PICASSO, COUPP and PICO - Search for Dark Matter with Bubble Chambers Robert Filgas (on behalf of the PICO Collaboration) Institute of Experimental and Applied Physics (IEAP), Czech Technical University in Prague International Conference on New Frontiers in Physics, July 31 – August 6 2014, Kolymvari, Greece

C. Amole, M. Besnier, G. Caria, G. Giroux, A. Kamaha, A. Noble I. Lawson, E. Vázquez Jáuregui M. Ardid, M. Bou-Cabo, I. Felis D.M. Asner, J. Hall D. Baxter, C.E. Dahl, M. Jin, J. Zhang S. Gagnebin, C. Krauss, D. Marlisov, P. Mitra P. Bhattacharjee. M. Das, S. Seth R. Filgas, S. Pospisil, I. Stekl E. Behnke, H. Borsodi, O. Harris, I. Levine, E. Mann, J. Wells S.J. Brice, D. Broemmelsiek, P.S. Cooper, M. Crisler, W.H. Lippincott, M.K. Ruschman, A. Sonnenschein K. Clark F. Debris, M. Fines-Neuschild, C.M. Jackson, M. Lafrenière, M. Laurin, L. Lessard, J.-P. Martin, M.-C. Piro, A. Plante, O. Scallon, N. Starinski, V. Zacek J.I. Collar, R. Neilson, A.E. Robinson N. Dhungana, J. Farine, R. Podviyanuk, U. Wichoski D. Maurya, S. Priya

Superheated liquids Detection of elastic scattering of WIMPs on target nucleus Critical energy deposition within small volume induces bubble nucleation Advantages of bubble chambers: Insensitive to electron and gamma background Easily scalable to large sizes at reasonable cost Interchangeable liquids, excellent SD sensitivity Drawbacks: Threshold detectors – lack of direct energy measurement Sensitive to alpha radiation

PICASSO Acrylic container ø14 × 40 cm 4.5 liters 200 µm droplets 200 µm droplets of C4F10 dispersed in polymerised gel Droplets superheated at ambient temperature and pressure Operating temperature determines energy threshold (20 °C – 50 °C) Events recorded by 9 piezo-electric transducers Bubbles recompressed back to droplets by pressurization (6 bar) Low threshold Eth (45 °C) = 2 keV 4.5 liters 200 µm droplets of C4F10 Stainless steel frame 9 piezo-sensors

PICASSO 32 modules housed in 8 temperature and pressure control units Radiopure acrylic/steel vessels constructed at IEAP Total of 2.6 kg of active mass 40-50 hour runs of data-taking followed by 11 hours of recompression Located at SNOlab, Canada 2 km of rock and 50 cm of UPW shield against neutrons

PICASSO response At operating temperatures (< 50 °C) insensitive to electrons and gamma radiation Alpha particles are dominant background The acoustic signal from bubble formation is recorded and used for analysis Neutrons Alphas 50 GeV WIMPs Gammas, electrons

Neutron – alpha discrimination PICASSO pioneered the use of acoustic sensors to reject alphas Events initiated by alpha-decays include both long alpha tracks (~10 microns) and recoiling daughter nuclei - acoustic pulses are several times louder than those emitted by single nuclear recoil events > 99.4% alpha discrimination! Confirmed by COUPP detector

COUPP Superheated fluid contained within single pressure-controlled vessel Uses CF3I – probes both SD and SI sectors Cameras – excellent position reconstruction - able to identify multiple scattering events characteristic of neutrons Located at SNOLAB COUPP 4 kg -> PICO 2L 2013 – COUPP 60 kg

COUPP Energy threshold determined by pressure rather than temperature Rapid repressurization after each nucleation The detector consists of a quartz (synthetic fused silica) bell jar, a stainless steel diaphragm/bellows, seals, and highly purified fluids.

PICASSO + COUPP = PICO PICO combines operational principle of COUPP bubble chamber and fluid handling expertise from PICASSO Uses C3F8 better SD sensitivity due to higher fluorine concentration higher efficiency for detecting fluorine recoils lower threshold without gamma and electron background

PICO PICO 2L (former COUPP 4kg) running at SNOLAB Collaboration goal is to build PICO 250L - C3F8 or CF3I target fluid Background needs to be less than 1 event/year IEAP part of the material assay group to test U/Th contamination of all materials (dominant neutron source) and to devise Radon-cleaning facility (dominant alpha source)

PICO 2L VIDEO

Results Spin dependent Spin independent Results to be published soon: PICASSO run ended Dec 2013 COUPP 60 kg 1st run ended May 2014 PICO 2L 1st run ended May 2014

Extra Slides

SD and SI Reaction classified as spin-dependent (SD), when the sign of the scattering amplitude depends on the relative orientation of particle spin