Dynamical solution of the strong CP problem within QCD ?

TL;DR

This paper presents a lattice QCD study showing that the vacuum angle θ is renormalized and flows to zero at low energies, providing a nonperturbative solution to the strong CP problem and indicating CP conservation in strong interactions.

Contribution

The study demonstrates, using the gradient flow method, that the vacuum angle θ in QCD is renormalized and naturally flows to zero, offering a novel nonperturbative resolution to the strong CP problem.

Findings

θ is renormalized in QCD.

θ flows to zero in the infrared limit.

CP symmetry is conserved in strong interactions.

Abstract

The strong CP problem is inseparably connected with the topology of gauge fields and the mechanism of color confinement, which requires nonperturbative tools to solve it. In this talk I present results of a recent lattice investigation of QCD with the $\theta$ term in collaboration with Yoshifumi Nakamura. The tool we are using to address the nonperturbative properties of the theory is the gradient flow, which is a particular realization of momentum space RG transformations. The novel result is that within QCD the vacuum angle $\theta$ is renormalized, together with the strong coupling constant, and flows to $\theta = 0$ in the infrared limit. This means that CP is conserved by the strong interactions.