A note on instantons, θ-dependence and strong CP

A

bstract

I review the standard instanton framework for determining the θ -dependence of instanton-dominated correlation functions in QCD. I then contrast these well-established semiclassical results with the recent assertion of refs. [1, 2] that θ -phases are absent and that strong interactions preserve CP for all values of θ . In all scenarios considered, the θ -dependence is shown to be intrinsically non-trivial, being governed either by instantons in the weak-coupling regime or by alternative, non-perturbative considerations when the theory is strongly coupled.