When CP requires $\bar\theta=0$, not $\bar\theta=\pi$

Abstract

Imposing CP or P forces the QCD topological angle to be either $\bar\theta=0$ or $\bar\theta=\pi$. However, only the former is phenomenologically viable. This implies that the assumption of CP or P alone cannot provide a framework for unambiguously solving the Strong CP problem. We show that $\bar\theta=0$ is naturally selected when the assumption of CP is combined with the hypothesis that the Standard Model is embedded in a suitable gauge group.