MRI Consortium: Development of Instrumentation for the PICO-500 Bubble Chamber

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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 in acoustic discrimination against alpha particles and particulate-induced backgrounds 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/c^2. The group has aided the broader scientific community by fabricating specially designed transducers. Several astrophysics experiments are now using acoustic particle identification. The group has also been working with a luthier to develop techniques to measure wood properties and design jazz arch top guitars with predictable sound. A liquid xenon bubble chamber experiment was outfitted with cryogenic-safe transducers to correlate scintillation light with bubble images. The PSU group has developed novel radiopure piezoceramics with properties suggesting ten times greater sensitivity than the best current PZT formulations.This award provides funds to enable the Indiana University South Bend (IUSB), North Eastern Illinois University (NEIU), and Penn State groups to develop hardware and software components of the ton-scale PICO-500 bubble chamber which will be the most sensitive detector for a wide variety of dark matter particle candidates and will serve over 20 research groups around the world. PICO uses superheated liquids to search for the tiny energy depositions left by nuclei recoiling due to collisions with dark matter. These depositions cause the liquid to explosively transform into gas. The IUSB group has designed and fabricated all of the acoustic transducers for the previous detectors. In 2008 the group discovered that it is possible to differentiate the sounds of bubbles created by nuclear recoils from those initiated by alpha decay (the main background). The NEIU team has worked on improving the control and display software necessary for operating the detector. The PSU team has been responsible for fabricating piezoelectric ceramics from radiopure oxides.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.