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.
It is believed that WIMPs will interact with normal matter through a process of elastic nuclear scattering. The dominant source of background for a dark matter detector comes in the form of gamma-ray radiation which will recoil off electrons. Through measuring both the charge and light produced in these interactions, it is possible to discriminate between the two. This is what is called a "two-phase" detection system. ZEPLIN-II was the first two-phase liquid xenon dark matter detector in the hunt for dark matter. ZEPLIN-II demonstrated a discrimination power of 98.5% meaning that 98.5% of gamma-ray events could be rejected as potential WIMP signals. Based in the Boulby underground laboratory, ZEPLIN-II ran for 31 days. After analysis was completed, no events above the predicted background were seen. This means that no claim of a positive dark matter measurement could be put forward. Given this null discovery, a competitive limit of 6.6x10^-7 pb upper limit for the WIMP-nucleon interaction cross-section. ZEPLIN-II taught the community valuable lessons about radon emanation from detector components, paving the way for advances in background reduction techniques still used in the leading experiments today. The discoveries about radon emanation also allowed the development of cutting edge analysis techniques designed to identify and remove unwanted sources of background from data. These techniques are still used. The successor to ZEPLIN-II, ZEPLIN-III was developed in parallel and began acquiring data immediately after the conclusion of the ZEPLIN-II program. After an initial engineering run produced a limit on the WIMP nucleon cross-section of 8.1×10^-8 , a second run of 320 days (the longest uninterrupted run of a liquid noble gas dark matter detector) produced the worlds best limit for a 10kg scale dark matter detector at 3.9×10^-8 pb and more than an order of magnitude improvement over ZEPLIN-II. ZEPLIN-III was also the first two-phase liquid xenon detector to demonstrate an electron/nuclear recoil discrimination power up to a level of 99.99%. The successes of and lessons learnt from the ZEPLIN dark matter program continue to influence the direction of direct dark matter detection experiments worldwide. Collaborators from these projects are now involved in multiple two-phase dark matter detection experiments, imparting their knowledge and experience to a new generation of young scientists.
