Lepton Flavor Non-Universality in B-meson Decays from a U(2) Flavor Model

Abstract

We address the recent anomalies in semi-leptonic $B$-meson decays using a model of fermion masses based on the $U(2)$ flavor symmetry. The new contributions to $b \to s \ell \ell$ transitions arise due to a tree-level exchange of a $Z^\prime$ vector boson gauging a $U(1)$ subgroup of the flavor symmetry. They are controlled by a single parameter and are approximately aligned to the Standard Model prediction, with constructive interference in the $e$-channel and destructive interference in the $\mu$-channel. The current experimental data on semi-leptonic $B$-meson decays can be very well reproduced without violating existing constraints from flavor violation in the quark and lepton sectors. Our model will be tested by new measurements of $b \to s \ell \ell$ transitions and also by future electroweak precision tests, direct $Z^\prime$ searches, and $\mu$-$e$ conversion in nuclei.