Global Quantization in Gauge Orbit Space with Magnetic Monopoles As a   Solution to Strong CP Problem and the Relevance to $U_A(1)$ Problem
  We generalize our discussions and give more general physical applications of a new solution to the strong CP problem with magnetic monopoles as originally proposed by the author$^1$. Especially, we will discuss about the global topological structure in the relevant gauge orbit spaces to be clarified. As it is shown that in non-abelian gauge theories with a $\theta$ term, the induced gauge orbit space with gauge potentials and gauge functions restricted on the space boundary $S^2$ has a magnetic monopole structure and the gauge orbit space has a vortex structure if there is a magnetic monopole in the ordinary space. The Dirac's quantization conditions in the quantum theories ensure that the vacuum angle $\theta$ in the gauge theories must be quantized. The quantization rule is given by $\theta=2\pi/n~(n\neq 0)$ with n being the topological charge of the magnetic monopole. Therefore, the strong CP problem is automatically solved in the presence of a magnetic monopole of charge $\pm 1$ with $\theta=\pm 2\pi$, or magnetic monopoles of very large total topological charge ($|n|\geq 10^92\pi$) if it is consistent with the abundance of magnetic monopoles. Where in the first case with a magnetic monopole of topological charge 1 or -1, we mean the strong CP-violation can be only very small by the measurements implemented so far. Since $\theta=\pm 2\pi$ correspond to different monopole sectors, the CP can not be conserved exactly in strong interactions in this case. In the second case, the strong CP cannot be conserved either for large but finite n. The fact that the strong CP-violation measured so far can be only so small or vanishing may be a signal for the existence of magnetic monopoles. We also conjecture that the parity violation and CP violation in weak interaction fundamentally may intimately 
Huazhong Zhang