Consequences of a Dark Disk for the Fermi and PAMELA Signals in Theories
  with a Sommerfeld Enhancement
  Much attention has been given to dark matter explanations of the PAMELA
positron fraction and Fermi electronic excesses. For those theories with a
TeV-scale WIMP annihilating through a light force-carrier, the associated
Sommerfeld enhancement provides a natural explanation of the large boost factor
needed to explain the signals, and the light force-carrier naturally gives rise
to hard cosmic ray spectra without excess pi0 gamma rays or anti-protons. The
Sommerfeld enhancement of the annihilation rate, which at low relative
velocities v scales as 1/v, relies on the comparatively low velocity dispersion
of the dark matter particles in the smooth halo. Dark matter substructures in
which the velocity dispersion is smaller than in the smooth halo have even
larger annihilation rates. N-body simulations containing only dark matter
predict the existence of such structures, for example subhalos and caustics,
and the effects of these substructures on dark matter indirect detection
signals have been studied extensively. The addition of baryons into
cosmological simulations of disk-dominated galaxies gives rise to an additional
substructure component, a dark disk. The disk has a lower velocity dispersion
than the spherical halo component by a factor ~6, so the contributions to dark
matter signals from the disk can be more significant in Sommerfeld models than
for WIMPs without such low-velocity enhancements. We consider the consequences
of a dark disk on the observed signals of cosmic rays as measured by Fermi and
PAMELA in models where the WIMP annihilations are into a light boson. We find
that both the PAMELA and Fermi results are easily accomodated by scenarios in
which a disk signal is included with the standard spherical halo signal. Limits
from extrapolations to the center of the galaxy can also be modified.
