PICO-500: Improving Acoustic Particle Identification in Superheated-Liquid WIMP Detectors

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Multiple astronomical observations have established that about 85% of the matter in the universe is not made of known particles. 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. PICO, a merger of the PICASSO and COUPP collaborations, employs room-temperature super-heated liquids to search for dark matter particle candidates. Previous efforts by PICO at SNOLAB have resulted in a world-leading sensitivity to spin-dependent WIMP-proton couplings. Recent progress has been made in acoustic discrimination against alpha particles and particulate-induced backgrounds. These developments will enable the next-generation of PICO bubble chambers to also lead in sensitivity to spin-independent couplings in the WIMP mass range below 10 GeV/c2, thus contributing substantially to the search to address the fundamental question of what Dark Matter actually is.This award will generate substantial broader impacts in the areas of education and mentoring. The PICO collaboration has a long history of commitment to providing undergraduates with laboratory research opportunities, and to the mentoring and professional advancement of graduate students and postdocs.Superheated liquids provide the best discrimination of any dark matter detector against dominant electron recoil backgrounds (10^-10 at ~ keVnr threshold, i.e., seven orders of magnitude better than liquid-xenon detectors). PICO chambers aim to perform an extensive probing of the remaining supersymmetric dark matter parameter space via spin-dependent interactions, an approach that is highly complementary to other experiments. This maximizes the chances of WIMP discovery. The activities envisioned during the period of support include participation in the design of PICO-500, a large (500 liter) chamber recently approved for construction by the Canadia Foundation for Innovation, and an expansion of PICO's acoustic detection methods to a promising higher frequency range, thus far unexplored.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.