Semileptonic B-physics anomalies: A general EFT analysis within U(2)n flavor symmetry

Abstract

We analyze the recent hints of lepton flavor universality violations in semileptonic B decays within a general effective field theory (EFT) based on a U(2)n flavor symmetry acting on the light generations of Standard Model (SM) fermions. We analyze in particular the consistency of these anomalies with the tight constraints on various low-energy observables in B and τ physics. We show that, with a moderate fine-tuning, a consistent picture for all low-energy observables can be obtained under the additional dynamical assumption that the new physics (NP) sector is coupled preferentially to third-generation SM fermions. We discuss how this dynamical assumption can be implemented in general terms within the EFT, and we identify a series of observables in τ decays which could provide further evidence of this NP framework.