Northwestern Events Calendar

Nov
30
2015

Jianjie Zhang: Latest results from PICO

When: Monday, November 30, 2015
4:00 PM - 5:00 PM CT

Where: Technological Institute, F160, 2145 Sheridan Road, Evanston, IL 60208 map it

Audience: Public

Contact: Liz Lwanga   (847) 491-3645

Group: Physics and Astronomy High Energy Physics Seminars

Category: Academic

Description:

Title: Latest results from PICO

 

Speaker: Jianjie Zhang, Northwestern University
 

Abstract: The PICO experiment operates superheated bubble chamber detectors in the SNOLAB underground laboratory to search for the Weakly Interacting Massive Particles (WIMPs), a well motivated dark matter candidate. Two bubble chambers, a 2-liter one (PICO-2L) filled with C3F8 and a 30-liter one (PICO-60) filled with CF3I, are deployed to maximize the WIMP parameter space coverage. Both chambers exhibit excellent electron recoil and alpha rejection capabilities down to low thresholds. The first run of PICO-2L observed twelve single nuclear-recoil event candidates in the 211.5 kg-day exposure. The candidate events exhibit a significant time clustering inconsistent with a WIMP hypothesis, and the data set the most sensitive direct detection constraints on WIMP-proton spin-dependent scattering to date. That result has recently been passed by PICO-60, the largest bubble chamber to search for dark matter. The first PICO-60 physics run took an exposure of 3,415 kg-days and observed a large population (~2k) of unknown background events. The acoustic, spatial, and timing behaviors of these events are sufficiently different from those expected from a dark matter signal to allow for analysis cuts to remove all background events while retaining 48.2% of the exposure, or an effective 27% after taking the appropriate trials penalty. While the backgrounds in both PICO-2L and PICO-60 were distinguishable from the WIMP hypothesis, future progress relies on identifying and eliminating the source of these events.  A growing body of evidence points to micron-scale particulate playing the key role in producing these events.  Most recently, a new run of PICO-2L with improved material selection and cleanliness controls has succeeded in reducing the observed background by at least an order of magnitude, to a level consistent with the expected neutron background in that detector.  We are now working to reproduce this result in PICO-60 to show that the bubble chamber technology is converging toward a background free experiment.

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