What it takes to measure a fundamental difference between dark matter and baryons: the halo velocity anisotropy

Numerous ongoing experiments are aimed at detecting WIMP dark matter particles from the galactic halo directly through WIMP-nucleon interactions. Once such a detection is established a confirmation of the galactic origin of the signal is needed. This requires a direction-sensitive detector. We show that such a detector can measure the velocity anisotropy beta of the galactic halo. Cosmological N-body simulations predict the dark matter anisotropy to be nonzero, beta similar to 0.2. Baryonic matter has beta = 0 and therefore a detection of a nonzero beta would be strong proof of the fundamental difference between dark and baryonic matter. We estimate the sensitivity for various detector configurations using Monte Carlo methods and we show that the strongest signal is found in the relatively few high recoil energy events. Measuring beta to the precision of similar to 0.03 will require detecting more than 10(4) WIMP events with nuclear recoil energies greater than 100 keV for a WIMP mass of 100 GeV and a S-32 target. This number corresponds to similar to 10(6) events at all energies. We discuss variations with respect to input parameters and we show that our method is robust to the presence of backgrounds and discuss the possible improved sensitivity for an energy-sensitive detector.