Study of Argon for WIMP Dark Matter Detectors and Earth Sciences

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Evidence for the existence of dark matter is now compelling, but its composition remains a fundamental mystery. Particularly intriguing is the possibility that dark matter consists of elementary particles in the form of Weakly Interacting Massive Particles (WIMPs). WIMPs may be detected by their collisions with nuclei, but the expected low rate and low energy of the recoil nuclei present daunting size and background requirements for the detector. Liquid argon is an excellent scintillation and ionization detector and has unique features that make it particularly attractive for detection of WIMP dark matter. The authors propose to contribute to the development of liquid argon as a detector for WIMPs. During the past year the Princeton group participated in development and tests of the 3.2-kg WARP liquid argon detector at the Gran Sasso underground laboratory in Italy. The results of the tests are very positive and demonstrate the feasibility of a sensitive WIMP search with a large-scale LAr detector. They propose to increase their involvement in the development and operation of larger liquid argon detectors, with the next step being a 140-kg detector. Except for critical electronics that is requested in this proposal, the 140-kg WARP detector is funded and under construction. Commissioning is expected to occur in Summer 2007. The research plan consists of the following three goals: 1) Pulse Shape Discrimination: further development of pulse shape discrimination with new 0.5-1 GHz transient digitizers (1-2 ns digitization). 2) Light Collection: the PIs propose to investigate ways to improve the light collection system, including the possibility of adding an array of PMTs in liquid argon at the bottom of the detector. 3) Underground Argon: the radioactive 39Ar found in commercial argon, which is due to cosmic ray interactions with the argon in the atmosphere, produces a significant beta background in argon detectors. It is likely that argon with a low content of 39Ar could be obtained from natural gas wells. Gas that originates from the Earth's mantle, such as at the CO2 gas wells in Northeast New Mexico, should have a very low level of 39Ar. The development of US sources for geological argon would have many broader impacts applications to low background counting, in addition to WIMP searches, and would be a major asset for the upcoming Deep Underground Science and Engineering Laboratory (DUSEL). The study of geological argon also constitutes a good example of scientific cooperation between two of the disciplines involved in the DUSEL, physics and geology.