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

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) x U( 2)(l) 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(*), R-K(*), B((B) over bar (c, u) -> l (v) over bar), B((B) over bar -> pi l (v) over bar), B(B -> pi l (l) over bar), B(B-s,B-d -> l (l) over bar (()'())), as well as various polarization asymmetries in (B) over bar -> D-(*())tau(v) over bar decays, will allow to prove or falsify this general hypothesis independently of its dynamical origin. (C) 2019 The Author(s). Published by Elsevier B.V.