QCD, Strong CP and Axions
  The physical origin of the strong CP problem in QCD, rooted in the structures of the vacuum of the standard model, is reviewed. The chiral solution to this problem, with its accompanying axion, is explained and various characteristics of axions are detailed. Although visible axion models are excluded experimentally, models with very light and very weakly coupled axions are still tenable. The astrophysical and cosmological implications of such axion models are discussed, along with ongoing experimental attempts to detect such, so called, invisible axions. 
R. D. Peccei