R. Gornea

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.

Suitability of superheated droplet detectors for dark matter search

Abstract We have measured the sensitivity of superheated droplet detectors in different radiation environments and we discuss the suitability of this detection technique for the search of weakly interacting cold dark matter particles. In particular, we show that our new proposed detector based on superheated carbo-fluorates can be operated at ambient pressure and room temperature in a mode where it is almost exclusively sensitive to the nuclear recoils following neutralino interaction, which allows a powerful background discrimination. The potential of the detection technique is shown by deriving a limit on the spin-dependent neutralino cross-section, making use of the very favourable neutralino– 19 F interaction and background suppression.

Search for dark matter with the PICASSO experiment

Abstract The superheated droplet technique exploits the fact that bubble formation is triggered by a heat spike produced during energy deposition of charged particles traversing a metastable liquid. This process allows for an efficient suppression of low-ionizing backgrounds. Moreover the possibility of using cheap room temperature superheated liquids such as fluorinated halocarbons, as well as low energy thresholds render this technique interesting for large mass, direct neutralino searches.

Status of the Picasso project

The Picasso project is a dark matter search experiment based on the superheated droplet technique. Preliminary runs performed at the Picasso Laboratory in Montreal showed the suitability of this detection technique to the search for weakly interacting cold dark matter particles. In July 2002, a new phase of the project started. A batch of six 1-liter detectors with an active mass of approximately 40 g was installed in a gallery of the SNO observatory in Sudbury, Ontario, Canada at a depth of 6,800 feet (2,070 m). We give a status report on the new experimental setup, data analysis, and preliminary limits on spin-dependent neutralino interaction cross section.

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.

Data acquisition system for PICASSO experiment

This paper describes the data acquisition system (DAQ) developed for the PISASSO project (Project In Canada to Search for Supersymmetric Objects) - the ongoing low background experiment located about 2 km underground at the SNOLAB, Ontario, Canada. This experiment employs an unconventional radiation detecting technique using piezo sensors to detect production of sound by superheated liquid when nuclear recoil takes place in the active region of the detector. Related to such technique specifics of the electronics requirements are discussed in details. The evolution of the DAQ solutions is presented for three different stages of the experiment corresponding to increased active mass of the detector during each stage. Emphasis is given to the current version of DAQ designed for a total of 32 detectors (288 channels). A brief description of the future scaling to 4000 channel system is given as well.

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.