Uncertainties in Direct Dark Matter Detection in Light of Gaia's Escape
  Velocity Measurements
  Direct detection experiments have set increasingly stringent limits on the
cross section for spin-independent dark matter-nucleon interactions. In
obtaining such limits, experiments primarily assume the standard halo model
(SHM) as the distribution of dark matter in our Milky Way. Three astrophysical
parameters are required to define the SHM: the local dark matter escape
velocity, the local dark matter density and the circular velocity of the sun
around the center of the galaxy. This paper studies the effect of the
uncertainties in these three astrophysical parameters on the XENON1T exclusion
limits using the publicly available DDCalc code. We compare limits obtained
using the widely assumed escape velocity from the RAVE survey and the newly
calculated escape velocity by Monari $et$ $al.$ using Gaia data. Our study
finds that the astrophysical uncertainties are dominated by the uncertainty in
the escape velocity (independent of the best fit value) at dark matter masses
below 6 GeV and can lead to a variation of nearly 6 orders of magnitude in the
exclusion limits at 4 GeV. Above a WIMP mass of 6 GeV, the uncertainty becomes
dominated by the local dark matter density, leading to uncertainties of factors
of $\sim$10 (3) at 6 (15) GeV WIMP mass in the exclusion limits. Additionally,
this work finds that the updated best fit value for the escape velocity based
on Gaia data leads to only very minor changes to the effects of the
astrophysical uncertainties on the XENON1T exclusion limits.
