Freeze-in Production of Scalar Singlet Dark Matter

Abstract

The mystery of dark matter (DM) has intrigued scientists for many decades. The type of DM particles that has been studied the most is Weakly Interacting Massive Particles (WIMPs). These particles are assumed to have been in thermal equilibrium with the visible sector heat bath at early times, and experienced freeze-out when the expansion rate of the universe became bigger than the interaction rate. However, the null result from DM searches and the shrinking parameter space for WIMP models have made it crucial to look at other models. A fascinating model to study is one that includes a Feebly Interacting Massive Particle (FIMP). These particles are assumed to never be in thermal equilibrium with the visible sector heat bath and are produced by the freeze-in mechanism. They naturally evade experimental constraints while explaining the observed relic density. Since FIMPs are never in thermal equilibrium with the visible sector, any thermal effect on the FIMP abundance will have to be accounted for. This is the main goal of this thesis. I consider the thermal effects coming from electroweak symmetry breaking (EWSB), thermal contribution to the masses and the QCD phase transition in the early universe for the scalar singlet model, where I include both higher order correction to the Standard Model (SM) couplings and quantum statistical effects. These effects result in an important correction to the DM relic abundance when the dominant contribution does not arise from Higgs boson decays, which happens when decays into scalar singlets are kinetically forbidden or if the reheating temperature after inflation is much smaller than the Higgs boson mass. Freeze-in has also been implemented in the FORTRAN package DarkSUSY together with the thermal effects for the scalar singlet model.