Strong CP problem, electric dipole moment, and fate of the axion

TL;DR

This paper explores the long-distance effects of the theta term in QCD, proposing a natural solution to the strong CP problem by showing that confinement is lost for theta > 0, which conflicts with the axion solution.

Contribution

It introduces a lattice-based analysis of long-distance QCD with a theta term, revealing confinement loss for theta > 0 and challenging the axion extension of the Standard Model.

Findings

Color fields are screened for theta > 0

Confinement is lost at theta > 0

Results align with neutron electric dipole moment measurements

Abstract

Three hard problems! In this talk I investigate the long-distance properties of quantum chromodynamics in the presence of a topological theta term. This is done on the lattice, using the gradient flow to isolate the long-distance modes in the functional integral measure and tracing it over successive length scales. It turns out that the color fields produced by quarks and gluons are screened, and confinement is lost, for vacuum angles theta > 0, thus providing a natural solution of the strong CP problem. This solution is compatible with recent lattice calculations of the electric dipole moment of the neutron, while it excludes the axion extension of the Standard Model.