Magnetic Monopoles As a New Solution to Strong CP Problem
  A non-perturbative solution to strong CP problem is proposed. It is shown that the gauge orbit space with gauge potentials and gauge tranformations restricted on the space boundary in non-abelian gauge theories with a $\theta$ term has a magnetic monopole structure if there is a magnetic monopole in the ordinary space. The Dirac's quantization condition in the corresponding quantum theories ensures that the vacuum angle $\theta$ in the gauge theories must be quantized. The quantization rule is derived as $\theta=2\pi/n~(n\neq 0)$ with n being the topological charge of the magnetic monopole. Therefore, we conclude that the strong CP problem is automatically solved non-perturbatively with the existence of a magnetic monopole of charge $\pm 1$ with $\theta=\pm 2\pi$. This is also true when the total magnetic charge of monopoles are very large ($|n|\geq 10^92\pi$) if it is consistent with the abundance of magnetic monopoles. This implies that the fact that the strong CP violation can be only so small or vanishing may be a signal for the existence of magnetic monopoles. 
H. Zhang