A natural and simple UV completion of the QCD axion model

TL;DR

This paper presents a straightforward UV-complete model for the QCD axion that naturally addresses the origin, scale, and quality of the PQ symmetry using extra-dimensional branes and gauge dynamics.

Contribution

It introduces a simple UV completion of the QCD axion model utilizing brane separation and gauge condensates to solve key challenges in PQ symmetry realization.

Findings

The model naturally generates the PQ symmetry breaking scale.

The 5th dimension stabilization ensures PQ symmetry quality.

Extra quarks are separated by branes in a Randall-Sundrum setup.

Abstract

The novel PQ mechanism replaces the strong CP problem with some challenges in a model building. In particular, the challenges arise regarding i) the origin of an anomalous global symmetry called a PQ symmetry, ii) the scale of the PQ symmetry breaking, and iii) the quality of the PQ symmetry. In this letter, we provide a natural and simple UV completed model that addresses these challenges. Extra quarks and anti-quarks are separated by two branes in the Randall-Sundrum ${\bf R}^4 \times S^1 / {\bf Z}_2$ spacetime while a hidden SU($N_H$) gauge field condensates in the bulk. The brane separation is the origin of the PQ symmetry and its breaking scale is given by the dynamical scale of the SU($N_H$) gauge interaction. The (generalized) Casimir force of SU($N_H$) condensation stabilizes the 5th dimension, which guarantees the quality of the PQ symmetry.