Request for continued operation and study of the ZEPLIN II dark matter detector

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Large scale astronomical observations, including expansion, galaxy clustering, gravitational lensing and microwave background, indicate that the universe is dominated by unidentified dark matter whose total mass exceeds that of normal (baryonic) matter by a factor 5-10. A favored dark matter candidate is a massive particle interacting only weakly with normal matter but nevertheless producing rare nuclear recoils detectable in suitable targets such as liquid xenon.This UCLA group requests funding for the continued running and performance improvements for the 32 kg two-phase ZEPLIN II liquid Xenon dark matter detector which is now running in the UK Boulby Mine. The program is based on a new technique for identifying low energy nuclear recoils from dark matter collisions, using scintillation and ionization processes in liquid xenon. After completion of laboratory tests in 2005, the detector was installed underground and initial runs with neutron and gamma sources show primary S1 and secondary S2 scintillation pulses in accordance with expectation, and with neutron and gamma populations having different mean values of the parameter S2/S1. New work needed includes the investigation and prevention of radon influx into the detector, removal of radon decay products from interior surfaces, improvements to data acquisition, improved event selection, and reduction of energy threshold by a more efficient trigger. The detection of particle dark matter would open a new window in astronomy and particle physics, with more advanced detectors able to determine both details of the particle properties and the spectrum and flow of the dark matter in our Galaxy. It will thus transform and extend world activities in the particle physics and astronomical communities. Moreover, the techniques themselves can be further scaled up and applied to other fundamental experiments such as double beta decay, and solar neutrinos. As with other particle detection techniques, new methods of position sensitivity and particle discrimination may give rise to new medical diagnostic techniques.