Results are presented from the first underground data run of ZEPLIN-II, a 31 kg two-phase xenon detector developed to observe nuclear recoils from hypothetical weakly interacting massive dark matter particles. Discrimination between nuclear recoils and background electron recoils is afforded 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 event. This ratio is calibrated for different incident species using an AmBe neutron source and 60Co γ-ray sources. From our first 31 live days of running ZEPLIN-II, the total exposure following the application of fiducial and stability cuts was 225 kg × days. A background population of radon progeny events was observed in this run, arising from radon emission in the gas purification getters, due to radon daughter ion decays on the surfaces of the walls of the chamber. An acceptance window, defined by the neutron calibration data, of 50% nuclear recoil acceptance between 5 keVee and 20 keVee, 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% c.l. upper limit to the number of nuclear recoils of 10.4 events in this acceptance window, which converts to a WIMP-nucleon spin-independent cross-section with a minimum of 6.6 × 10-7 pb following the inclusion of an energy-dependent, calibrated, efficiency. A second run is currently underway in which the radon progeny will be eliminated, thereby removing the background population, with a projected sensitivity of 2 × 10-7 pb for similar exposures as the first run.
|Original language||English (US)|
|Number of pages||16|
|State||Published - Nov 2007|
Bibliographical noteFunding Information:
This work has been funded by the UK Particle Physics and Astronomy Research Council (PPARC), the US Department of Energy (Grant Nos. DE-FG03-91ER40662 and DE-FG03-95ER40917) and the US National Science Foundation (Grant No. PHY-0139065). We acknowledge support from the Central Laboratories for the Research Councils (CCLRC), the Engineering and Physical Sciences Research Council (EPSRC), the ILIAS integrating activity (Contract R113-CT-2004-506222), the INTAS programme (Grant No. 04-78-6744) and the Research Corporation (Grant No. RA0350). We also acknowledge support from Fundação para a Ciência e a Tecnologia (Project POCI/FP/FNU/63446/2005), the Marie Curie International Reintegration Grant (Grant No. FP6-006651) and a PPARC PIPPS award (Grant PP/D000742/1). We would like to gratefully acknowledge the strong support of Cleveland Potash Ltd., the owners of the Boulby mine, and J. Mulholland and L. Yeoman, the underground facility staff. We would like to thank the team from ITEP, Moscow, led by Dr. D. Akimov for their valuable contribution to the ZEPLIN programme. We acknowledge valuable advice from Professor R. Cousins on the application and extension of the Feldman–Cousins tables.
- Dark matter
- Liquid xenon
- Radiation detectors