Cosmologically safe QCD axion without fine-tuning

TL;DR

This paper introduces a novel QCD axion model utilizing extra dimensions and branes, eliminating fine-tuning issues and addressing the domain wall problem through a new symmetry-breaking mechanism.

Contribution

It presents a simple, fine-tuning free QCD axion model based on extra dimensions, monopole condensation, and brane-localized quarks, offering a new origin for the PQ symmetry.

Findings

The model naturally explains the PQ symmetry origin.

It avoids the cosmological domain wall problem.

The model is free from fine-tuning issues.

Abstract

Although QCD axion models are widely studied as solutions to the strong CP problem, they generically confront severe fine-tuning problems to guarantee the anomalous PQ symmetry. In this letter, we propose a simple QCD axion model without any fine-tunings. We introduce an extra dimension and a pair of extra quarks living on two branes separately, which is also charged under a bulk Abelian gauge symmetry. We assume a monopole condensation on our brane at an intermediate scale, which implies that the extra quarks develop the chiral symmetry breaking and the PQ symmetry is broken. In contrast to the original Kim's model, our model explains the origin of the PQ symmetry thanks to the extra dimension and avoids the cosmological domain wall problem because of the chiral symmetry breaking in the Abelian gauge theory.