1 “ Liquid Xenon R & D for Future Large - Scale Dark - Matter Detectors ”

A strong theoretical candidate for weakly interacting massive particles (WIMPs) is the neutralino. The neutralino is the lightest supersymmetric particle (LSP), from supersymmetry (SUSY) theory, that would be formed in the early universe and subsequently cluster in galaxies with normal matter. This could account for the ~90% non-luminous mass inferred in our Galaxy from the motion of stars and gas, and forming a dark halo extending to 150-200 kpc. If the dark matter consists of nuetralinos, these could in princple be detected by collisions with normal matter, producing nuclear recoil typically in the range 1-30 keV and with predicted rates 10 – 10/kg/day. Experiments so far have achieved sensitivities down to ~ 1/kg/day without observing a nuclear recoil signal, although the DAMA group have abserved a seasonal variation in background which they claim could be caused by dark matter events. There are therefore intensive efforts to reach lower levels of sensitivity in the direct observation of nuclear recoils. The ZEPLIN-II detector (with 35-kg of liquid xenon medium), which currently has the best possibility of reaching the lowest limit, will be installed and taking data at the UK Boulby Mine next year (2002). The current research and development program (for which an NSF proposal is pending) compris es the operation of the 35-kg ZEPLIN-II detector at Boulby. ZEPLIN II will cover well below the DAMA regions. However, to cover completely the predicted SUSY dark matter regions, a large one-ton scale detector with low background, low energy threshold and high background discrimination is needed. With supplemental funding from the DOE Advanced Detector Research Program, we will be able to study the performance of the ZEPLIN-II detector and carry out a series of tests to optimize the design for the one-ton detector. These tests will include: 1. photon amplification in liquid and gas xenon using a CsI internal photo-cathode, 2. ionization yield in liquid and gas xenon due to recoil nuclei at low energies. 3. low radioactive background photon readout devices R&D and, Kr removal from xenon. 4. optimization of the ZEPLIN-II operation and combine 1, 2, 3, for the ZEPLIN II scale up. If accomplished, the one-ton xenon detector (with low energy threshold, extremely low background, Kr free and large mass) will be the best option for the future US national underground science laboratory for dark matter search. Liquid-noble-gas time-projection-chamber (TPC) technology has been utilized for a couple of key frontiers in high-energy physics experiments, including the ICARUS experiment. The ZEPLIN-II detector that is under construction at the UCLA Dark Matter Laboratory fully utilizes this technology. The UCLA group's participation in the ICARUS experiment (using liquid Ar) and in its R&D on a liquid xenon dark matter detector, has shown that the UCLA group is best suited to carry out the next generation dark matter detector R&D and operation. In addition, with new institutions (TAMU, Princeton) joining the quest, we are forming the best team with expertise from various high-energy physics experiments for this effort. A preliminary test at CERN in the gaseous xenon chamber showed that a CsI internal photo-cathode produced approximately 10 times overall amplification, taking CsI quantum efficiency into account. With repeated measurements of the experiment, the firm limit of amplification ratio can be achieved for the double-phase (liquid and gas) detector. This will lower the detector energy threshold significantly. Large Area Avalanche Photo Diode (LAAPD) and Gas Electron Multipliers (GEM) are of new devices, which if carefully optimized, could be used as single photon readout devices with very low radioactive background. PMTs currently used in ZEPLIN II may become the limiting background sources in the