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

Abstract

We show that when the QCD axion is directly coupled to quarks with ci/f∂μaqiγμγ5qi, such as in Dine-Fischler-Srednicki-Zhitnitsky models, the dominant production mechanism in the early Universe at temperatures 1 GeVT100 GeV is obtained via qiqi↔ga and qig↔qia, where g are gluons. The production of axions through such processes is maximal around T≈mi, where mi are the different heavy quark masses. This leads to a relic axion background that decouples at such temperatures, leaving a contribution to the effective number of relativistic degrees of freedom, which can be larger than the case of decoupling happens the electroweak phase transition, ΔNeff0.027. Our prediction for the t quark is 0.027≤ΔNeff≤0.036 for 106 GeVf/ct4×108 GeV and for the b quark is 0.027≤ΔNeff≤0.047 for 107 GeVf/cb3×108 GeV. For the c quark the window can only be roughly estimated as 0.027