U(2) Is Right for Leptons and Left for Quarks

We posit that the distinct patterns observed in fermion masses and mixings are due to a minimally broken $\mathrm{U}(2{)}_{q+e}$ flavor symmetry acting on left-handed quarks and right-handed charged leptons, giving rise to an accidental $\mathrm{U}(2{)}^5$ symmetry at the renormalizable level without imposing selection rules on the Weinberg operator. We show that the symmetry can be consistently gauged by explicit examples and comment on realizations in SU(5) unification. Via a model-independent analysis of a standard model viewed as an effective field theory, we find that selection rules due to $\mathrm{U}(2{)}_{q+e}$ enhance the importance of charged lepton flavor violation as a probe, where significant experimental progress is expected in the near future.

Figure 1

Comparative constraints on SMEFT operators from flavor and $CP$ violation: minimally broken $\mathrm{U}(2{)}_{q+e}$ (blue), MFV (green), flavor anarchy (red). Here, $Q=q$, $u$, $d$ and $L=\ell ,e$. See the main text for details.