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

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.

Comments: 5 pages; v2: expanded discussion, to appear in EPJC