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

TL;DR

This paper explores how future lepton colliders can detect a neutral scalar that causes lepton flavor violation, potentially explaining the muon g-2 anomaly and surpassing current constraints.

Contribution

It demonstrates the potential of future colliders to probe flavor-violating scalars and tests the scalar-loop explanation of the muon g-2 anomaly.

Findings

Large parameter space can be probed beyond current constraints

On-shell production can test muon g-2 explanation

Future colliders offer promising LFV detection capabilities

Abstract

Many new physics scenarios beyond the 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. $e^+ e^- \to \ell_\alpha^\pm \ell_\beta^\mp (+H)$ with $\alpha\neq \beta$. 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 longstanding muon $g-2$ anomaly can be directly tested in the on-shell mode.