Refined Bounds on MeV-scale Thermal Dark Sectors from BBN and the CMB

TL;DR

This paper provides refined bounds on MeV-scale thermal dark matter particles based on Big Bang Nucleosynthesis and CMB data, showing current constraints and future prospects for lighter particles.

Contribution

It offers a comprehensive and precise analysis of BBN and CMB constraints on MeV-scale BSM particles, establishing bounds independent of particle properties and exploring future observational improvements.

Findings

BBN sets a lower bound of 0.4 MeV on dark matter mass at 2σ.

Current BBN and CMB data constrain electrophilic and neutrinophilic species to >3.7 MeV.

Future CMB missions could improve bounds to 10-15 MeV.

Abstract

New light states thermally coupled to the Standard Model plasma alter the expansion history of the Universe and impact the synthesis of the primordial light elements. In this work, we carry out an exhaustive and precise analysis of the implications of MeV-scale BSM particles in Big Bang Nucleosynthesis (BBN) and for Cosmic Microwave Background (CMB) observations. We find that, BBN observations set a lower bound on the thermal dark matter mass of $m_\chi > 0.4\,\text{MeV}$ at $2\sigma$. This bound is independent of the spin and number of internal degrees of freedom of the particle, of the annihilation being s-wave or p-wave, and of the annihilation final state. Furthermore, we show that current BBN plus CMB observations constrain purely electrophilic and neutrinophilic BSM species to have a mass, $m_\chi > 3.7\,\text{MeV}$ at $2\sigma$. We explore the reach of future BBN measurements and show that upcoming CMB missions should improve the bounds on light BSM thermal states to $m_\chi > (10-15)\,\text{MeV}$. Finally, we demonstrate that very light BSM species thermally coupled to the SM plasma are highly disfavoured by current cosmological observations.