PICASSO, an Experiment for Direct Search of Dark Matter

There is strong evidence that a large fraction of the matter in the Universe is non-luminous and non-baryonic. The most recent evidence is based on the measurement of the cosmic radiative background (CMB) by WMAP. The mass components of the Universe are ~ 4% of baryonic (visible) matter, 23% of cold dark matter and 73% of dark energy. The lightest neutralino, ?, predicted in supersymmetric (SUSY) extensions of the Standard Model (SM) emerges as the most promising candidate for dark matter. In a direct dark matter search experiment, the detection reaction of neutralinos is elastic scattering off detector atomic nuclei. The detection of neutralinos is through the measurement of the energy deposited by the recoiling nucleus. The rate of interaction of neutralino with ordinary matter is a fraction of event per day per kilogram. Therefore, it is necessary to build detectors with large active mass and extremely low intrinsic background and, in practice, to operate the experiment in a deep underground site. The PICASSO experiment is searching for cold dark matter through the direct detection of neutralinos via their spin-dependent interactions with nuclei. The experiment is installed in the Sudbury Neutrino Observatory Laboratory at a depth of 2070 m (6000 mwe). PICASSO makes use of the superheated droplet technique with C4F10 as the active material and searches for ? interactions on 19F. The experimental technique used by PICASSO is reviewed and results of the measurements are presented in terms of limits on the spin-dependent ?-proton and ?-neutron cross sections. The results exclude regions of spin dependent ?-nucleon interaction parameter space. Future phases of PICASSO are briefly discussed.