C. Levy

Constraints on low-mass WIMP interactions on 19F from PICASSO

Abstract Recent results from the PICASSO dark matter search experiment at SNOLAB are reported. These results were obtained using a subset of 10 detectors with a total target mass of 0.72 kg of 19F and an exposure of 114 kgd. The low backgrounds in PICASSO allow recoil energy thresholds as low as 1.7 keV to be obtained which results in an increased sensitivity to interactions from Weakly Interacting Massive Particles (WIMPs) with masses below 10 GeV/c 2 . No dark matter signal was found. Best exclusion limits in the spin dependent sector were obtained for WIMP masses of 20 GeV/c 2 with a cross section on protons of σ p S D = 0.032 pb (90% C.L.). In the spin independent sector close to the low mass region of 7 GeV/c 2 favoured by CoGeNT and DAMA/LIBRA, cross sections larger than σ p S I = 1.41 × 10 − 4 pb (90% C.L.) are excluded.

Dark matter spin-dependent limits for WIMP interactions on 19F by PICASSO

The PICASSO experiment at SNOLAB uses super?heated C4F10 droplets suspended in a gel as a target sensitive to WIMP?proton spin?dependent elastic scattering. The phase II setup has been improved substantially in sensitivity by using an array of 32 detectors with an active mass of ~65 g each and largely reduced background. First results are presented for a subset of two detectors with target masses of 19F of 65 g and 69 g respectively and a total exposure of 13.75 ? 0.48 kgd. No dark matter signal was found and for WIMP masses around 24 GeV/c2 new limits have been obtained on the spin?dependent cross section on 19F of ?F = 13.9 pb (90% C.L.) which can be converted into cross section limits on protons and neutrons of ?p = 0.15 pb and ?n = 2.45 pb respectively (90% C.L). The obtained limits on protons restrict recent interpretations of the DAMA/LIBRA annual modulations in terms of spin-dependent interactions.

Discrimination of nuclear recoils from alpha particles with superheated liquids

The PICASSO collaboration observed for the first time a significant difference between the acoustic signals induced by neutrons and alpha particles in a detector based on superheated liquids. This new discovery offers the possibility of improved background suppression and could be especially useful for dark matter experiments. This new effect may be attributed to the formation of multiple bubbles on alpha tracks, compared to single nucleations created by neutron-induced recoils.

New insights into particle detection with superheated liquids

We report new results obtained from calibrations of superheated liquid droplet detectors used in dark matter searches with different radiation sources (n, α, γ). In particular, detectors were spiked with α-emitters located inside and outside the droplets. It is shown that the responses have different temperature thresholds, depending on whether α-particles or recoil nuclei create the signals. The measured temperature threshold for recoiling 210Pb nuclei from 214Po α-decays was found to be in agreement with test beam measurements using mono-energetic neutrons. A comparison of the threshold data with theoretical predictions shows deviations, especially at high temperatures. It is shown that signals produced simultaneously by recoil nuclei and α-particles have more acoustic energy than signals produced by one or the other separately. A model is presented that describes how the observed intensities of particle-induced acoustic signals can be related to the dynamics of bubble growth in superheated liquids. A growth scenario that is limited by the inertia of the surrounding liquid shows a trend that is supported by the data. An improved understanding of the bubble dynamics is an important first step in obtaining better discrimination between particle types interacting in detectors of this kind.

Dark matter search with PICASSO

PICASSO at SNOLAB searches primarily for spin-dependent WIMP interactions on 19F using the superheated droplet technique. This technique is based on the bubble chamber principle, where phase transitions in superheated liquid droplets can be triggered by WIMP induced nuclear recoils. The physics of the detection process allows a highly efficient suppression of backgrounds from cosmic muons, γ- and β-rays. We will discuss qualitatively recent progress in PICASSO and its sensitivity reach for spin-dependent and spin-independent WIMP searches.

Optimization of Neutron Shielding for the PICASSO Experiment

The PICASSO experiment searches WIMPs via their spin‐dependent interactions with nuclei. At their operating temperatures, PICASSO threshold detectors are not sensitive to γ‐rays, while it is necessary to suppress neutron and α‐ray backgrounds. The different types of neutron shielding (water, polyethylene, borated water and polyethylene doped with different compounds of boron and lithium) were studied via the Monte Carlo method using MCNPX code. Efficiencies of these different types of neutron shielding were compared from the point of view of neutron suppression as well as number of newly born y‐rays. The neutrons and γ‐rays were followed up to the PICASSO detector active material. The most suitable neutron shielding for the PICASSO experiment was determined following these simulations.