This proposal will fund the search led by Princeton University for evidence to the existence of dark matter that is made of Weakly Interacting Massive Particles (WIMPs). WIMPs may be detectable by their collisions with nuclei on Earth, but the low expected rate of such collisions and low energy of the recoil nuclei requires massive detectors with extremely low background rates, located in a deep underground laboratory. The development of a Deep Underground Science and Engineering Laboratory (DUSEL) will enable the deployment within the U.S. of WIMP detectors several orders of magnitude more sensitive than those operating today. This proposal plans to use Liquid argon and xenon as the active media in Time Projection Chamber (TPC)where the scintillation and ionization can be independently detected and spatially resolved through large volumes of liquid. The discrimination of nuclear recoils from background is possible using the relative size and time dependence of these signals. By exploiting these methods and the self-shielding capability of the dense liquids, the leading xenon and argon TPC experiments have already achieved competitive sensitivity to WIMPs. This proposal plans to develop two important new technologies that will greatly enhance the power of the present collaboration to design the best xenon and argon detector system for the discovery and identification of WIMPs: (1) the development within the Xenon collaboration of a new low radioactivity, high-quantum-efficiency hybrid photomultiplier tube, the Quartz Photon Intensifying Detector or Qupid, designed at UCLA in partnership with Hamamatsu and (2) the discovery of underground sources of argon gas low in the isotope 39Ar by the Princeton and Notre Dame groups, supported by the NSF. This proposal will also prepare the way for a following generation of still-larger detectors (10 ton xenon TPC, 50 ton depleted argon TPC). These will be required to detect WIMP dark matter if the cross section is below the 10 to the minus 47 cm**2, or to perform high statistics, detailed studies of WIMP properties if WIMP dark matter is discovered using DUSEL-ISE experiments.
BROADER IMPACT: The proposed activity will advance the development of DUSEL and its scientific and educational mission in a variety of ways: (1) it will help the visibility of DUSEL as an international facility, through cooperation and partnership of US universities and national laboratories with European and Japanese groups; (2) it will offer an excellent opportunity for the training of students, who will have a chance to contribute to the success of a cutting edge project in fundamental science and advanced engineering; (3) it will benefit society through commercial applications of noble liquid detectors and associated technologies in areas ranging from national security to medical imaging; (4) it will support continued development of successful EPO programs such as the Princeton-Abruzzo-South Dakota summer school for high school students.
This grant initially supported the design of the infrastructure required to support two twin, 3rd generation dark matter detectors to be deployed at DUSEL, with two-phase xenon and argon as target. Upon closing of the DUSEL project, the effort shifted towards the support of the development of larger detectors of the DarkSide and XENON families at LNGS. The XENON family of detectors led for long time the direct search for dark matter, until the recent publication of the LUX results. The DarkSide family of detectors is leading the development of argon dark matter searches. Among other tasks, the MAX collaboration confronted the key question of whether cosmogenic backgrounds could prevent the development of 3rd generation experiments at DUSEL depth. The requirement to offset cosmogenic background drove the design of the active vetoes of MAX, which influenced subsequently the design of the active vetoes for DarkSide-50. Upon closure of the DUSEL project, the focus shifted on operation of 2nd and 3rd generation experiments at LNGS depth. The problem was successfully addressed and the results have filled a gap of knowledge that would have otherwise hampered the progress of the overall field. Exploiting data from the Borexino solar neutrino experiment, MAX collaborators were able to measure the most relevant distributions of cosmogenic backgrounds. Knowledge of these distributions alone allowed calculation of the effectiveness of active vetoes of any reasonable size. In addition, our FLUKA- and GEANT4-based Monte Carlo codes, following an intense effort to improve the underlying code describing the fundamental interactions in the muon-induced showers, have been validated by an excellent agreement achieved with the distributions measured in Borexino. The improved Monte Carlo has been used to estimate the cosmogenic background for 2nd and 3rd generation detectors at LNGS depth. This study determined that the combination of the active neutron and muon veto designed for DarkSide-G2 is sufficient to completely abate the cosmogenic neutron background for an exposure in excess of 100 ton-years. This shows that the same scheme would be adequate not only for operation of 2nd generation detectors, but also for operation of 3rd generation detectors at LNGS depth. The design of the infrastructure required to support two twin, 3rd generation dark matter detectors to be deployed at DUSEL, with two-phase xenon and argon as target, was conveniently recycled to serve as the basis for the design of the infrastructure of DarkSide-50. The 1st generation dark matter detector DarkSide-50 is installed within two nested active vetoes, a 30-ton liquid scintillator neutron veto and a 1,000-ton water Cerenkov muon veto, whose design was strongly influenced by the preparatoty work developed for DUSEL. The actives vetoes for DarkSide-50 were also designed to support a possible second generation detector, DarkSide-G2.
