Collaborative Research: DarkSide-20k

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Multiple astronomical observations have established that about 85% of the matter in the universe is not made of normal atoms, but must be otherwise undetected elementary "dark matter" particles that do not emit or absorb light. Deciphering the nature of this so-called 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 are the dark matter, then their presence in our galaxy may be detectable via scattering from atomic nuclei in detectors located deep underground to help reject backgrounds due to cosmic rays. Direct detection of WIMP dark matter would solve a fundamental mystery in particle physics and cosmology, providing a unique window to learning about the primary matter constituent of the Universe and of physics beyond the Standard Model of particle physics. This award is to support the NSF-funded DarkSide (DS) groups for the construction of DarkSide-20k (DS-20k), a direct WIMP search using a Liquid Argon Time Projection Chamber (LAr TPC) with an active (fiducial) mass of 23 tons (20 tons).DarkSide collaborators have proposed a technology for separation of He-3 at large He-4 production plants that could alleviate future shortages of the rare helium isotope. The Urania project will establish a long-term facility that will be able to produce hundreds of tons of low-radioactivity underground argon (UAr), and will supply UAr for several programs beyond DarkSide-20k, such as for radiometry, Ar-39 age dating of ground water, and National Security treaty verification. The DarkSide program will advance technologies for detection of Ar-37, used for nuclear test ban verification, and Ar-39, used for radiometric dating. The Aria project will further reduce the Ar-39 in UAr by isotope separation, yielding as by-product the ability to separate important stable isotopes such as O-18, N-15, and C-13. These are used to produce radionuclides for medical positron emission tomography (PET). Technologies for advanced gas purification, precision cleaning, production of ultra-pure titanium, and reduction of radioactivity in silicon chips will also be developed.The significant improvement in sensitivity is made possible by technology developed within several world-wide programs toward WIMP recoil detection with liquid argon as the target, and in particular within the DarkSide-10 and DarkSide-50 programs, including operation of the DarkSide-50 LAr TPC with both atmospheric argon (AAr) and underground argon (UAr) fills. Liquid argon is an excellent medium for WIMP detection due to its efficient conversion of energy from nuclear recoils into both ionization and scintillation. The argon scintillation time profile ('pulse shape') depends on the nature of the ionizing particle, providing powerful particle discrimination to suppress background from natural radioactivity. Using AAr in DarkSide-50, it was demonstrated that pulse shape analysis suppresses beta/gamma backgrounds by a factor in excess of 1.5 x 10^7, one of the most powerful background rejection factors among all dark matter detectors.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.