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 a large enough boost factor to explain the recent PAMELA, ATIC, and PPB-BETS anomalies. We also calculate the coupling of the annihilation process, which is useful for an appropriate model building to give the desired dark matter relic density.