Sommerfeld enhancements with vector, scalar, and pseudoscalar force carriers

The first AMS-02 measurement confirms the existence of an excess in the cosmic-ray positron fraction previously reported by the PAMELA and Fermi-LAT experiments. If interpreted in terms of thermal dark matter (DM) annihilation, the AMS-02 result still suggests that the DM annihilation cross section in the present day should be significantly larger than that at freeze-out. The Sommerfeld enhancement of the DM annihilation cross section is a possible explanation, which is however subject to the constraints from DM thermal relic density, mainly due to the annihilation of DM particles into force-carrier particles introduced by the mechanism. We show that the effects of the Sommerfeld enhancement and the relic density constraints depend significantly on the nature of the force carrier. Three scenarios where the force carrier is a vector boson, scalar, and pseudoscalar particle are investigated and compared. The results show that for the case with a vector force carrier, the Sommerfeld enhancement can marginally account for the AMS-02 data for the DM particle annihilating into 2 mu final states, while for the scalar force carrier the allowed Sommerfeld enhancement factor can be larger by a factor of 2. For the case with a pseudoscalar force carrier, the Sommerfeld enhancement factor can be very large in the resonance region, and it is possible to accommodate the AMS-02 and Fermi-LAT result for a variety of DM annihilation final states.