TPPC weekly seminar: Wenyuan Ai

Wednesday 15 Dec 2021, 14:00 → 15:00 Europe/London

Speaker: Wenyuan Ai, King's College London

Title: A new look at the strong CP problem

Abstract:
In this talk, I will first give a pedagogical review of the strong CP problem and then move on to some subtleties that have been overlooked so far. QCD theta vacuum and instantons induce in the Lagrangian a CP-odd topological term which is naively thought to incur CP violation. However, whether or not such a term really incurs CP violation depends on how instantons affect the fermion correlation functions. There are two potential sources of CP violation, namely complex chiral phases in the quark mass matrix and the so-called 't Hooft vertex, which arises from instanton effects. Only one of them can be removed by field redefinitions, meaning that QCD should violate CP unless the chiral phases in these two terms happen to be aligned. In this talk, we identify the origin of the commonly believed misalignment: it comes from the coherence between different topological sectors in the path integral. However, if one takes the infinity spacetime volume limit before the summation over different topological sectors, the coherence disappears. We thus obtain alignment between the aforementioned chiral phases and CP is conserved in strong interactions.