Galactic Positron Excess from Selectively Enhanced Dark Matter
  Annihilation
  Precision measurements of the positron flux in cosmic ray have revealed an
unexplained bump in the spectrum around $E\simeq 300\,\mathrm{GeV}$, not
clearly attributable to known astrophysical processes. We propose annihilation
of dark matter of mass $m_\chi = 780\,\mathrm{GeV}$ with a late-time cross
section $\sigma v = 4.63\times 10^{-24}\,\mathrm{cm^3\,s^{-1}}$ as a possible
source. The nonmonotonic dependence of the annihilation rate on dark matter
velocity, owing to a selective $p$-wave Sommerfeld enhancement, allows such a
large signal from the Milky Way without violating corresponding constraints
from CMB and dwarf galaxy observations. We briefly explore other signatures of
this scenario, and outline avenues to test it in future experiments.
