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

TL;DR

This paper investigates how future measurements of various B meson decays can test the hypothesis that non-standard flavor effects are governed by a specific U(2) flavor symmetry, providing a way to confirm or refute this model.

Contribution

It proposes a framework linking flavor symmetry assumptions to specific predictions for B decay observables, enabling experimental tests of non-standard flavor structures.

Findings

Future measurements can confirm or falsify the U(2) flavor symmetry hypothesis.

Predictions relate flavor symmetry to leptonic and semileptonic B decay observables.

The approach is independent of the underlying dynamical origin of effects.

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 $R_{D^{(*)}}$, $R_{K^{(*)}}$, ${\mathcal B}(\bar B_{c,u}\to \ell \bar \nu)$, ${\mathcal B}(\bar B \to \pi \ell \bar\nu)$, ${\mathcal B}(B \to \pi \ell \bar\ell)$, ${\mathcal B}(B_{s,d}\to\ell\bar\ell^{(\prime)})$, as well as various polarization asymmetries in $\bar B\to D^{(*)} \tau \bar \nu$ decays, will allow to prove or falsify this general hypothesis independently of its dynamical origin.