Freezing-in a hot bath: resonances, medium effects and phase transitions
  Relic density calculations of dark matter freezing out from the primordial
plasma have reached a high level of sophistication, with several numerical
tools readily available that match the observationally required accuracy. Dark
matter production via the freeze-in mechanism, on the other hand, is sensitive
to much higher temperatures than in the freeze-out case, implying both
technical and computational difficulties when aiming for the same level of
precision. We revisit the formulation of freeze-in production in a way that
facilitates the inclusion of in-medium corrections like plasma effects and the
spin statistics of relativistic quantum gases, as well as the temperature
dependence of dark matter production rates induced by the electroweak and
strong phase transitions, and we discuss in detail the additional complications
arising in the presence of $s$-channel resonances. We illustrate our approach
in the context of Higgs portal models, and provide the most accurate
calculation to date of the freeze-in abundance of Scalar Singlet dark matter.
We explore in particular the case of small reheating temperatures, for which
the couplings implied by the freeze-in mechanism may be testable at the LHC.
Together with this article we present a major update 6.3 of DarkSUSY with the
added capability of performing general freeze-in calculations, including all
complications mentioned above.
