Lepton Flavor Violation Induced by a Neutral Scalar at Future Lepton Colliders

Many new physics scenarios beyond standard model often necessitate the existence of a (light) neutral scalar $H$, which might couple to the charged leptons in a flavor violating way, while evading all existing constraints. We show that such scalars could be effectively produced at future lepton colliders, either on shell or off shell depending on their mass, and induce lepton flavor violating (LFV) signals, i.e., ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\ell}}_{\ensuremath{\alpha}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\ell}}_{\ensuremath{\beta}}^{\ensuremath{\mp}}(+H)$ with $\ensuremath{\alpha}\ensuremath{\ne}\ensuremath{\beta}$. We find that a large parameter space of the scalar mass and the LFV couplings can be probed well beyond the current low-energy constraints in the lepton sector. In particular, a scalar-loop induced explanation of the long-standing muon $g\ensuremath{-}2$ anomaly can be directly tested in the on-shell mode.