Probing ALP Lepton Flavour Violation at $μ$TRISTAN

Abstract

Axion-like particles (ALPs) with lepton flavour violating (LFV) interactions are predicted within a wide range of flavoured ALP models. The proposed $μ$TRISTAN high-energy $e^-μ^+$ and $μ^+μ^+$ collider will provide a good opportunity to explore flavour physics in the charged lepton sector. In this work, based on a model-independent effective Lagrangian describing the ALP leptonic interactions, we investigate the potential of $μ$TRISTAN to probe ALP LFV couplings. We analyse the testability of selected ALP production channels with potential sensitivity at $μ$TRISTAN, considering different beams and collision energies, including $e^- μ^+ \to a γ$, $e^- μ^+ \to e^- τ^+ a$, $μ^+ μ^+ \to μ^+ τ^+ a$, and $e^- μ^+ \to τ^- μ^+ a$. The produced ALP $a$ is either long-lived or can promptly decay to flavour violating or conserving charged lepton final states. In particular, combining the above LFV ALP production modes with a suitable LFV decay mode, one can identify signatures that are virtually free of Standard Model background. We show the resulting sensitivity of $μ$TRISTAN to LFV ALP couplings and compare it with multiple low-energy leptonic constraints and the future improvements thereof. We find that $μ$TRISTAN can be generally complementary to searches for low-energy LFV processes and measurements of the leptonic magnetic dipole moments and has the capability to explore unconstrained parameter space for ALP masses in the $\mathcal{O}(1)$ to $\mathcal{O}(100)$~GeV range. In the light ALP regime, however, the parameter space that $μ$TRISTAN is sensitive to, has been already excluded by low-energy searches for LFV decays.