Dark Matter Direct Detection Experiments Are Sensitive to the Millicharged Background

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∼10^{-12} across 9 orders of magnitude in mass, corresponding to a dark matter subfraction as low as ∼10^{-3}.