A Flavorful Factoring of the Strong CP Problem

TL;DR

This paper proposes a novel flavor model embedding QCD into a trinified gauge group, linking the strong CP problem to quark flavor structure, and predicts a neutron EDM close to current experimental bounds.

Contribution

It introduces a flavor model that extends the ${m_u=0}$ solution, connecting the strong CP problem with quark flavor structure through a trinified gauge symmetry.

Findings

$ar heta$ is predicted to be near current experimental bounds.

The model naturally explains small third-generation mixing angles.

Predicted neutron EDM is within reach of upcoming experiments.

Abstract

Motivated by the intimate connection between the strong CP problem and the flavor structure of the Standard Model, we present a flavor model that revives and extends the classic ${m_u=0}$ solution to the strong CP problem. QCD is embedded into a $SU(3)_1\times SU(3)_2 \times SU(3)_3$ gauge group, with each generation of quarks charged under the respective $SU(3)$. The non-zero value of the up-quark Yukawa coupling (along with the strange quark and bottom-quark Yukawas) is generated by contributions from small instantons at a new scale $M \gg \Lambda_{QCD}$. The Higgsing of $SU(3)^3\to SU(3)_c$ allows dimension-5 operators that generate the Standard Model flavor structure and can be completed in a simple renormalizable theory. The smallness of the third generation mixing angles can naturally emerge in this picture, and is connected to the smallness of threshold corrections to $\bar\theta$. Remarkably, $\bar\theta$ is essentially fixed by the measured quark masses and mixings, and is estimated to be close to the current experimental bound and well within reach of the next generation of neutron and proton EDM experiments.