Directional dark matter detection sensitivity of a two-phase liquid
  argon detector
  We examine the sensitivity of a large scale two-phase liquid argon detector
to the directionality of the dark matter signal. This study was performed under
the assumption that, above 50 keV of recoil energy, one can determine (with
some resolution) the direction of the recoil nucleus without head-tail
discrimination, as suggested by past studies that proposed to exploit the
dependence of columnar recombination on the angle between the recoil nucleus
direction and the electric field. In this paper we study the differential
interaction recoil rate as a function of the recoil direction angle with
respect to the zenith for a detector located at the Laboratori Nazionali del
Gran Sasso and we determine its diurnal and seasonal modulation. Using a
likelihood-ratio based approach we show that, with the angular information
alone, 100 events are enough to reject the isotropic hypothesis at three
standard deviation level. For an exposure of 100 tonne years this would
correspond to a spin independent WIMP-nucleon cross section of about 10^-46
cm^2 at 200 GeV WIMP mass. The results presented in this paper provide strong
motivation for the experimental determination of directional recoil effects in
two-phase liquid argon detectors.
