A Search for Weakly-Interacting Particle Dark Matter with DarkSide

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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 will fund the continued participation of this group in the DarkSide-50 (DS-50) Dark Matter experiment at the Gran Sasso underground national laboratory in Italy. A detailed understanding of the DS-50 backgrounds and performance is key to the design of future liquid argon (LAr) dark matter detectors with substantially better sensitivity for heavy WIMPs. The UMass effort is centered on studies of radon, alpha-decays, and surface backgrounds to maximize the DS-50 physics output and to understand scale-up implications for such future, larger detectors.The PI has a diverse population of students, graduate and undergraduate, working in his lab on hardware and software projects. Their work reaches beyond the UMass walls, as they experience research at the forefront of particle physics in most cases for the first time. Fully involved in international research collaborations, they have the opportunity to travel to prime science laboratories and institutions, meet fellow students and scientists of international renown, and have hands-on experiences on leading experiments. The PI has an established track record for promoting diversity in his research group.DS-50 searches for WIMPs with a dual-phase liquid argon Time Projection Chamber (LAr TPC) in which WIMPs could deposit energy into both scintillation and ionization signals. Exploiting the different time profile ("pulse shape") of the scintillation light of nuclear and electron recoils, DS-50 has demonstrated beta/gamma background suppression of 1.5x10^7. The continued run of DS-50 will include the full information of ionization signals in a background-free dark matter search with spin-independent WIMP-nucleon cross section sensitivity of ~­10x^-45 cm^2 for 100 GeV mass WIMPs.