Astronomical observations tell us that approximately 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 so, then the presence of WIMPs in our Milky Way may be detectable via scattering from atomic nuclei in a terrestrial detector. The Cryogenic Dark Matter Search (CDMS) Collaboration has pioneered the use of low-temperature phonon-mediated devices to search for the rare scattering of WIMPs on nuclei and to distinguish them from background interactions.
This award provides support for the UCD group that, in conjunction with the initial SuperCDMS project grants, will enable (a) the completion of the CDMSII experiment, which is fully constructed and acquiring science data at the Soudan Mine, (b) the testing and underground operation at Soudan of two new ?SuperTowers,? each comprised of six next-generation detectors and (c) a modest effort of long-range research and development to develop and test new hardware and detectors for possible followup experiments. Specifically, this proposal will support the PI's activities in the CDMS projects as well as the group's long-term development and implementation of a two-stage superconducting quantum interference device (SQUID) pre-amplifier phonon-readout system and development of a SQUID-based ionization-readout system for future detector technology.
As part of the Broader Impacts of this program, the technical development will push the envelope of phonon-mediated detectors, which have increasing utility in astrophysics, from optical astronomy to x-rays and cosmic microwave backgrounds studies, as well as other branches of physics such as quantum computing.
Science Impact: Most of our knowledge of the universe at large is derived through astronomical observations through either ground- or space-based telescopes. All this knowledge is therefore based on electromagnetic radiation—photons, or light—of various energies and wavelengths received from space. Any component of the universe that doesn’t interact with light is therefore extremely difficult to study. Despite this difficulty, there is an accumulation of evidence that a significant portion of the matter in the universe (more than 80%) interacts gravitationally but not electromagnetically; it is, essentially, dark matter. (There’s also ‘dark energy,’ but that’s another field of study…) None of the standard models of physics contains particles consistent with observations of mass distributions inferred from the astronomical observations. No such particles have been observed in terrestrial particle accelerators. The direct detection of particles responsible for this dark matter is therefore of great importance to our understanding of our universe. The Cryogenic Dark Matter Search (CDMS) collaboration operates experiments whose goal is the direct detection of dark matter in terrestrial laboratories. CDMS experiments are designed to be particularly sensitive to one candidate for dark matter, weakly interacting massive particles (WIMPs). The CDMS experiments have some sensitivity to other candidates, such as axions, as well, but they excel at background-free searches for WIMPs. The SuperCDMS experiment aims to measure the recoil energy imparted to a nucleus due to collisions with WIMPs by employing detectors which are highly sensitive to the ionization and phonon signals that results from a WIMP-on-nucleus collision. The detectors, known as iZIP (interleaved Z-sensitive Ionization Phonon) detectors, feature state-of-the-art superconducting thin films deposited on 600 g germanium crystals to accurately measure information about the WIMP collisions. The SuperCDMS experiment is located at the Soudan Underground Laboratory in Minnesota wherein it will operate a total detector mass of ~10kg. This underground location provides essential shielding from cosmic rays, which are a source of background signals in the WIMP search. More than a dozen institutions participate in this effort with support from the National Science Foundation and the Department of Energy. With such a broad institutional participation, institutions contribute to the collaboration through activities focusing on their individual expertise. The Principal Investigator’s (Huber’s) group at the University of Colorado Denver (UCD) specializes in the cryogenic signal readout elements; specifically, the preamplifiers that allow the detector signals to be measured by room-temperature electronics. These preamplifiers are arrays of superconducting quantum interference devices (SQUIDs), and Huber’s laboratory is known locally as the SQUID Lab. The UCD group contributes to the design, fabrication, and operation of the SQUID preamplifiers, a critical component of the signal readout chain. The UCD group also contributes to operation of the experiment at the underground laboratory in Minnesota and research and development into next-generation experiments that will prove even more sensitive than the existing detectors. Additional information on the CDMS collaboration and its activities can be found at http://cdms.berkeley.edu. Broader Impact: The University of Colorado Denver is an urban campus in the University of Colorado system and, as such, its student body comprises both traditional and non-traditional students, and students from many groups underrepresented in science, technology, engineering, and math (STEM) disciplines. The PI offers research opportunities to outstanding students from this diverse student population, primarily at the undergraduate level in Physics and Electrical Engineering, but also in other disciplines as warranted and at the Master’s level in Electrical Engineering. All the SQUID preamplifiers now in use at the underground site as well as the test facilities were initially verified by undergraduate student interns in the PI’s laboratory. Several students have pursued graduate studies and some are employed at local high-tech companies.
