Thermal axion production at low temperatures: a smooth treatment of the QCD phase transition

TL;DR

This paper develops a continuous model for thermal axion production across the QCD phase transition, incorporating heavy quark effects at high temperatures and hadron scattering at low temperatures, to refine estimates of axion contributions to cosmology.

Contribution

It introduces a smooth, unified calculation of axion production rates across the QCD transition, improving upon previous segmented approaches.

Findings

Continuous axion production rate across the QCD phase transition.

Quantified axion contribution to the effective number of neutrino species.

Extended calculations to include heavy quark masses at high temperatures.

Abstract

We study thermal axion production around the confinement scale. At higher temperatures, we extend current calculations to account for the masses of heavy quarks, whereas we quantify production via hadron scattering at lower temperatures. Matching our results between the two opposite regimes provides us with a continuous axion production rate across the QCD phase transition. We employ such a rate to quantify the axion contribution to the effective number of neutrino species.