On Axion Thermalization in the Early Universe
Eduard Masso, Francesc Rota, Gabriel Zsembinszki
TL;DR
The study investigates whether primordial axions could reach thermal equilibrium in the early universe and how this impacts their relic abundance. By extending Turner's analysis to include gluon- and quark-initiated processes through the axion-gluon coupling and computing thermally averaged rates with Debye screening, the authors solve the Boltzmann evolution for $Y=n_a/s$ and derive a central bound on the Peccei-Quinn scale: $F_a < 1.2\times 10^{12}\ \mathrm{GeV}$ for thermalization, corresponding to a present-day thermal axion density of $n_{a0} \approx 7.5\ \mathrm{cm}^{-3}$. The result, with $\Gamma$ scaling as $T^3$ and enhanced color/flavor factors, yields a significantly larger window for thermalization than previous estimates, and implies that any non-thermal axions produced during the thermal era would become part of the thermal population. This has important implications for axion dark matter scenarios, particularly regarding the expected density from string decay for models with $F_a$ in the thermalizing range.
Abstract
We reanalyze the conditions under which we have a primordial thermal population of axions. Compared with previous studies, we find other processes, involving gluons and quarks, that dominate at high temperatures. We conclude that if the Peccei-Quinn scale fulfills $F_a < 1.2 \times 10^{12} \GeV$ there is thermal axion production. In this case, a period in the early universe exists where axions interact with the QCD plasma and we point out that non-thermal axions produced before the end of this period will thermalize. This could lead to a reduction of the expected density of axions from string decay in models with $F_a < 1.2 \times 10^{12} \GeV$.