On the minimal mass of thermal dark matter and the viability of millicharged particles affecting 21cm cosmology

TL;DR

This paper determines the minimal mass of thermal dark matter compatible with cosmological data, analyzes the impact of millicharged particles on 21cm cosmology, and provides detailed models and experimental implications.

Contribution

It offers the first fully self-consistent calculation of dark matter abundance considering sector temperatures and explores the viability of millicharged particles affecting 21cm signals.

Findings

Minimal dark matter mass is constrained by cosmological observations.

Millicharged particles with sub-% charge can influence 21cm cosmology without conflicting with freeze-out.

Accurate thermal annihilation cross sections are provided for various velocity dependencies.

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 and 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.