Assessing Astrophysical Uncertainties in Direct Detection with Galaxy
  Simulations
  We study the local dark matter velocity distribution in simulated Milky
Way-mass galaxies, generated at high resolution with both dark matter and
baryons. We find that the dark matter in the Solar neighborhood is influenced
appreciably by the inclusion of baryons, increasing the speed of dark matter
particles compared to dark matter-only simulations. The gravitational potential
due to the presence of a baryonic disk increases the amount of high velocity
dark matter, resulting in velocity distributions which are more similar to the
Maxwellian Standard Halo Model than predicted from dark matter-only
simulations. Further, the velocity structures present in baryonic simulations
possess a greater diversity than expected from dark matter-only simulation. We
show the impact on the direct detection experiments LUX, DAMA/Libra, and CoGeNT
using our simulated velocity distributions, and explore how resolution and halo
mass within the Milky Way's estimated mass range impact the results. A
Maxwellian fit to the velocity distribution tends to overpredict the amount of
dark matter in the high velocity tail, even with baryons, and thus leads to
overly optimistic direct detection bounds on models which are dependent on this
region of phase space for an experimental signal. Our work further demonstrates
that it is critical to transform simulated velocity distributions to the lab
frame of reference, due to the fact that velocity structure in the Solar
neighborhood appears when baryons are included. There is more velocity
structure present when baryons are included than in dark matter-only
simulations. Even when baryons are included, the importance of the velocity
structure is not as apparent in the Galactic frame of reference as in the Earth
frame.
