Production of Dark Matter in Cosmological Models with Low Reheating Temperature

Abstract

In standard cosmology it is assumed that dark matter (DM) is thermally produced during the radiation dominated era after an inflationary period. This has led to the prediction of weakly interactive massive particles (WIMPs) as the most popular candidates for DM. However, current indirect, direct, and collider searches for the WIMPs have not found any clear evidence yet. This issue motivates us to investigate cosmological scenarios with matter domination before big bang nucleosynthesis (BBN). The theories unifying the four fundamental forces of nature at high energy scales, like superstring theory, can have proposals for the early universe cosmology after the inflationary reheating and before BBN. In such cases the universe can be dominated by heavy long–lived fields (moduli) with low reheating temperatures which can decay to DM and radiation. To explore such scenarios, we precisely compute the degrees of freedom (DoF) of standard model particles using lattice quantum chromodynamics (QCD) results for QCD equation of state around the temperature of quark–gluon confinement. Afterwards, we study the production of DM in an early matter dominated era from the annihilation of thermal bath particles and decay of moduli, using the computed DoF and assuming initially vanishing and nonvanishing radiation and DM densities. In that case new production mechanisms for DM beyond the thermal WIMP production are also possible. Finally, we focus on studying the production of neutralinos, as the most popular DM candidate in supersymmetric models, in low reheating scenarios and its status in current experimental searches.