Subatomic, High Energy and Plasma Physics seminar: Francesco Sannino

Abstract: Why do strong interactions appear to preserve CP, although QCD allows its violation? I will revisit this question through the effective description of the QCD vacuum, connecting the physical θ angle, the axial anomaly and the neutron electric dipole moment. After introducing the essential ingredients, I will discuss recent work on the tension between the resolution of the U(1) problem and the absence of strong CP violation. Using anomalous Ward identities and a one-particle-irreducible effective action, we show that, under mild assumptions, eliminating CP violation at small nonzero θ would require suppressing the very anomaly contribution needed to account for the pseudoscalar spectrum. Topological susceptibility provides the link between these two questions. I will conclude by explaining how the Peccei–Quinn mechanism and the QCD axion resolve this tension, and which challenges remain.