Bounds on charged-lepton flavor violations via resonant scattering

TL;DR

This paper investigates the potential to detect charged-lepton flavor violations through resonant scattering processes in high-luminosity lepton-photon and electron-muon collisions, proposing methods to overcome experimental limitations.

Contribution

It introduces a novel approach to probe flavor violations via resonant scattering channels and extends analysis to radiative return processes to mitigate beam energy spread issues.

Findings

Computed upper bounds on cross sections based on current decay limits.

Identified conditions under which future colliders could observe flavor violations.

Proposed radiative return as a method to improve sensitivity in experiments.

Abstract

We explore the possibility of probing flavor violations in the charged-lepton sector by means of high-luminosity lepton-photon and electron-muon collisions, by inverting initial and final states in a variety of decay channels presently used to bound such violations. In particular, we analyse the resonant lepton, $\gamma\, \ell \to \ell^{\prime}$, and neutral-meson, $e^- \mu ^+ \to \phi,\eta,\pi^0\!$, scattering channels, whose cross sections are critically dependent on the colliding-beams energy spread, being particularly demanding in the case of leptonic processes. For these processes, we compute upper bounds to the cross-section corresponding to present limits on the inverse decay channel rates. In order to circumvent the beam energy spread limitations we extend the analysis to processes in which a photon accompanies the resonance in the final state, compensating the off-shellness effects by radiative return. These processes might be studied at future facilities with moderate energies, in case lepton-photon and electron-muon collisions with sufficiently high luminosity will be available.