Strong CP violation and axion

Abstract

The strong CP violation and the visible, invisible axion were studied. The role of Peccei-Quinn symmetry was analyzed with respect to the SU(2) $\times U(1) standard model and the SU(N) GUTs. The visible axion was thought to be absent in real world. Hence the invisible axion is thoroughly investigated in connections with the family problem, the domain wall problem, the baryon asymmetry, and the monopole puzzle in GUTs; SU(7) and SU(9), and SU$^\star$(7).