Leptonic meson decays into invisible ALP

Abstract

The theoretical calculation of pseudo–scalar leptonic decay widths into an invisible ALP, M→ℓνℓa, is reviewed. Assuming generic flavor–conserving ALP couplings to SM fermions and a generic ALP mass, ma, the latest experimental results for pseudo–scalar leptonic decays are used to provide updated bounds on the ALP–fermion Lagrangian sector. Constrains on the ALP-quark couplings obtained from these channels are not yet competitive with the ones derived from FCNC processes, like M→Pa decays. These leptonic decays can, however, provide the most stringent model–independent upper bounds on ALP-leptons couplings for ma in the (sub)–GeV range.