Why PQ?

Abstract

I discuss how the solution of the $U(1{)}_A$ problem of QCD through the existence of the $\theta$-vacuum gave rise to the strong CP problem. After examining various suggested solutions to this problem, I conclude that the only viable solution still is the one that involves the existence of a spontaneously broken chiral symmetry: $U(1{)}_{PQ}$

Comments: Invited talk given at the Axion 2010 Conference, January 15-17, 2010, Univ. of Florida, Gainsville, FL