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

DOI10.1088/1475-7516/2021/03/075zbMATH Open1504.83036arXiv2012.09083OpenAlexW3110683411MaRDI QIDQ5099165

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Publication date: 31 August 2022

Published in: (Search for Journal in Brave)

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 $TllM$, the energy density of a feebly interacting state builds up and increases over $TgtrsimM$. 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 $2o2$ 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 $DeltaM$ $-$ 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 $1leftrightarrow2$ and $2o2$ high-temperature contributions to dark-matter production give $mathcalO(10)$ (20%) corrections for $DeltaM/M=0.1$ ($DeltaM/M=10$) 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.

Full work available at URL: https://arxiv.org/abs/2012.09083

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