Leptonic Meson Decays into Invisible ALP

The theoretical calculation of pseudo-scalar leptonic decay widths into an invisible ALP, $M \to \ell\, \nu_\ell\, a$, is reviewed. Assuming generic flavour-conserving ALP couplings to SM fermions and a generic ALP mass, $m_a$, 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 $M\to P\,a$ decays. These leptonic decays can, however, provide the most stringent model-independent upper bounds on ALP-leptons couplings for $m_a$ in the (sub)-GeV range.