# Why PQ - High Energy Physics - Phenomenology

Abstract: I discuss how the solution of the $U1 A$ problem of QCD through theexistence of the $\theta$-vacuum gave rise to the strong CP problem. Afterexamining various suggested solutions to this problem, I conclude that the onlyviable solution still is the one that involves the existence of a spontaneouslybroken chiral symmetry: $U1 {PQ}$

Author: R. D. Peccei

Source: https://arxiv.org/