Dark matter

QCD matter

Quantum chromodynamics

Quantum mechanics

Particle physics

Quark–gluon plasma

Observable

Axion

Physics

Physical Review Letters

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 obtained via ${q}_{i}{\overline{q}}_{i}\ensuremath{\leftrightarrow}ga$ and ${q}_{i}g\ensuremath{\leftrightarrow}{q}_{i}a$, where $g$ are gluons. The production of axions through such processes is maximal around $T\ensuremath{\approx}{m}_{i}$, where ${m}_{i}$ 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, $\mathrm{\ensuremath{\Delta}}{...

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