On Axion Thermalization in the Early Universe

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 $F_a<1.2\times 10^{12}\text{GeV}$ 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 $F_a<1.2\times 10^{12}\text{GeV}$.

Comments: 13 pages, 3 figures. Latex-file. References added and other minor corrections. Accepted for publication in Phys.Rev.D. Revised version matches publication in journal