Optical dispersion of composite particles consisting of millicharged constituents

TL;DR

This paper calculates how composite dark matter with millicharged constituents disperses light, potentially causing observable time lags in astrophysical signals, and compares this effect to Lorentz invariance violation signals.

Contribution

It provides the first detailed computation of the optical dispersion caused by composite millicharged dark matter and assesses its impact on astrophysical observations.

Findings

Dispersion due to dark matter is much smaller than LIV effects below the Planck scale.

The optical index of refraction for atomic and bound dark matter systems is quantitatively modeled.

Time lag effects from dark matter dispersion are negligible compared to potential LIV signals.

Abstract

Composite dark matter (DM) comprised of electrically charged constituents can interact with the electromagnetic field via the particle's dipole moment. This interaction results in a dispersive optical index of refraction for the DM medium. We compute this refractive index for atomic dark matter and more strongly bound systems, modeled via a harmonic oscillator potential. The dispersive nature of the index will result in a time lag between high and low energy photons simultaneously emitted from a distant astrophysical observable. This time lag, due to matter dispersion, could confound potential claims of Lorentz invariance violation (LIV) which can also result in such time lags. We compare the relative size of the two effects and determine that the dispersion due to DM is dwarfed by potential LIV effects for energies below the Planck scale.