The topological susceptibility in finite temperature QCD and axion cosmology

TL;DR

This study investigates how the topological susceptibility in finite temperature QCD varies with temperature, providing insights relevant for axion dark matter models and cosmological constraints.

Contribution

It presents lattice QCD results on topological susceptibility across a range of temperatures, highlighting the transition from non-perturbative to dilute instanton gas behavior.

Findings

Topological susceptibility follows dilute instanton gas approximation above 250 MeV.

Below 250 MeV, the susceptibility decreases more slowly with temperature.

Results inform bounds on axion decay constant and oscillation temperature in cosmology.

Abstract

We study the topological susceptibility in 2+1 flavor QCD above the chiral crossover transition temperature using Highly Improved Staggered Quark action and several lattice spacings, corresponding to temporal extent of the lattice, $N_\tau=6,8,10$ and $12$. We observe very distinct temperature dependences of the topological susceptibility in the ranges above and below $250$ MeV. While for temperatures above $250$ MeV, the dependence is found to be consistent with dilute instanton gas approximation, at lower temperatures the fall-off of topological susceptibility is milder. We discuss the consequence of our results for cosmology wherein we estimate the bounds on the axion decay constant and the oscillation temperature if indeed the QCD axion is a possible dark matter candidate.