Axion thermalization in the early universe

We reanalyze the conditions under which we have a primordial thermal population of axions. We extend a previous study and take into account processes involving gluons and quarks. We conclude that if the PecceiQuinn scale satisfies Fa,1.231012 GeV there is thermal axion production. In this case, a period in the early universe exists where axions would interact with the QCD plasma and we point out that nonthermal axions produced before the end of this period would thermalize.The work of E.M. and F.R. is partially supported by the CICYT Research Project AEN99-0766, by the DGR Project 2001 SGR 00188, and by the EU network on Supersymmetry and the Early Universe ~HPRN-CT-2000-00152!. G.Z. is grateful for hospitality at the Grup de Fisica Teorica of the Universitat Autonoma de Barcelona