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PICO is an experiment searching for direct evidence of dark matter using a bubble detector of chlorofluorocarbon (Freon) as the active mass. It is located at SNOLAB in Canada.

It was formed in 2013 from the merger of two similar experiments, PICASSO' and COUPP.[1][2]

PICASSO (Project In Canada to Search for Supersymmetric Objects, or Projet d'Identification de CAndidats Supersymétriques SOmbres in French) was an international collaboration with members from the Université de Montréal, Queen's University, Indiana University South Bend and Czech Technical University in Prague, University of Alberta, Laurentian University and BTI, Chalk River, Ontario. PICASSO is predominantly sensitive to spin-dependent interactions of Weakly Interacting Massive Particles (WIMPs) with fluorine atoms.

COUPP (Chicagoland Observatory for Underground Particle Physics) was a similar project with members from Fermilab, University of Chicago, and Indiana University. Prototypes were tested in the MINOS experiment far hall, with a scaled-up experiment also operating at SNOLAB. It used trifluoroiodomethane (CF3I as the medium.

Principle

A bubble detector is a radiation sensitive device that uses small droplets of superheated liquid that are suspended in a gel matrix.[3] It uses the principle of a bubble chamber but since only the small droplets can undergo a phase transition at a time, the detector can stay active for much longer periods than a classic bubble chamber. When enough energy is deposited in a droplet by ionizing radiation the superheated droplet undergoes a phase transition and becomes a gas bubble. The PICASSO detectors contain Freon droplets with an average diameter of 200 µm. The bubble development in the detector is accompanied by an acoustic shock wave that is picked up by piezo-electric sensors. The main advantage of the bubble detector technique is that the detector is almost insensitive to background radiation. The detector sensitivity can be adjusted by changing the temperature of the droplets. Freon-loaded detectors are typically operated at temperatures between 15–55 °C (60–130 °F).

The validity of the bubble detector concept has been shown in several publications.[4] There is another similar experiment using this technique in Europe called SIMPLE.
Results

PICASSO reports results (November 2009) for spin-dependent WIMP interactions on 19F. No dark matter signal has been found, but for WIMP masses of 24 GeV/c2 new stringent limits have been obtained on the spin-dependent cross section for WIMP scattering on 19F of 13.9 pb (90% CL). This result has been converted into a cross section limit for WIMP interactions on protons of 0.16 pb (90% CL). The obtained limits restrict recent interpretations of the DAMA/LIBRA annual modulation effect in terms of spin dependent interactions.[5]

New results were published in May 2012, using 10 detectors with total exposure 14 kg·d, to constrain low-mass WIMP interaction on 19F.[6] The best spin-dependent limits were obtained for a 20 GeV/c2 WIMP mass: 0.032 pb (90% C.L.) for proton cross section. For the Spin-independent near 7 GeV low mass region cross section: 1.41×10⁻⁴ pb upper limit (90% C.L.)
References

"PICO". SNOLAB. Retrieved 2015-12-01.
"PICO/PICASSO". Institute of Particle Physics. Retrieved 2015-12-02. "The PICASSO and COUPP collaborations merged in 2013 to form the PICO collaboration."
http://www.bubbletech.ca/radiation_detectors_files/bubble_detectors.html
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=find+collaboration+picasso&FORMAT=WWW&SEQUENCE=
S. Archambault; et al. (PICASSO Collaboration) (2009). "Dark matter spin-dependent limits for WIMP interactions on 19F by PICASSO". Physics Letters B 682 (2): 185–92. arXiv:0907.0307v2. Bibcode:2009PhLB..682..185A. doi:10.1016/j.physletb.2009.11.019.

PICASSO Collaboration (3 May 2012). "Constraints on low-mass WIMP interactions on 19F from PICASSO". Physics Letters B 711 (2): 153–161. arXiv:1202.1240. Bibcode:2012PhLB..711..153A. doi:10.1016/j.physletb.2012.03.078.