A broad range of observations, from galaxies and superclusters to distant supernovae and the cosmic microwave background radiation, tell us that about 85% of the matter in the universe is not made of ordinary particles, but exists in some dark form. Deciphering the nature of this dark matter is of fundamental importance to cosmology, astrophysics, and high-energy particle physics. A leading hypothesis is that it is comprised of Weakly Interacting Massive Particles, or WIMPs, that were produced moments after the Big Bang. If WIMPs exist and are the dark matter, then their presence in our Milky Way may be detectable via scattering from atomic nuclei in a terrestrial detector.
This proposal requests the renewal of the base program of the UC Berkeley group to support their participation in the Cryogenic Dark Matter Search (CDMS) CDMS-II & SuperCDMS 25 kg projects. This includes (a) the completion of the CDMS-II experiment, which is fully constructed and acquiring science data at the Soudan Mine in Minnesota, and (b) the testing and underground operation in the CDMS-II infrastructure at Soudan of two new SuperTowers.
This program will have broader impact than improving limits or detecting WIMPs. Not only are its scientific results important for astrophysics and particle physics, but its technological development will also push the envelope of phonon-mediated detectors, whose applications are increasingly widespread, and contribute to the development of non-dissipative readout techniques. Furthermore, they continue to support the public outreach program at Soudan, which hosts some 60,000 visitors and tens of classes each year for tours of the mine and laboratory.
Weakly Interacting Massive Particles (WIMPs) are one of the main candidates to explain the mysterious dark matter in the universe, through new particle physics at the "TeV scale." A number of experiments are attempting to detect the WIMPs from the halo of our galaxy through their scattering in sophisticated detectors located deep underground. Over the last 20 years, the National Science Foundation and the Department of Energy have supported a number of experiments to check this hypothesis. The Cryogenic Dark Matter Search, CDMS, uses sophisticated low temperature detectors currently operating underground in the Soudan (MN) mine, to detect interactions of these putative WIMPs and distinguish them from experimental background. This "Experimental Particle Cosmology" award from the NSF to UC Berkeley, supported three different activities: the Berkeley participation in the analysis of data taken by CDMS, the testing of the detectors built for the next generation of the experiment, and the R&D of technologies which could be used in the longer term. The main science result of this research period has been the publication in Science in March 2010 of the full data of the CDMS II experiment. In this highly visible article (>439 citations), we reported our observations of two events, when we expected 0.8 background events. These events could be background at a 23% confidence level and certainly do not provide a compelling evidence for WIMPs. These results, however, provides a strong incentive to increase the sensitivity of such WIMP searches. In the same period, we also published a number of analyses, in particular for the incompatibility of our results with an excited dark matter scenario, or a low mass WIMP of roughly seven times the mass of the proton. In this research period, we also tested in Berkeley some 30 detectors in about 40 dilution refrigerator runs, and investigated their performance. Most of these detectors are now running at Soudan in the new SuperCDMS payload. Our long term R&D focused on two directions: the development of large diameter Germanium crystals with no dislocation, and the development of Kinetic Inductance readout for phonons sensors. Both technologies could enable the fabrication of ton scale targets at relatively low costs. In this research period we have made significant progress, identifying key challenges that we are now addressing within the next NSF award.
