Freeze-in produced dark matter in the ultra-relativistic regime

Abstract

When dark matter particles only feebly interact with plasma constituents in the early universe, they never reach thermal equilibrium. As opposed to the freeze-out mechanism, where the dark matter abundance is determined at T M , the energy density of a feebly interacting state builds up and increases over T > ∼ M . In this work, we address the impact of the high-temperature regime on the dark matter production rate, where the dark and Standard Model particles are ultra-relativistic and nearly light-like. In this setting, multiple soft scatterings, as well as 2 → 2 processes, are found to give a large contribution to the production rate. Within the model we consider in this work, namely a Majorana fermion dark matter of mass M accompanied by a heavier scalar -with mass splitting ∆M -which shares interactions with the visible sector, the energy density can be dramatically underestimated when neglecting the high-temperature dynamics. We find that the overall effective 1 ↔ 2 and 2 → 2 high-temperature contributions to dark-matter production give O(10) (20%) corrections for ∆M/M = 0.1 (∆M/M = 10) to the Born production rate with in-vacuum masses and matrix elements. We also assess the impact of bound-state effects on the late-time annihilations of the heavier scalar, in the context of the super-WIMP mechanism.

The Higgs Portal to Cosmology

“…The relevant constraints on feebly interacting DM have been reviewed in [315]. Related relativistic analyses have recently appeared in [318], [319]. Finally, the micrOMEGAs tool has been updated to include the freeze-in option [320].…”