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

Zhong Zhang, Yu Zhang, Zeren Simon Wang

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, $γe^- \to e^+μ^-μ^-$, mediated by an axionlike particle (ALP) with flavor-violating $e$-$μ$ 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.

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

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, , mediated by an axionlike particle (ALP) with flavor-violating - 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 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 or hadron-collider environments, establishing electron-photon collisions as a uniquely powerful probe of charged lepton flavor violation.

Paper Structure

This paper contains 4 sections, 9 equations, 4 figures, 2 tables.

Table of Contents

  1. Photon spectrum from laser Compton backscattering
  2. Monte Carlo simulation setup
  3. Fiducial volumes and decay treatment
  4. Estimate of detector-induced backgrounds for the prompt searches

Figures (4)

  • Figure 1: Feynman diagrams for the signal process with on-shell ALP production.
  • Figure 2: Cross sections of $\gamma e^- \to e^+ \mu^- \mu^-$ as functions of $m_a$ for BESIII, STCF, and ILC, including detector acceptance and reconstruction efficiencies.
  • Figure 3: Proper decay length of the ALP as a function of its mass for representative values of $g_{ae\mu}$, illustrating the transition between prompt and non-prompt regimes relevant for different collider setups.
  • Figure 4: Projected $95\%$ C.L. sensitivities to $g_{ae\mu}$ as functions of $m_a$ at BESIII, STCF, and ILC. For comparison, the projected Belle II sensitivity in the $e^+ e^-$ mode with an integrated luminosity of $50~\mathrm{ab}^{-1}$Endo:2020mev is also shown. Existing constraints from muonium--antimuonium oscillations Willmann:1998gd, leptonic $g-2$Fan:2022etoMuong-2:2025xykAliberti:2025beg, and LEP ALEPH:2013dgf are included. The solid (dashed) curves correspond to the prompt (non-prompt) search strategies.