Collaborative Research: The DarkSide Dark-Matter Search Using Liquid Argon

A large body of astronomical observations, including galactic rotation curves, strong gravitational lensing, and the anisotropy of the cosmic microwave background, indicates that known matter only accounts for a small fraction of the observed mass-energy of the Universe. A strong candidate for the remaining mass is weakly interacting Dark Matter particles with properties different from ordinary matter. Direct (non-gravitational) detection of dark matter would be a transformational discovery and would represent a watershed moment in the history of science, opening the gates to the investigation of the dark universe. The goal of this award is the testing, on-site installation, commissioning, and transitioning to operations of DarkSide-20k (DS-20k), an experiment capable of pushing the sensitivity of direct dark matter searches beyond that of any current or presently funded experiment. Broader scientific and technological impacts from the project include, among others: the production of low-radioactivity underground argon (UAr) for DS-20k and for nuclear test ban verification and radiometric dating; the continued study of the underground gas source discovered during the UAr exploration, a source which supplies 15 % of the current U.S. helium consumption; and the continued development of 3Dπ, a LAr-based TOF-PET system that can enhance cancer screening while lowering radiation dose.A leading dark matter candidate is a weakly interacting massive particle, or WIMP, a thermal relic of the Big Bang that has a sub-electroweak-scale self-annihilation cross section and a mass in the GeV/c2 to TeV/c2 range. The motion of galactic halo WIMPs relative to a detector on Earth could result in WIMP-nucleus elastic collisions detectable by a low-background, low-energy-threshold detector capable of unambiguously identifying a small number of nuclear recoils from WIMP collisions over the course of a very large exposure. Thanks to its excellent ionization response and unique scintillation light emission characteristics, liquid argon can provide outstanding sensitivity for WIMP-nuclear collisions and strong background suppression. The U.S. NSF groups involved in this program have led the conception and development of the DarkSide-20k liquid argon dark matter search. With support from a separate Mid-scale award, they are responsible for the construction and delivery of the most critical DS-20k components: the liquid argon time projection chamber (LAr TPC) that serves as the heart of the DS-20k dark matter detector along with its critical ancillary components, including the high-voltage delivery system, and the associated cryogenic, purification, and calibration systems. The U.S. NSF groups are also leading the activities for the procurement of the required ultra-low radioactivity argon, which is extracted from a special underground source in Colorado, with support from other US and international funding agencies (U.S. Department of Energy, Istituto Nazionale di Fisica Nucleare of Italy, and Canada Foundation for Innovation). This award will support the U.S. groups for the final testing, on-site installation, commissioning, and transition to operations of the DS-20k detector and preparation for science analyses. The DarkSide-20k experiment will utilize a two-phase time projection chamber with a 50-tonne fill (20-tonne fiducial) to extend the exploration for dark matter, reaching, in a 10 years run, a cross section of 4.9×10−48 cm2 for a 90 % C.L. exclusion and 1.6×10−47 cm2 at a 5 σ discovery significance for a 1 TeV/c2 WIMP, well beyond any current or presently funded experiment. This will lead to either discovery, confirmation, or exclusion of the WIMP dark matter hypothesis down to the level where coherent scatters from atmospheric neutrinos become an unavoidable background. DarkSide-20k will also be sensitive to a galactic supernova neutrino burst originating anywhere in the Milky Way Galaxy.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.