Limits on WIMP-nucleon cross-sections from the ZEPLIN-II data

Abstract

Results are presented from ZEPLIN-II, the world-first 31 kg two-phase xenon dark matter detector. Discrimination between nuclear recoils and background electron recoils is achieved by recording both the scintillation and ionisation signals generated within the liquid xenon, with the ratio of these signals being different for the two classes of events. This ratio is calibrated for different incident species using neutron and γ-ray sources. Background populations of gamma-induced events and radon progeny events were observed in the data. An acceptance window, defined by the neutron calibration data, of 50% nuclear recoil acceptance between 5 keV and 20 keV electron equivalent energy, had an observed count of 29 events, with a summed expectation of 28.6±4.3 γ-ray and radon progeny induced background events. These figures provide a 90% CL upper limit to the number of nuclear recoils of 10.4 events in this acceptance window. This corresponds to a WIMP-nucleon spin-independent cross-section with a minimum of 6.6 × 10-7 pb, and to a WIMP neutron spin-dependent cross-section of 7 × 10-2 pb at a WIMP mass of around 65 GeV.