The PICO-2L program: Towards Background-Free Dark Matter Direct Detection Using Bubble Chambers

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Amole, Chanpreet
PICO , PICO-2L , Dark Matter , Direct Detection , Bubble Chamber , WIMP , SNOLAB , C. Amole , Amole
The PICO-2L program commenced in 2013 with the deployment of a 2 litre bubble chamber filled with superheated C3F8 and over the next three years, conducted two separate WIMP-search experiments. The first run was conducted at 4 different energy thresholds ranging from 3.2 keV to 8.1 keV and a total of 12 candidate events were observed during a total exposure of 211.5 kg-days. Due to the non-uniform time distribution of the candidate events, no evidence for a WIMP-signal was claimed and upper limits on WIMP-nucleon scattering cross-sections were reported. These results represented the world leading limits at the time on spin-dependant WIMP- proton cross-sections with a 90% confidence upper limit of σ < 9.7 × 10^−40 cm^2 for a 30 GeV/c^2 WIMP. PICO-2L was redeployed in 2015 with modifications that aimed to control particulate contamination in the superheated fluid, and a follow- up WIMP-search experiment was conducted. During 129 kg-day exposure with the detector operating at an energy threshold of 3.3 keV, only 1 candidate event was observed, consistent with the predicted background rate from neutrons. These data showed that the measures taken to control the particulate contamination resulted in the eradication of the previously observed anomalous background events and provided the most stringent limits on WIMP-proton spin-dependent cross-sections with a 90% confidence upper limit of σ < 5.9 × 10^−40 cm2 for a 30 GeV/c^2 WIMP.
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