Flavoured Warped Axion

TL;DR

This paper proposes a 5D extension of the DFSZ axion model that addresses the axion quality and fermion mass hierarchy problems, predicting flavour-dependent axion-fermion couplings that could be tested in future experiments.

Contribution

It introduces a 5D framework with bulk and boundary Higgs fields to generate flavour-dependent axion couplings, providing a novel approach to axion phenomenology.

Findings

Off-diagonal axion-fermion couplings can be as low as 10^{11} GeV.

The model explains fermion mass hierarchies via bulk Higgs localization.

Future flavour experiments can probe these off-diagonal couplings.

Abstract

We consider a 5D extension of the DFSZ axion model that addresses both the axion quality and fermion mass hierarchy problems, and predicts flavour-dependent, off-diagonal axion-fermion couplings. The axion is part of a 5D complex scalar field charged under a U(1)$_{PQ}$ symmetry that is spontaneously broken in the bulk, and is insensitive to explicit PQ breaking on the UV boundary. Bulk Standard Model fermions interact with two Higgs doublets that can be localized on the UV boundary or propagate in the bulk to explain the fermion masses and mixings. When the Higgs doublets are localized on the UV boundary, they induce flavour diagonal couplings between the fermions and the axion. However, when the Higgs doublets propagate in the bulk, the overlap of the axion and fermion profiles generates flavour off-diagonal couplings. The effective scale of these off-diagonal couplings in both the quark and lepton sectors can be as small as $10^{11}$ GeV, and therefore will be probed in future precision flavour experiments.