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The PICASSO Dark Matter Physics Program at SNOLAB

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The PICASSO Dark Matter Physics Program at SNOLAB 33RD INTERNATIONAL COSMIC RAY CONFERENCE,RIO DE JANEIRO 2013 THE ASTROPARTICLE PHYSICS CONFERENCE The PICASSO Dark Matter Physics Program at SNOLAB A.J. NOBLE1 FOR THE PICASSO COLLABORATION: S. ARCHAMBAULT2, E. BEHNKE3, P. BHATTACHARJEE4, S. BHATTACHARYA4, X. DAI1, M. DAS4, A. DAVOUR1, F. DEBRIS2, N. DHUNGANA5, J. FARINE5, S. GAGNEBIN6, G. GIROUX2, E. GRACE3, C. M. JACKSON2, A. KAMAHA1, C. KRAUSS6, S. KUMARATUNGA2, M. LAFRENIRE2, M. LAURIN2, I. LAWSON7, L. LESSARD2, I. LEVINE3, C. LEVY1, R. P. MACDONALD6, D. MARLISOV6, J.-P. MARTIN2, P. MITRA6, A. J. NOBLE1, M.-C. PIRO2, R. PODVIYANUK5, S. POSPISIL8, S. SAHA4, O. SCALLON2, S. SETH4, N. STARINSKI2, I. STEKL8, U. WICHOSKI5, T. XIE1, V. ZACEK2 1 Department of Physics, Queens University, Kingston, K7L 3N6, Canada 2 Departement´ de Physique, Universite´ de Montreal,´ Montreal,´ H3C 3J7, Canada 3 Department of Physics & Astronomy, Indiana University South Bend, South Bend, IN 46634, USA 4 Saha Institute of Nuclear Physics, Centre for AstroParticle Physics (CAPP), Kolkata, 700064, India 5 Department of Physics, Laurentian University, Sudbury, P3E 2C6, Canada 6 Department of Physics, University of Alberta, Edmonton, T6G 2G7, Canada 7 SNOLAB, 1039 Regional Road 24, Lively ON, P3Y 1N2, Canada 8 Institute of Experimental and Applied Physics, Czech Technical University in Prague, Prague, Cz-12800, Czech Republic [email protected] Abstract: PICASSO is a dark matter search experiment currently operational at the SNOLAB International Facility for Astroparticle Physics, located 2 km underground in Sudbury, Canada. PICASSO is based on the superheated bubble technique. With good control of radiological backgrounds and a detector design that stabilizes the superheated liquid even when very close to the critical temperature, PICASSO has achieved very low nuclear recoil thresholds. This allowed the experiment to produce world-leading results with particular sensitivity to low mass WIMP signals in both spin-dependent and spin-independent interactions. The 2012 results and the future plans of the PICASSO collaboration are presented. Keywords: Dark Matter, Superheated Droplet Detector, Bubble Detector, Spin Dependent, PICASSO, SNOLAB 1 Introduction ties available to get adequate shelter from cosmic rays. It The past few years have been witness to great advances is not known if spin-independent or spin-dependent WIM- in the field of Astroparticle Physics. Through a variety of P interactions are more likely. Nor is the mass very con- astronomical observations of very high precision, we have strained. Hence there are a range of experiments current- come to understand our Universe as one dominated by dark ly operational, or planned, with different target materials, matter and dark energy. Despite the gravitational evidence spin sensitivity, and strategies to mitigate against potential for dark matter in galaxy and cluster halos, no irrefutable backgrounds. This has led to a significant world wide enter- evidence exists for the direct detection of dark matter on prise in the search for dark matter in direct detection experi- earth. Experiments with increasing levels of sophistication ments. and sensitivity are exploring the expected parameter space PICASSO is a direct detection experiment with very u- for dark matter. nique characteristics. It is one of only a few experiments A number of experiments now see a few events seemingly with significant sensitivity to spin-dependent interaction- inconsistent with background models at low particle masses s, and the superheated liquid technique has the advantage [1, 2, 3, 4], but these are in tension with other experimental of being largely insensitive to the backgrounds from gam- results which appear to exclude them [5, 6, 7, 8]. To ma rays, electrons and minimum ionizing particles which resolve these issues, experiments with significantly greater plague most other detector types. In addition, the detection sensitivity are required. There is a major international effort mechanism, low threshold, and use of 19F as a target mate- afoot to accomplish this, with experiments employing a rial make this experiment particularly sensitive to low mass- variety of techniques. Direct detection experiments search es, where there appear to be some hints for dark matter. for evidence of dark matter in the form of some unknown Weakly Interacting Massive Particle (WIMP) expected to The PICASSO experiment has had a program of gradual- scatter weakly off nuclei in matter. In these experiments the ly increasing the sensitive mass, decreasing the background- recoiling nucleus is expected to have energies from a few s, and becoming ever more sensitive through a series of de- KeV to order 100 keV, depending on the mass of the WIMP tector upgrades. Working in collaboration with COUPP, a and the angle of scatter. The event rates could be as low as next generation superheated liquid detector, with a mass or a few events per year per tonne of target material. order 500 Kg is now being designed. The following section- To detect such rare low-energy events, experiments need s will explain the principle of operation for PICASSO, the to be very large, must have ultra-low levels of radioactive results obtained so far, the status of the current phase, and backgrounds, and they typically operate for several years. the ambitious plans for a joint COUPP/PICASSO program They tend to be located in the deepest underground facili- based on the superheated liquid technology. PICASSO Dark Matter Program 33RD INTERNATIONAL COSMIC RAY CONFERENCE,RIO DE JANEIRO 2013 2 Principle of Operation these by surrounding the entire apparatus with large tanks The current form of the PICASSO experiment is 32 individ- of water which effectively thermalize fast neutrons. ual detectors, each 4.5` in volume, containing tiny droplets In the current configuration, the dominant source of back- ground is due to a-particles in the materials of the detector. of C4F10 in a polymerized gel. The loading is limited to about 2% C F by weight in each detector, and this can- Although great advances have been made by fabricating 4 10 the detectors from ultra pure materials, this is still the sen- not be increased without damaging the gel as bubbles for- sitivity limiting background for the experiment. Figure 2 m. A version of the detector is shown in figure 1. When shows the detector response as measured with g, neutron, the detectors are active, the 200 mm f C F droplets are 4 10 and a calibration sources, as well as the expected response maintained in a superheated∼ state by controlling the tem- of the detector to a 50 GeV/c2 WIMP. By controlling the perature and pressure of the detector. When a small, but backgrounds and by measuring the bubble formation rate sufficient amount of energy is deposited locally within the as a function of temperature, it is possible to distinguish a superheated droplets they undergo a rapid phase transition. WIMP signal from the irreducible background. The amount of energy required depends on pressure and Data are collected in runs of about 60 hours, followed by temperature, and these can be varied to control the thresh- 15 hour compression periods where the detectors are reset old sensitivity of the detectors. by driving the bubbles back to the liquid phase and curing As the energy (heat) must be deposited within a critical the gel matrix. radius to begin bubble growth, the operating conditions can be set so that only heavily ionizing radiation creates a phase transition. This is one of the unique features of these Temperature (°C) 70 60 50 40 30 20 detectors, as in contrast to most other detectors they are 1.2 held blind to the troublesome background gamma, elec- tron and minimum ionizing particles. The noise created 1 by the phase transition can be detected using an array of 9 strategically located piezo-electric transducers mounted on 0.8 the walls of the detector. Hence the data in PICASSO are digitized acoustic waveforms from particle induced bubble 0.6 production collected over a range of temperatures. 0.4 Detector Response 0.2 0 10­1 1 10 102 Threshold Energy (keV) Fig. 2: Response to different kinds of particles in superheat- Figure 2:ed Response C F . Fromto different left to kinds right: of particles1.75 MeV in superheated-rays and mini-C4F10. From left to right: 1.75 MeV4 γ10-rays and minimum ionizing particlesg (dot-dashed); 19F recoils modeled assumingmum the scattering ionizing particles of a 50 GeV/c (dot-dashed)2 WIMP only (red); observable poly-energetic at very neutrons from an AcBe sourcehigh (dotted); temperatures;α particles19F at recoils the Bragg modeled peak from assuming241Am the decays scat- (open triangles); and 210Pbtering recoil of nuclei a 50 from GeV/c2262RaWIMP spikes (red); (full dots). poly-energetic neutrons from an AcBe source (dotted); a particles at the Bragg peak 241 210 becomesfrom sensitiveAm to decays smaller (open dE/dx triangles); on the andα track.Pb It recoil is important nu- to note 226 in Fig. 2,clei that from the responseRa spikes remains (full dots). flat The from last 1 two120 correspond keV. This has been confirmedto a withsources numerous in the gel, detectors and within with the large droplet,α−background respectively. and indicates that the detectors are within an uncertainty of less than 3% fully sensitive Fig. 1: An isolated 4.5` PICASSO detector with piezoto energy depositions above threshold. A more detailed discussion can be sensors mounted on the walls. The inset shows the dropletsfound in3 [12]. Results from PICASSO 2012 embedded within the gel matrix. Since the detectors are fully sensitive to α particles over the entire range of the WIMPThe most sensitivity, recent resultsα particles from PICASSO are the most [7] were important published background for this kindin of 2012 dark based matter on asearch. set of 10 However ”golden the detectors” shapes - of ones the which WIMP (essentially The PICASSO detectors are designed to be sensitive had lower backgrounds and reproducible behavior.
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