Same-sign dimuon probe of charged lepton flavor violation at electron-photon colliders

TL;DR

This paper proposes a new, clean method to detect charged lepton flavor violation via same-sign dimuon signatures in electron-photon collisions mediated by axion-like particles, with high sensitivity at future colliders.

Contribution

It introduces a novel experimental signature for lepton flavor violation in electron-photon collisions, leveraging ALP mediation and collider setups like STCF and ILC.

Findings

STCF and ILC can probe couplings below current bounds.

The channel has minimal SM background and high sensitivity.

Electron-photon collisions offer a unique environment for this search.

Abstract

Observation of charged lepton flavor violation would constitute unambiguous evidence for physics beyond the Standard Model (SM). We identify a previously unexplored same-sign dimuon signature in electron-photon collisions, $\gamma e^- \to e^+\mu^-\mu^-$, mediated by an axionlike particle (ALP) with flavor-violating $e$-$\mu$ couplings. The absence of irreducible SM backgrounds and the on-shell production of the ALP render this channel intrinsically clean and highly sensitive, with only small residual backgrounds arising from detector effects. Such collisions can be realized via laser Compton backscattering at $e^+e^-$ colliders including BEPC-II with the BESIII detector, STCF, and ILC. We find that STCF and ILC can probe couplings one to two orders of magnitude below existing bounds. This combination of resonant production, vanishing irreducible background, and same-sign topology would be difficult to achieve in conventional $e^+e^-$ or hadron-collider environments, establishing electron-photon collisions as a uniquely powerful probe of charged lepton flavor violation.