sensitive to an ALP coupling to right-handed leptonic currents. I will also compare the sensitivity of LFV experiments to the leptonic ALP couplings with astrophysical constraints, such as bounds from stellar evolution, and with the regions of the parameter space where the ALP constitutes a dark matter candidate.