Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Turning off the Lights: How Dark is Dark Matter?

Samuel D. McDermott, Hai-Bo Yu, Kathryn M. Zurek

TL;DR

This paper investigates whether dark matter could carry a small electromagnetic charge (εe) and still satisfy cosmological and astrophysical constraints. It shows that velocity-dependent photon-mediated scattering yields stringent bounds from the recombination epoch and structure formation, requiring ε ≲ 10^-6–10^-4 depending on DM mass, while late-time halo and cluster observations further tighten constraints. The analysis demonstrates that DM with ε large enough to yield detectable direct-detection signals would be evacuated from the Galactic disk by magnetic fields and supernova shocks, effectively eliminating such signals in current or upcoming experiments. Overall, millicharged DM faces severe constraints across cosmology, astrophysics, and direct detection, challenging its viability and its use to explain anomalies like DAMA/CoGeNT.

Abstract

We consider current observational constraints on the electromagnetic charge of dark matter. The velocity dependence of the scattering cross-section through the photon gives rise to qualitatively different constraints than standard dark matter scattering through massive force carriers. In particular, recombination epoch observations of dark matter density perturbations require that $ε$, the ratio of the dark matter to electronic charge, is less than $10^{-6}$ for $m_X = 1 GeV$, rising to $ε< 10^{-4}$ for $m_X = 10 TeV$. Though naively one would expect that dark matter carrying a charge well below this constraint could still give rise to large scattering in current direct detection experiments, we show that charged dark matter particles that could be detected with upcoming experiments are expected to be evacuated from the Galactic disk by the Galactic magnetic fields and supernova shock waves, and hence will not give rise to a signal. Thus dark matter with a small charge is likely not a source of a signal in current or upcoming dark matter direct detection experiments.

Turning off the Lights: How Dark is Dark Matter?

TL;DR

This paper investigates whether dark matter could carry a small electromagnetic charge (εe) and still satisfy cosmological and astrophysical constraints. It shows that velocity-dependent photon-mediated scattering yields stringent bounds from the recombination epoch and structure formation, requiring ε ≲ 10^-6–10^-4 depending on DM mass, while late-time halo and cluster observations further tighten constraints. The analysis demonstrates that DM with ε large enough to yield detectable direct-detection signals would be evacuated from the Galactic disk by magnetic fields and supernova shocks, effectively eliminating such signals in current or upcoming experiments. Overall, millicharged DM faces severe constraints across cosmology, astrophysics, and direct detection, challenging its viability and its use to explain anomalies like DAMA/CoGeNT.

Abstract

We consider current observational constraints on the electromagnetic charge of dark matter. The velocity dependence of the scattering cross-section through the photon gives rise to qualitatively different constraints than standard dark matter scattering through massive force carriers. In particular, recombination epoch observations of dark matter density perturbations require that , the ratio of the dark matter to electronic charge, is less than for , rising to for . Though naively one would expect that dark matter carrying a charge well below this constraint could still give rise to large scattering in current direct detection experiments, we show that charged dark matter particles that could be detected with upcoming experiments are expected to be evacuated from the Galactic disk by the Galactic magnetic fields and supernova shock waves, and hence will not give rise to a signal. Thus dark matter with a small charge is likely not a source of a signal in current or upcoming dark matter direct detection experiments.

Paper Structure

This paper contains 13 sections, 33 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Models and General Considerations
  3. Relic Density Constraints
  4. Structure Formation and CMB Constraints
  5. Decoupling at the Recombination Epoch
  6. Effect on the Dark Matter Virialization
  7. Dark Matter Halo Constraints
  8. Elliptical galaxies
  9. The Bullet Cluster
  10. Direct Detection of Charged Dark Matter
  11. Direct Detection Basics
  12. Evacuation of Charged DM from the Disk
  13. Conclusions

Figures (2)

  • Figure 1: Constraints from various sources, from top to bottom: (i) Scattering in the bullet cluster and NGC720, (ii) DM as a charged thermal relic, and (iii) DM virial processes, and (iv) recombination epoch.
  • Figure 2: CoGeNT (blue), DAMA (green) allowed regions at 99% C.L. The CDMS-Si (yellow) line is included as a sample exclusion at $99\%$ C.L. In the gray area, the charged DM is evacuated from the Galactic disk. Also shown the bound from Recombination epoch (red). Below the dotted line (black), charged DM may diffuse to the disk.