Search for Lepton Flavor Violating Decay at FASER

TL;DR

This paper evaluates FASER's potential to detect charged lepton flavor violation mediated by a light boson, exploring parameter space beyond current bounds and highlighting its unique sensitivity to rare muon decays.

Contribution

It provides the first detailed sensitivity analysis of FASER2 for various CLFV interactions mediated by light bosons, extending the search beyond existing experimental limits.

Findings

FASER2 can probe new regions of parameter space for CLFV interactions.

FASER2 is sensitive to very small couplings undetectable by current muon decay experiments.

Potential to observe CLFV decays of light bosons not accessible to other experiments.

Abstract

FASER is one of the promising experiments which search for long-lived particles beyond the Standard Model. In this paper, we consider charged lepton flavor violation (CLFV) via a light and weakly interacting boson and discuss the detectability by FASER. We focus on four types of CLFV interactions, i.e., the scalar-, pseudoscalar-, vector-, and dipole-type interaction, and calculate the sensitivity of FASER to each CLFV interaction. We show that, with the setup of FASER2, a wide region of the parameter space can be explored. Particularly, it is found that FASER2 has a sensitivity to very small coupling regions in which the rare muon decays, such as $\mu \rightarrow e\gamma$, cannot place bounds, and that there is a possibility to detect CLFV decays of the new light bosons.