Relic density and CMB constraints on dark matter annihilation with Sommerfeld enhancement

TL;DR

This paper investigates how Sommerfeld enhancement affects dark matter relic density and CMB spectral distortions, showing that annihilation cross sections can be significantly suppressed and that certain parameter regimes are constrained by COBE/FIRAS data.

Contribution

It provides a detailed calculation of relic density adjustments and CMB distortions due to Sommerfeld enhancement, highlighting the impact of model parameters and resonance effects.

Findings

Annihilation cross section can be suppressed by up to an order of magnitude.

CMB mu-distortions can be significant near resonance conditions.

Certain parameter regions are excluded by COBE/FIRAS constraints.

Abstract

We calculate how the relic density of dark matter particles is altered when their annihilation is enhanced by the Sommerfeld mechanism due to a Yukawa interaction between the annihilating particles. Maintaining a dark matter abundance consistent with current observational bounds requires the normalization of the s-wave annihilation cross section to be decreased compared to a model without enhancement. The level of suppression depends on the specific parameters of the particle model, with the kinetic decoupling temperature having the most effect. We find that the cross section can be reduced by as much as an order of magnitude for extreme cases. We also compute the mu-type distortion of the CMB energy spectrum caused by energy injection from such Sommerfeld-enhanced annihilation. Our results indicate that in the vicinity of resonances, associated with bound states, distortions can be large enough to be excluded by the upper limit |mu|<9.0x10^(-5) found by the COBE/FIRAS experiment.