Relic abundance of MeV millicharged particles

TL;DR

This paper calculates the relic abundance of MeV millicharged particles, exploring their potential as dark matter candidates and how existing limits constrain their parameter space.

Contribution

It provides a detailed calculation of MCP relic abundance near the electron mass and discusses conditions under which they could constitute dark matter.

Findings

MCP relic abundance depends on mass ratio to electron.

For certain mass ratios, MCPs could account for dark matter.

Existing limits challenge the viability of MCPs as dark matter.

Abstract

The relic abundance of light millicharged particles (MCPs) with the electric charge $e' = 5\cdot 10^{-5} e$ and with the mass slightly below or above the electron mass is calculated. The abundance depends on the mass ratio $\eta=m_X/m_e$ and for $\eta<1$ can be high enough to allow MCPs to be the cosmological dark matter or to make a noticeable contribution to it. On the other hand, for $\eta \gtrsim 1$ the cosmological energy density of MCPs can be quite low, $\Omega_X h_0^2 \approx 0.02$ for scalar MCPs, and $\Omega_X h_0^2 \approx 0.001$ for spin 1/2 fermions. But even the lowest value of $\Omega_X h_0^2$ is in tension with several existing limits on the MCP abundances and parameters. However, these limits have been derived under some natural or reasonable assumptions on the properties of MCPs. If these assumptions are relaxed, a patch in the mass-charge plot of MCPs may appear, permitting them to be dark matter particles.