CP-Conservation in QCD and why only "invisible" Axions work
  Among solutions of the strong CP problem, the "invisible" axion in the narrow axion window is argued to be the remaining possibility among natural solutions on the smallness of $\bar{\theta}$. Related to the gravity spoil of global symmetries, some prospective invisible axions from theory point of view are discussed. In all these discussions, including the observational possibility, cosmological constraints must be included. 
Jihn E. Kim
hep-ph astro-ph.CO hep-ex