A Search for Dark Matter Annihilation in the Milky Way Halo
  The Milky Way halo is the brightest source of dark matter annihilation on the
sky. Indeed, the potential strength of the Galactic dark matter signal can
supersede that expected from dwarf galaxies and galaxy groups even in regions
away from the Inner Galaxy. In this paper, we present the results of a search
for dark matter annihilation in the smooth Milky Way halo for $|b| > 20^\circ$
and $r < 50^\circ$ using 413 weeks of Fermi Pass 8 data within the energy range
of $\sim$0.8-50 GeV. We exclude thermal dark matter with mass below $\sim$70
GeV that annihilates to $b\bar{b}$ at the 95% confidence level using the p6v11
cosmic-ray foreground model, providing the strongest limits on the annihilation
cross section in this mass range. These results exclude the region of dark
matter parameter space that is consistent with the excess of $\sim$GeV photons
observed at the Galactic Center for the $b\bar{b}$ annihilation channel and,
for the first time, start probing the $\tau^+\tau^-$ explanation. We explore
how these results depend on uncertainties in the foregrounds by varying over a
set of reasonable models.
