A Realistic U(2) Model of Flavor

TL;DR

This paper introduces a realistic $U(2)$ flavor model compatible with $SU(5)$ GUTs that explains fermion mass hierarchies and mixings without supersymmetry, fitting all flavor observables including neutrinos.

Contribution

It presents a novel $U(2)$ flavor model that naturally accounts for fermion hierarchies and mixing angles, and explores a variant with $D_6 imes U(1)_F$ symmetry linking to neutrino masses and axion dark matter.

Findings

Successfully fits all Standard Model flavor observables.

Provides a mechanism for large lepton mixing from small quark mixing.

Offers a solution to the Strong CP Problem and Dark Matter via the QCD axion.

Abstract

We propose a simple $U(2)$ model of flavor compatible with an $SU(5)$ GUT structure. All hierarchies in fermion masses and mixings arise from powers of two small parameters that control the $U(2)$ breaking. In contrast to previous $U(2)$ models this setup can be realized without supersymmetry and provides an excellent fit to all SM flavor observables including neutrinos. We also consider a variant of this model based on a $D_6 \times U(1)_F$ flavor symmetry, which closely resembles the $U(2)$ structure, but allows for Majorana neutrino masses from the Weinberg operator. Remarkably, in this case one naturally obtains large mixing in the lepton sector from small mixing in the quark sector. The model also offers a natural option for addressing the Strong CP Problem and Dark Matter by identifying the Goldstone boson of the $U(1)_F$ factor as the QCD axion.