The axion flavour connection

TL;DR

This paper explores how a local flavour symmetry in the Standard Model can naturally produce an axion that solves the strong CP problem while explaining quark mass hierarchies.

Contribution

It demonstrates a framework where flavour symmetries lead to an accidental global symmetry with a high-quality Peccei-Quinn symmetry, addressing the strong CP problem.

Findings

A simple model based on $SU(3)\times SU(2) \times U(1)_F$ illustrates the mechanism.

The model naturally generates quark mass hierarchies.

The approach ensures a high-quality axion solution to the strong CP problem.

Abstract

A local flavour symmetry acting on the quarks of the Standard Model can automatically give rise to an accidental global $U(1)$ which remains preserved from sources of explicit breaking up to a large operator dimension, while it gets spontaneously broken together with the flavour symmetry. Such non-fundamental symmetries are often endowed with a mixed QCD anomaly, so that the strong CP problem is automatically solved via the axion mechanism. We illustrate the general features required to realise this scenario, and we discuss a simple construction based on the flavour group $SU(3)\times SU(2) \times U(1)_F$ to illustrate how mass hierarchies can arise while ensuring at the same time a high quality Peccei-Quinn symmetry.