Astrophysical constraints on millicharged atomic dark matter

TL;DR

This paper uses astrophysical observations to set constraints on the electric charge of dark matter particles that form bound states, focusing on their dispersive effects on light from sources like the Crab Pulsar.

Contribution

It introduces a method to constrain dark atom millicharges using frequency-dependent light delay measurements from astrophysical sources.

Findings

Dark electron and proton millicharges are constrained to be smaller than e for dark atom masses below 100 keV.

Future gamma-ray burst observations could improve constraints on dark atom millicharges in the keV mass range.

Constraints are competitive with collider bounds for certain dark matter mass ranges.

Abstract

Some models of inelastic dark matter posit the existence of bound states under some new $U(1)'$ gauge symmetry. If this new dark photon kinetically mixes with the standard model photon, then the constituent particles in these bound states can acquire a fractional electric charge. This electric charge renders a dark-matter medium dispersive. We compute this frequency-dependent index of refraction for such a medium and use the frequency-dependent arrival time of light from astrophysical sources to constrain the properties of dark atoms in the medium. Using optical-wavelength observations from the Crab Pulsar, we find the electric millicharge of dark (electrons) protons to be smaller than the electric charge $e$ for dark atom masses below 100 keV, assuming a dark fine structure constant $\boldsymbol{\alpha}=1$. We estimate that future broadband observations of gamma-ray bursts can produce constraints on the millicharge of dark atoms with masses in the keV range that are competitive with existing collider constraints.