CMB Constraints on WIMP Annihilation: Energy Absorption During the Recombination Epoch

TL;DR

This paper calculates how dark matter annihilation affects the ionization history during recombination and uses CMB data to constrain dark matter models, especially those explaining cosmic ray excesses.

Contribution

It provides detailed energy absorption rates for various dark matter annihilation channels and masses, and applies these to constrain dark matter models with CMB observations.

Findings

WMAP data constrains dark matter annihilation models explaining cosmic ray excesses.

Planck data will further tighten constraints on dark matter annihilation.

Sommerfeld-enhanced models must be near saturation to fit constraints.

Abstract

We compute in detail the rate at which energy injected by dark matter annihilation heats and ionizes the photon-baryon plasma at z ~ 1000, and provide accurate fitting functions over the relevant redshift range for a broad array of annihilation channels and DM masses. The resulting perturbations to the ionization history can be constrained by measurements of the CMB temperature and polarization angular power spectra. We show that models which fit recently measured excesses in 10-1000 GeV electron and positron cosmic rays are already close to the 95% confidence limits from WMAP. The recently launched Planck satellite will be capable of ruling out a wide range of DM explanations for these excesses. In models of dark matter with Sommerfeld-enhanced annihilation, where sigma v rises with decreasing WIMP velocity until some saturation point, the WMAP5 constraints imply that the enhancement must be close to saturation in the neighborhood of the Earth.