Minimal mass of thermal dark matter and the viability of millicharged particles affecting 21-cm cosmology

Author:

Chu Xiaoyong1ORCID,Pradler Josef12ORCID

Affiliation:

1. Institute of High Energy Physics, Austrian Academy of Sciences, Nikolsdorfergasse 18, 1050 Vienna, Austria

2. University of Vienna, Faculty of Physics, Boltzmanngasse 5, A-1090 Vienna, Austria

Abstract

Thermal freeze-out offers an attractive explanation of the dark matter density free from fine-tuning of initial conditions. For dark matter with a mass below tens of MeV, photons, electrons, and neutrinos are the only available direct Standard Model annihilation products. Using a full three-sector abundance calculation, we determine the minimal mass of dark matter, allowing for an arbitrary branching into electrons/photons and neutrinos that is compatible with current cosmological observations. The analysis takes into account the heat transfer between the various sectors from annihilation elastic scattering, representing the first fully self-consistent analysis that tracks the respective sectors’ temperatures. We thereby provide accurate thermal annihilation cross sections, particularly for velocity-dependent cases, and deduce the sensitivity of current and upcoming CMB experiments to MeV thermal dark matter. In the latter context, we also establish the fine-tuned parameter region where a tiny admixture of neutrinos in the final states rules in MeV-scale p-wave annihilating DM into electrons. Finally, we show that a sub-% millicharged dark matter with an interaction strength that interferes with 21-cm cosmology is still allowed when freeze-out is supplemented with annihilation into neutrinos. For all cases considered, we provide concrete particle physics models and supplement our findings with a discussion of other relevant experimental results. Published by the American Physical Society 2024

Funder

National Science Foundation

European Commission

European Research Council

Research Network Quantum Aspects of Spacetime

Publisher

American Physical Society (APS)

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