Improved dark matter limits from pulse shape discrimination in a low background sodium iodide detector at the Boulby mine

We report new limits on nuclear scattering events resulting from Galactic dark matter in the form of hypothetical weakly interacting massive particles. By pulse shape analysis of 6 months' underground data from a 6 kg sodium iodide crystal, over the gamma-equivalent energy range 4-25 keV, we achieve a sensitivity to nuclear recoil events a factor 10-40 below the gamma and beta background. Limits obtained for spin-dependent dark matter interactions improve on Ge detectors by a factor 50. Limits for coherent interactions improve marginally on Ge for masses > 50 GeV, with foreseeable improvements offering further gains of 1-2 orders of magnitude.