Thermal axion production in the primordial quark-gluon plasma

TL;DR

This paper calculates the thermal production rate of axions in the early universe's quark-gluon plasma, providing insights into their abundance and role as dark matter candidates.

Contribution

It introduces a gauge-invariant, systematic field theoretical approach to compute axion production rates in the primordial plasma.

Findings

Axion yield and decoupling temperature are quantified for masses below 10 meV.

The study shows axions can remain relativistic today for certain parameters.

Results support axions as viable dark matter components.

Abstract

We calculate the rate for thermal production of axions via scattering of quarks and gluons in the primordial quark-gluon plasma. To obtain a finite result in a gauge-invariant way that is consistent to leading order in the strong gauge coupling, we use systematic field theoretical methods such as hard thermal loop resummation and the Braaten-Yuan prescription. The thermally produced yield, the decoupling temperature, and the density parameter are computed for axions with a mass below 10 meV. In this regime, with a Peccei-Quinn scale above 6x10^8 GeV, the associated axion population can still be relativistic today and can coexist with the axion cold dark matter condensate.