QCD topology with electromagnetic fields and the axion-photon coupling

TL;DR

This paper investigates how electromagnetic fields influence QCD topology and the axion-photon coupling, using lattice simulations to analyze topological responses and susceptibilities across temperatures.

Contribution

It provides a lattice-based analysis of the linear response of QCD topology to electromagnetic fields and connects this to the axion-photon coupling correction.

Findings

Linear response of topological charge to weak electromagnetic fields.

Magnetic field dependence of topological susceptibility near T_c.

Quantitative relation between electromagnetic fields and QCD topological sectors.

Abstract

The introduction of non-orthogonal electric and magnetic fields in the QCD vacuum enhances the weight of topological sectors with a nonzero topological charge. For weak fields, there is a linear response for the expectation value of the topological charge. We study this linear response and relate it to the QCD correction to the axion-photon coupling. We also analyse the magnetic field dependence of the topological susceptibility for a range of temperatures around $T_c$. In this work we use lattice simulations with improved staggered quarks at physical masses, including background magnetic and (imaginary) electric fields.