Chiral observables and topology in hot QCD with two families of quarks

TL;DR

This study uses lattice QCD simulations to analyze chiral and topological properties of hot QCD with four flavors, providing insights into the topological susceptibility and implications for axion dark matter.

Contribution

First lattice QCD study with four dynamical flavors at high temperatures, confirming continuum estimates and exploring topological susceptibility relevant for axion physics.

Findings

Topological susceptibility decays as a power law above twice the critical temperature.

The pseudocritical temperature aligns with previous continuum results.

Results support instanton-based models near the transition temperature.

Abstract

We present results on QCD with four dynamical flavors in the temperature range $150$ MeV $\lesssim T \lesssim 500$ MeV. We have performed lattice simulations with Wilson fermions at maximal twist and measured Polyakov loop, chiral condensate and disconnected susceptibility, on lattices with spacings as fine as 0.065 fm. For most observables spacing effects are below statistical errors, which enables us to identify lattice results with continuum estimates. Our estimate of the pseudocritical temperature compares favorably with continuum results from staggered and domain wall fermions, confirming that a dynamical charm does not contribute in the transition region. From the high temperature behaviour of the disconnected chiral susceptibility we infer the topological susceptibility, which encodes relevant properties of the QCD axion, a plausible Dark Matter candidate. The topological susceptibility thus measured exhibits a power-law decay for $T/T_c \gtrsim 2$, with an exponent close to the one predicted by the Dilute Instanton Gas Approximation (DIGA). Close to $T_c$ the temperature dependent effective exponent seems to approach the DIGA result from above, a behaviour which would support recent analytic calculations based on an Instantons-dyons model. These results constrain the mass of a hypothetic QCD post-inflationary axion, once an assumption concerning the relative contribution of axions to Dark Matter is made.