I. Levine

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.

Improved spin dependent limits from the PICASSO dark matter search experiment

Abstract The PICASSO experiment reports an improved limit for the existence of cold dark matter WIMPs interacting via spin-dependent interactions with nuclei. The experiment is installed in the Sudbury Neutrino Observatory at a depth of 2070 m. With superheated C4F10 droplets as the active material, and an exposure of 1.98 ± 0.19 kg day , no evidence for a WIMP signal was found. For a WIMP mass of 29 GeV / c 2 , limits on the spin-dependent cross section on protons of σ p = 1.31 pb and on neutrons of σ n = 21.5 pb have been obtained at 90% C.L. In both cases, some new parameter space in the region of WIMP masses below 20 GeV / c 2 has now been ruled out. The results of these measurements are also presented in terms of limits on the effective WIMP-proton and neutron coupling strengths a p and a n .

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.

Response of superheated droplet detectors of the PICASSO dark matter search experiment

We present results of systematic studies of the radiation response of superheated liquid droplet detectors, which are used in the PICASSO dark matter search experiment. This detection technique is based on the phase transitions of superheated liquid Freon droplets dispersed and trapped in a polymerized gel. Phase transitions can be induced by nuclear recoils following particle interactions and, in particular, interactions with Weakly Interacting Massive Particles (WIMPs). These detectors are threshold devices since a minimal energy deposition is necessary to induce a phase transition and their sensitivity to various types of radiation depends strongly on the operating temperature and pressure. The sensitivity to neutrons, α-particles and γ-rays was determined as a function of these operating parameters and the results are compared with simulations. In particular, we present a complete characterization of the response of detector modules already in use for a dark matter search at the SNO site to detect WIMPs and discuss possible background sources.

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.

Simulation of special bubble detectors for PICASSO

The PICASSO project is a cold dark matter (CDM) search experiment relying on the superheated droplet technique. The detectors use superheated freon liquid droplets (active material) dispersed and trapped in a polymerised gel. This detection technique is based on the phase transition of superheated droplets at about room temperature and ambient pressure. The phase transition is induced by nuclear recoils when an atomic nucleus in the droplets interacts with incoming subatomic particles. This includes CDM particles candidate as the neutralino (a yet-to-discover particle predicted in extensions of the standard model of particle physics). Simulations performed to understand the detector response to neutrons and alpha particles are presented along with corresponding data obtained at the Montreal Laboratory.

CHARACTERIZATION OF THE RESPONSE OF SUPERHEATED DROPLET (BUBBLE) DETECTORS

The PICASSO project is a cold dark matter (CDM) search experiment relying on the superheated droplet technique. The detectors use superheated freon liquid droplets (active material) dispersed and trapped in a polymerized gel. This detection technique is based on the phase transition of superheated droplets at room or moderate temperatures. The phase transitions are induced by nuclear recoils when undergoing interactions with particles, including CDM candidates such as the neutralinos predicted by supersymmetric models. The suitability of the technique for this purpose has been demonstrated by R&D studies performed over several years on detectors of various composition and volume. Simulations performed to understand the detector response to neutrons and alpha particles are presented along with corresponding data obtained at the Montreal Laboratory.

Status and future plans of the experiment PICASSO searching for Dark Matter

The PICASSO (Project in Canada to Search for Supersymmetric Objects) is looking for non‐baryonic cold dark matter via nuclear scattering in high‐purity large‐mass superheated droplets detectors. The collaboration has finished the Phase I of the experiment (3 detector modules, around 7 g of active mass, each). Currently the second phase of the experiment is being set up at SNOLAB in which 32‐detector modules (of 4.5 l volume each) is being installed. Upon completion, the underground detection array will have a total of 2 kg of active mass. Expected performance and future plans are also discussed.