Abstract
We study production of scalar dark matter via the freeze-in mechanism in the relativistic regime, focusing on the simplest Higgs portal model. We derive the corresponding relativistic reaction rates based on the Bose–Einstein statistics taking into account the thermal mass effects as well as the change in the Higgs degrees of freedom at the electroweak phase transition. The consequent constraints on the Higgs portal coupling are obtained.
Highlights
The symmetry s → −s stabilizes the scalar which plays the role of dark matter (DM)
There are two reactions in which DM is produced: Higgs annihilation hh → ss and Higgs decay h → ss, if allowed kinematically. The latter is only possible at temperatures below the electroweak scale, so we distinguish (a) symmetric phase, T TEW and H = 0 (b) broken phase, T < TEW and H = 0
We find that if mh > 2ms, the DM yield is dominated by the decay in the broken phase and the annihilation processes can be neglected altogether
Summary
We consider the simplest real scalar dark matter model with the Higgs portal interaction. The gauge bosons are massless and there are 4 massive Higgs degrees of freedom hi such that In this case, dark matter is produced through annihilation, hihi → ss. While in the broken phase relativistic effects are unimportant, at high temperature they are significant In this case, it is important to take into account the thermal mass corrections which represent the leading thermal effects. For λhs in the range of interest, this effect is negligible unless s has an MeV (or below) mass For such light dark matter only the decay production mode in the broken phase is important, so only the total m2s matters. For heavier DM, we make no distinction between ms0 and ms
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