L. Lessard

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 kgu2009day , 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.

Final Results of the PICASSO Dark Matter Search Experiment

The PICASSO dark matter search experiment operated an array of 32 superheated droplet detectors containing 3.0xa0 kg of C 4 F 10 and collected an exposure of 231.4xa0kgd at SNOLAB between March 2012 and January 2014. We report on the final results of this experiment which includes for the first time the complete data set and improved analysis techniques including acoustic localization to allow fiducialization and removal of higher activity regions within the detectors. No signal consistent with dark matter was observed. We set limits for spin-dependent interactions on protons of σ p S D xa0=xa01.32xa0× xa010 −2 − 2 xa0pb (90%xa0C.L.) at a WIMP mass of 20xa0 GeV/c 2 . In the spin-independent sector we exclude cross sections larger than σ p S I xa0=xa04.86xa0× xa010 −5 − 5 xa0pbxa0(90% C.L.) in the region around 7xa0 GeV/c 2 . The pioneering efforts of the PICASSO experiment have paved the way forward for a next generation detector incorporating much of this technology and experience into larger mass bubble chambers.

Neutron response functions for superheated droplet detectors

Neutron response functions for superheated droplet detectors have been analyzed from threshold up to 2 MeV incident energy, taking into account a gradual threshold function, in terms of temperature and incident energy. A method for obtaining fitted parameters is described. Detection mechanisms up to 14 MeV are discussed.

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.

Design and tests of the z-coordinate drift chamber system for the opal central detector at LEP

Abstract A system of drift chambers has been designed to make high resolution measurements of the z -coordinates of charged tracks at the outer radius of the OPAL central detector at LEP. The unit module of this detector is a 25 cm drift length bidirectional cell with six sense wires in a thin 50 cm wide by 29 mm high drift slot. Tests indicate that the chamber has a wide and stable electric field operating range and its performance is unaffected by small misalignments between the drift electric field and an external magnetic field. The drift cell was found to have uniform acceptance up to its geometrical boundaries, and the z -resolution for beam tracks normal to the chamber was measured to be in the range of 40–175 μm.

New measurement of the beta decay rate of the16N(0−)-16O(0+) transition

The beta decay rate of the16N(0−; 120keV) →16O(0+, G.S.) transition has been remeasured. Our result (Λβ=0.48±0.024 s−1) is in excellent agreement with a previous measurement but strongly disagrees with another experimental value. The two agreeing results provide an experimental verification of the importance of meson exchange currents in 0+-0− weak transitions. A value ofgp/gA∼11–12 can be inferred from the correspondingΛμ rate.