Breit-Wigner Enhancement Considering the Dark Matter Kinetic Decoupling

TL;DR

This paper investigates how kinetic decoupling affects Breit-Wigner enhancement of dark matter annihilation, showing it significantly impacts relic density calculations and challenges explanations of cosmic ray excesses within minimal models.

Contribution

It analyzes the impact of kinetic decoupling on Breit-Wigner enhancement and its implications for dark matter relic density and cosmic ray excess explanations.

Findings

Kinetic decoupling occurs around 10 GeV, reducing relic density.

Breit-Wigner enhancement factor is limited to about 100.

Difficulty in explaining cosmic ray excesses with minimal models.

Abstract

In the paper we study the Breit-Wigner enhancement of dark matter (DM) annihilation considering the kinetic decoupling in the evolution of DM freeze-out at the early universe. Since the DM temperature decreases much faster (as $1/R^2$) after kinetic decoupling than that in kinetic equilibrium (as 1/R) we find the Breit-Wigner enhancement of DM annihilation rate after the kinetic decoupling will affect the DM relic density significantly. Focusing on the model parameters that trying to explain the anomalous cosmic positron/electron excesses observed by PAMELA/Fermi/ATIC we find the elastic scattering $Xf\to Xf$ is not efficient to keep dark matter in kinetic equilibrium, and the kinetic decoupling temperature $T_{kd}$ is comparable to the chemical decoupling temperature $T_f\sim O(10) GeV$. The reduction of the relic density after $T_{kd}$ is significant and leads to a limited enhancement factor $\sim O(10^2)$. Therefore it is difficult to explain the anomalous positron/electron excesses in cosmic rays by DM annihilation and give the correct DM relic density simultaneously in the minimal Breit-Wigner enhancement model.