Coloured gravitational instantons, the strong CP problem and the companion axion solution

TL;DR

This paper discusses how quantum gravity-induced CP violation affects the strong CP problem, suggesting that the standard axion solution is insufficient and proposing the need for an additional axion particle.

Contribution

It introduces the impact of coloured gravitational instantons on the strong CP problem and proposes that an extra axion is necessary for a complete solution.

Findings

Quantum gravity introduces new CP violation sources.

Coloured gravitational instantons link quantum gravity effects to strong CP violation.

An additional axion is required to resolve the strong CP problem.

Abstract

Quantum gravity introduces a new source of the combined parity (CP) violation in gauge theories. We argue that this new CP violation gets bundled with the strong CP violation through the coloured gravitational instantons. Consequently, the standard axion solution to the strong CP problem is compromised. Further, we argue that the ultimate solution to the strong CP problem must involve at least one additional axion particle.