Natural axion model from flavour

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 $U(1{)}_{\mathrm{PQ}}$ symmetry. Here we investigate how this $U(1{)}_{\mathrm{PQ}}$ 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.

21 pages, 4 figures, 1 table. Typo corrected and updated reference list in v2