SHM$^{++}$: A Refinement of the Standard Halo Model for Dark Matter
  Searches in Light of the Gaia Sausage
  Predicting signals in experiments to directly detect dark matter (DM)
requires a form for the local DM velocity distribution. Hitherto, the standard
halo model (SHM), in which velocities are isotropic and follow a truncated
Gaussian law, has performed this job. New data, however, suggest that a
substantial fraction of our stellar halo lies in a strongly radially
anisotropic population, the 'Gaia Sausage'. Inspired by this recent discovery,
we introduce an updated DM halo model, the SHM$^{++}$, which includes a
`Sausage' component, thus better describing the known features of our galaxy.
The SHM$^{++}$ is a simple analytic model with five parameters: the circular
speed, local escape speed and local DM density, which we update to be
consistent with the latest data, and two new parameters: the anisotropy and the
density of DM in the Sausage. The impact of the SHM$^{++}$ on signal models for
WIMPs and axions is rather modest since the multiple changes and updates have
competing effects. The largest change occurs for directional detectors which
have sensitivity to the full three-dimensional velocity distribution.
