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

Understand

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.