Observable Windows for the QCD Axion Through the Number of Relativistic Species

We show that when the QCD axion is directly coupled to quarks with {c}_{i}/f{\ensuremath{\partial}}_{\ensuremath{\mu}}a{\overline{q}}_{i}{\ensuremath{\gamma}}^{\ensuremath{\mu}}{\ensuremath{\gamma}}^{5}{q}_{i} such as in Dine-Fischler-Srednicki-Zhitnitsky models, the dominant production mechanism in the early Universe at temperatures 1\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}T\ensuremath{\lesssim}100\text{ }\text{ }\mathrm{GeV}is...