HEP Theory Journal Club

Absence of CP violation in the strong interactions

Gonzalo Alonso Alvarez

McGill

I will discuss 2001.07152, whose main claim is that the QCD vacuum angle does not give rise to any observable CP violation. If true, this would have important consequences for particle physics, in particular regarding the motivation for the existence of the QCD axion. So I invite you all to come and stress-test the argumentation together! Here the abstract of the paper for more details:

We derive correlation functions for massive fermions with a complex mass in the presence of a general vacuum angle. For this purpose, we first build the Green's functions in the one-instanton background and then sum over the configurations of background instantons. The quantization of topological sectors follows for saddle points of finite Euclidean action in an infinite spacetime volume and the fluctuations about these. For the resulting correlation functions, we therefore take the infinite-volume limit before summing over topological sectors. In contrast to the opposite order of limits, the chiral phases from the mass terms and from the instanton effects then are aligned so that, in absence of additional phases, these do not give rise to observables violating charge-parity symmetry. This result is confirmed when constraining the correlations at coincident points by using the index theorem instead of instanton calculus.