Natural axion model from flavour

TL;DR

This paper proposes a unified model linking the solution to the strong CP problem with the fermion mass hierarchy, introducing a four-Higgs-doublet framework that also provides a dark matter candidate and incorporates neutrino masses.

Contribution

It presents a novel four-Higgs-doublet model where a single $U(1)_{PQ}$ symmetry addresses both the strong CP problem and fermion mass hierarchy, with implications for dark matter and neutrino masses.

Findings

The model naturally explains fermion mass hierarchy with order-one Yukawa couplings.

The axion in the model is a viable dark matter candidate.

Neutrino masses are incorporated via the standard type-I seesaw mechanism.

Abstract

We explore a common symmetrical origin for two long standing problems in particle physics: the strong CP and the fermion mass hierarchy problems. The Peccei--Quinn mechanism solves the former one with an anomalous global $U(1)_{\rm PQ}$ symmetry. Here we investigate how this $U(1)_{\rm PQ}$ could at the same time explain the fermion mass hierarchy. We work in the context of a four-Higgs-doublet model which explains all quark and charged fermion masses with natural, i.e.\ order $1$, Yukawa couplings. Moreover, the axion of the model constitutes a viable dark matter candidate and neutrino masses are incorporated via the standard type-I seesaw mechanism. A simple extension of the model allows for Dirac neutrinos.