Constraints on lepton-flavor mixing with third-generation new physics

TL;DR

This paper investigates how an approximate flavor symmetry at the TeV scale constrains new physics models that mainly interact with third-generation fermions, using current data on lepton flavor observables.

Contribution

It derives bounds on the flavor mixing parameter from experimental data and discusses future prospects for tightening these constraints.

Findings

Most stringent bound on mixing parameter is |δ|<0.051 at 95% CL.

Current data from R_K(*) and B_s→μμ provide key constraints.

Future LFV searches could significantly improve sensitivity.

Abstract

We study the implications of an approximate $U(2)^5$ flavor symmetry at the TeV scale, under the assumption of new physics predominantly coupled to the third-generation fermions, focusing on the breaking of the $U(2)_\ell$ subgroup governing the mixing between second- and third-generation left-handed leptons. We derive constraints on the corresponding spurion parameter $\delta$ from current data on lepton flavor violating (LFV) and lepton flavor universality (LFU) observables, finding that $R_{K^{(*)}}$ and $\mathcal{B}(B_s \to \mu \mu)$ give the most stringent bound on $\delta$, yielding ${|\delta|<0.051}$ at 95% CL. In addition, we provide updated bounds for LFV decay rates and discuss prospects for future sensitivity improvements, finding that future LFV searches could further tighten constraints on the mixing between second- and third-generation leptons.