A Holographic Perspective on the Axion Quality Problem

TL;DR

This paper proposes a geometric, holographic solution to the axion quality problem by localizing the axion in a 5D AdS space, effectively suppressing gravitational violations of the PQ symmetry.

Contribution

It introduces a novel 5D holographic model where the axion's global symmetry is protected from quantum gravity effects through geometric sequestering.

Findings

The model successfully suppresses PQ symmetry violations.

Holographic duality relates the 5D model to 4D strong dynamics.

Provides a geometric mechanism for axion quality preservation.

Abstract

The axion provides a compelling solution to the strong CP problem as well as a candidate for the dark matter of the universe. However, the axion solution relies on the spontaneous breaking of a global $U(1)_{PQ}$ symmetry, which is also explicitly violated by quantum gravity. To preserve the axion solution, gravitational violations of the $U(1)_{PQ}$ symmetry must be suppressed to sufficiently high order. We present a simple, geometric solution of the axion quality problem by modelling the axion with a bulk complex scalar field in a slice of AdS$_5$, where the $U(1)_{PQ}$ symmetry is spontaneously broken in the bulk but explicitly broken on the UV brane. By localising the axion field towards the IR brane, gravitational violations of the PQ symmetry on the UV brane are sufficiently sequestered. This geometric solution is holographically dual to 4D strong dynamics where the global $U(1)_{PQ}$ is an accidental symmetry to sufficiently high order.