  Measurements of the cosmic ray antiproton spectrum can be used to search for
contributions from annihilating dark matter and to constrain the dark matter
annihilation cross section. Depending on the assumptions made regarding cosmic
ray propagation in the Galaxy, such constraints can be quite stringent. We
revisit this topic, utilizing a set of propagation models fit to the cosmic ray
boron, carbon, oxygen and beryllium data. We derive upper limits on the dark
matter annihilation cross section and find that when the cosmic ray propagation
parameters are treated as nuisance parameters (as we argue is appropriate), the
resulting limits are significantly less stringent than have been previously
reported. We also note (as have several previous groups) that simple
GALPROP-like diffusion-reacceleration models predict a spectrum of cosmic ray
antiprotons that is in good agreement with PAMELA's observations above ~5 GeV,
but that significantly underpredict the flux at lower energies. Although the
complexity of modeling cosmic ray propagation at GeV-scale energies makes it
difficult to determine the origin of this discrepancy, we consider the
possibility that the excess antiprotons are the result of annihilating dark
matter. Suggestively, we find that this excess is best fit for a dark matter
mass of approximately 35 GeV and annihilation cross section of approximately
1e-26 cm^3/s (to b-bbar), in good agreement with the mass and cross section
previously shown to be required to generate the gamma-ray excess observed from
the Galactic Center.
