The evidence is now overwhelming that ~ 25% of the mass-energy density of the universe is in the form of "dark matter". Discovering the nature of this dark matter is one of the most important questions in physics and cosmology. Perhaps the most compelling idea is that dark matter is composed of subatomic particles known as weakly interacting massive particles, or WIMPs, that were created in the big bang. They most likely have a mass roughly the same as an atom of gold, but are effectively incredibly small and difficult to detect. Such particles are a generic prediction of supersymmetry, the best-motivated theory for particle physics at higher energy scales than has been probed so far in accelerators. Probing for supersymmetry is one of the principle goals for the Large Hadron Collider at CERN. If WIMPs do exist, they must also be the dominant mass in our galaxy, and should be detectable from interactions in detectors on Earth. The rate of these interactions can be roughly predicted from the same physics that governs their production in the big bang, and is unfortunately small: somewhere between roughly 1 event/kg/month and 1 event/ton/year. Ambient backgrounds in particle detectors, both from radioactivity and cosmic rays, are much larger than this. Current searches have only just achieved sensitivity to the highest of these possible rates. The XENON collaboration has developed a new generation of dark matter detector based on liquid xenon. Like the current leading detectors, this technology has the ability to distinguish most radioactive backgrounds (gamma rays and betas), which cause an electron to recoil after the detector is struck, from WIMPs, which cause a nucleus to recoil. However it promises to more readily be scaled to the ton-scale needed to fully test the WIMP hypothesis than the current leading detectors, which have a mass of roughly 1 kg. The work funded here is participation in the XENON10, a ~10 kg prototype experiment which will be located in Gran Sasso, Italy. Our group is developing a system for removal of radioactive Kr which contaminates Xe, and is also developing a new method for removing other impurities that degrade the performance of the detector. Our group will also be involved in the design and construction of the detector, in particular wire grids used to measure ionization produced by WIMPs, and work on a CsI system for measuring scintillation. Finally, we will be involved in data taking operations and analysis. The nature of dark matter is one the most important questions in physics and cosmology, and is of great interest to the public at large. Our group will be actively engaged in public outreach, especially with local schools. Education and training of both undergraduate and graduate students is fundamental to this work, and students will have major roles in the proposed research. The technical aspects of experimental physics are a good foundation for a wide range of careers that benefit the public good. The specific techniques of very low background particle detection also have an important national security role in nuclear non-proliferation verification. Our group has been actively engaged in a joint effort between academic physicists and physicists in the defense non-proliferation community to develop advanced next-generation low radioactive background screening facilities. This work will indirectly benefit this effort.
