With or without U(2)? Probing non-standard flavor and helicity structures in semileptonic B decays

Abstract

Motivated by the recent hints of lepton flavor universality violation observed in semileptonic $B$ decays, we analyze how to test flavor and helicity structures of the corresponding amplitudes in view of future data. We show that the general assumption that such non-standard effects are controlled by a $U(2{)}_q\times U(2{)}_{\ell }$ flavor symmetry, minimally broken as in the Standard Model Yukawa sector, leads to stringent predictions on leptonic and semileptonic $B$ decays. Future measurements of $R_{D^{(\ast )}}$, $R_{K^{(\ast )}}$, $\mathcal{B}({\bar{B}}_{c,u}\to \ell \bar{\nu })$, $\mathcal{B}(\bar{B}\to \pi \ell \bar{\nu })$, $\mathcal{B}(B\to \pi \ell \bar{\ell })$, $\mathcal{B}(B_{s,d}\to \ell {\bar{\ell }}^{(\prime )})$, as well as various polarization asymmetries in $\bar{B}\to D^{(\ast )}\tau \bar{\nu }$ decays, will allow to prove or falsify this general hypothesis independently of its dynamical origin.

arXiv comment12 pages, 4 figures (v2: very minor modifications)

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