ALP production in Lepton Flavour Violating meson, tau and gauge boson decays

TL;DR

This paper explores new methods to detect axion-like particles with lepton-flavour-violating couplings in high-energy decays, focusing on their prompt decays and distinctive signatures above the muon mass.

Contribution

It introduces novel search strategies for LFV ALPs in meson, gauge boson, quarkonium, and tau decays, expanding the experimental reach above the muon threshold.

Findings

LFV ALPs can produce striking signatures with negligible Standard Model backgrounds.

Future colliders and experiments can significantly improve sensitivity to LFV ALPs.

Displaced vertex signatures in various decays offer promising detection channels.

Abstract

In this paper we study axion-like particles (ALPs) with lepton-flavour-violating (LFV) couplings in the mass regime above the muon threshold, $m_a>m_\mu$, where the strong bound from the exotic muon decay $\mu\to ea$ no longer apply and the decay channel $a\to e\mu$ 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 $\tau$ decays. The subsequent decay $a\to e\mu$ 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 $\tau$ decays (with displaced vertices) as a promising and largely unexplored avenue to test ALP interactions with charged leptons above the muon mass threshold.