Dark Matter and Dark Radiation

TL;DR

This paper investigates a hypothetical long-range dark electromagnetism that interacts only with dark matter, analyzing its astrophysical implications, constraints, and potential plasma effects, while ensuring consistency with existing cosmological and particle physics data.

Contribution

It introduces a new dark U(1) gauge field coupled to dark matter, analyzing its viability, constraints, and potential effects on dark matter behavior and structure formation.

Findings

Dark electromagnetism can exist with very small coupling constants.

Constraints from galactic dynamics limit the dark fine-structure constant to about 10^{-4}.

The model is consistent with structure formation and nucleosynthesis constraints.

Abstract

We explore the feasibility and astrophysical consequences of a new long-range U(1) gauge field ("dark electromagnetism") that couples only to dark matter, not to the Standard Model. The dark matter consists of an equal number of positive and negative charges under the new force, but annihilations are suppressed if the dark matter mass is sufficiently high and the dark fine-structure constant $\hat\alpha$ is sufficiently small. The correct relic abundance can be obtained if the dark matter also couples to the conventional weak interactions, and we verify that this is consistent with particle-physics constraints. The primary limit on $\hat\alpha$ comes from the demand that the dark matter be effectively collisionless in galactic dynamics, which implies $\hat\alpha \lesssim 10^{-4}$ for TeV-scale dark matter. These values are easily compatible with constraints from structure formation and primordial nucleosynthesis. We raise the prospect of interesting new plasma effects in dark matter dynamics, which remain to be explored.