Informing dark matter direct detection limits with the ARTEMIS
  simulations
  Dark matter (DM) direct detection experiments aim to place constraints on the
DM--nucleon scattering cross-section and the DM particle mass. These
constraints depend sensitively on the assumed local DM density and velocity
distribution function. While astrophysical observations can inform the former
(in a model-dependent way), the latter is not directly accessible with
observations. Here we use the high-resolution ARTEMIS cosmological
hydrodynamical simulation suite of 42 Milky Way-mass halos to explore the
spatial and kinematical distributions of the DM in the solar neighbourhood, and
we examine how these quantities are influenced by substructures, baryons, the
presence of dark discs, as well as general halo-to-halo scatter (cosmic
variance). We also explore the accuracy of the standard Maxwellian approach for
modelling the velocity distribution function. We find significant halo-to-halo
scatter in the density and velocity functions which, if propagated through the
standard halo model for predicting the DM detection limits, implies a
significant scatter about the typically quoted limit. We also show that, in
general, the Maxwellian approximation works relatively well for simulations
that include the important gravitational effects of baryons, but is less
accurate for collisionless (DM-only) simulations. Given the significant
halo-to-halo scatter in quantities relevant for DM direct detection, we
advocate propagating this source of uncertainty through in order to derive
conservative DM detection limits.
