Towards a full description of MeV dark matter decoupling: a self-consistent determination of relic abundance and $N_{\rm eff}$

TL;DR

This paper presents a comprehensive, self-consistent calculation of MeV-scale dark matter decoupling, accurately predicting relic abundance and $N_{ m eff}$ by tracking energy transfer among dark matter, neutrinos, and photons during freeze-out.

Contribution

It introduces a novel numerical method to precisely model the temperature evolution of three sectors during dark matter freeze-out, enabling accurate predictions of relic abundance and $N_{ m eff}$ for arbitrary annihilation channels.

Findings

Planck data excludes complex scalar dark matter with mass ≤ 7 MeV for certain annihilation channels.

First detailed treatment of dark matter temperature evolution involving three sectors.

Provides a framework for precise relic abundance and $N_{ m eff}$ predictions in MeV dark matter models.

Abstract

Thermal dark matter at the MeV mass-scale has its abundance set during the highly non-trivial epochs of neutrino decoupling and electron annihilation. The technical obstacles attached to solving Boltzmann equations of multiple interacting sectors being both relativistic and non-relativistic have to-date prevented the full treatment of this problem. Here, for the first time, we calculate the freeze-out of light dark matter, taking into account the energy transfer between the dark sector, neutrinos, and the electromagnetically interacting plasma from annihilation and elastic scattering processes alike. We develop a numerically feasible treatment that allows to track photon and neutrino temperatures across freeze-out and to arrive at a precision prediction of $N_{\rm eff}$ for arbitrary branching ratios of the dark matter annihilation channels. In addition, our treatment resolves for the first time the dark matter temperature evolution across freeze-out involving three sectors. It enters in the efficiency of velocity-dependent annihilation channels and for a flavor-blind $p$-wave annihilation into electron- and neutrino-pairs of all generations, we find the present Planck data excludes a complex scalar dark matter particle of mass of $m_\phi \leq 7$ MeV.