On Symmetric and Asymmetric Light Dark Matter

TL;DR

This paper explores constraints on light dark matter with masses from 1 MeV to 10 GeV, focusing on asymmetries, mediator properties, and implications for direct detection, collider, and astrophysical observations.

Contribution

It provides a comprehensive analysis of asymmetric light dark matter constraints, including mediator mass bounds and implications for direct detection experiments.

Findings

CMB constraints can be evaded with sufficient DM asymmetry.

Light mediators are required for the necessary annihilation cross sections.

Halo shape constraints set a lower bound on mediator mass.

Abstract

We examine cosmological, astrophysical and collider constraints on thermal dark matter (DM) with mass mX in the range 1 MeV to 10 GeV. Cosmic microwave background (CMB) observations, which severely constrain light symmetric DM, can be evaded if the DM relic density is sufficiently asymmetric. CMB constraints require the present anti-DM to DM ratio to be less than 2*10^{-6} (10^{-1}) for DM mass mX = 1 MeV (10 GeV) with ionizing efficiency factor f ~ 1. We determine the minimum annihilation cross section for achieving these asymmetries subject to the relic density constraint; these cross sections are larger than the usual thermal annihilation cross section. On account of collider constraints, such annihilation cross sections can only be obtained by invoking light mediators. These light mediators can give rise to significant DM self-interactions, and we derive a lower bound on the mediator mass from elliptical DM halo shape constraints. We find that halo shapes require a mediator with mass mphi > 4 * 10^{-2} MeV (40 MeV) for mX = 1 MeV (10 GeV). We map all of these constraints to the parameter space of DM-electron and DM-nucleon scattering cross sections for direct detection. For DM-electron scattering, a significant fraction of the parameter space is already ruled out by beam-dump and supernova cooling constraints.