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

2025/11/19 by Sebastiano Covone, Pol Morell, Covone, Sebastiano +3 · 1 citation
Physics and Astronomy · #Particle physics theoretical and experimental studies #Neutrino Physics Research #Particle Detector Development and Performance

paper · pdf · doi:10.1016/j.physletb.2026.140582

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)_ℓ subgroup governing the mixing between second- and third-generation left-handed leptons. We derive constraints on the corresponding spurion parameter δ from current data on lepton flavor violating (LFV) and lepton flavor universality (LFU) observables, finding that R_K^(*) and B(B_s → μμ) give the most stringent bound on δ, yielding |δ|<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.

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