Direct Detection of the Millicharged Background

TL;DR

This paper demonstrates that dark matter direct detection experiments can detect particles with tiny effective charges, sensitive to their early universe production, even if they are a small fraction of dark matter.

Contribution

It introduces a novel approach to detect millicharged particles via direct detection experiments, independent of their dark matter abundance.

Findings

Experiments can detect charges as small as 10^{-12}.

Sensitivity spans nine orders of magnitude in mass.

Detects sub-fractional dark matter components as low as 0.1%.

Abstract

We show that dark matter direct detection experiments are sensitive to the existence of particles with a small effective charge (for instance, via couplings to a kinetically mixed, low-mass dark photon). Our forecasts do not depend on these particles comprising a significant fraction of the dark matter. Rather, these experiments are sensitive to the irreducible abundance produced in the early universe through the freeze-in mechanism. We find that ongoing and proposed direct detection experiments will have world-leading sensitivity to effective charges $Q\sim 10^{-12}$ across nine orders of magnitude in mass, corresponding to a dark matter sub-fraction as low as $\sim 10^{-3}$.