Leptonic Meson Decays into Invisible ALP

TL;DR

This paper reviews the calculation of leptonic meson decays into an invisible axion-like particle (ALP), updating experimental bounds on ALP-fermion interactions and highlighting their significance for ALP-lepton couplings in the sub-GeV mass range.

Contribution

It provides a comprehensive theoretical review and updated experimental bounds on leptonic meson decays into ALPs, emphasizing their role in constraining ALP-lepton couplings.

Findings

Leptonic meson decays set the most stringent model-independent bounds on ALP-leptons for sub-GeV masses.

Current constraints on ALP-quark couplings are less competitive than FCNC process bounds.

Leptonic decays can be key in probing ALP interactions with leptons in certain mass ranges.

Abstract

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.