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 DM annihilation cross section as a possible
explanation for the observed excesses is subject to constraint from the DM
thermal relic density, mainly due to the annihilation of DM particles into
force-carrier particles introduced by the mechanism. We show that the effect of
the Sommerfeld enhancement and the relic density constraint depend
significantly on the nature of the force-carrier. Three cases where the
force-carrier is a vector boson, scalar and pseudoscalar particle are
investigated and compared with each other. The results show that for the vector
force-carrier, the Sommerfeld enhancement can marginally account for the AMS-02
data for $2\mu$ final states, while for scalar force-carrier the allowed
Sommerfeld enhancement can be larger by a factor of two. For the case with a
pesudoscalar force-carrier, the Sommerfeld enhancement can be large enough to
accommodate the AMS-02 and Fermi-LAT result for a variety of DM annihilation
final states.
