Standard model flavor from an $SU(2)$ symmetry

Abstract

We propose that the flavor structure of the standard model is based on a horizontal $SU(2)$ symmetry. It generically predicts (i) a parametrically small mass for the lightest charged fermions, (ii) small mixings in the quark sector, and (iii) suppression of flavor-changing neutral currents. Supplemented with the assumption of a strong hierarchy between the second- and third- generation masses, it also predicts (iv) a large $CP$-violating phase in the quark sector. Only Majorana neutrinos allow for large mixings in the lepton sector. In this case, this framework further predicts (v) near-maximal $\theta_{23}^l$, (vi) a normal hierarchy of neutrino masses, and (vii) large $CP$ violation in the lepton sector.