Enhancement of Dark Matter Annihilation via Breit-Wigner Resonance

The Breit-Wigner enhancement of the thermally averaged annihilation cross section <\sigma v> is shown to provide a large boost factor when the dark matter annihilation process nears a narrow resonance. We explicitly demonstrate the evolution behavior of the Breit-Wigner enhanced <\sigma v> as the function of universe temperature for both the physical and unphysical pole cases. It is found that both of the cases can lead an enough large boost factor to explain the recent PAMELA, ATIC and PPB-BETS anomalies. We also calculate the coupling of annihilation process, which is useful for an appropriate model building to give the desired dark matter relic density.