M. Kimmerle

EURECA — the European future of cryogenic dark matter searches

EURECA (European Underground Rare Event Calorimeter Array) is a new project, searching for dark matter, with largely the present groups of the CRESST and EDELWEISS experiments and already a few new groups. The aim is to explore scalar cross sections in the 10−9 - 10−10 pico-barn region with a target mass of up to one tonne. A major advantage of EURECA is our planned use of more that just one target material (multi target experiment for WIMP identification). In preparation for this large-scale experiment, R&D for EURECA is provided through the current phases of CRESST and EDELWEISS.

Discrimination of recoil backgrounds in scintillating calorimeters

The alpha decay of Po is a dangerous background to rare event searches. Here, we describe observations related to this alpha decay in the Cryogenic Rare Event Search with Superconducting Thermometers (CRESST). We find that lead nuclei show a scintillation light yield in our CaWO crystals of 0.0142±0.0013 relative to electrons of the same energy. We describe a way to discriminate this source of nuclear recoil background by means of a scintillating foil, and demonstrate its effectiveness. This leads to an observable difference in the pulse shape of the light detector, which can be used to tag these events. Differences in pulse shape of the phonon detector between lead and electron recoils are also extracted, opening the window to future additional background suppression techniques based on pulse shape discrimination in such experiments.

The CRESST Dark Matter Search

Abstract The aim of CRESST ( C ryogenic R are E vent S earch with S uperconducting T hermometers) is to search for particle dark matter via elastic scattering off nuclei. The experiment is located at the Laboratori Nazionali del Gran Sasso (LNGS), Italy, and it uses low-background cryogenic detectors with superconducting phase-transition thermometers for the direct detection of WIMP–nucleus scattering events.

Status of the CRESST Dark Matter Search

The CRESST experiment aims for a detection of dark matter in the form of WIMPs. These particles are expected to scatter elastically off the nuclei of a target material, thereby depositing energy on the recoiling nucleus. CRESST uses scintillating CaWO4 crystals as such a target. The energy deposited by an interacting particle is primarily converted to phonons which are detected by transition edge sensors. In addition, a small fraction of the interaction energy is emitted from the crystals in the form of scintillation light which is measured in coincidence with the phonon signal by a separate cryogenic light detector for each target crystal. The ratio of light to phonon energy permits the discrimination between the nuclear recoils expected from WIMPs and events from radioactive backgrounds which primarily lead to electron recoils. CRESST has shown the success of this method in a commissioning run in 2007 and, since then, further investigated possibilities for an even better suppression of backgrounds. Here, we report on a new class of background events observed in the course of this work. The consequences of this observation are discussed and we present the current status of the experiment.

The CRESST II Dark Matter Search

Direct Dark Matter detection with cryodetectors is briefly discussed, with particular mention of the possibility of the identification of the recoil nucleus. Preliminary results from the CRESST II Dark Matter search, with 730 kg-days of data, are presented. Major backgrounds and methods of identifying and dealing with them are indicated.

Composite CaWO4 Detectors for the CRESST‐II Experiment

CRESST‐II, standing for Cryogenic Rare Events Search with Superconducting Thermometers phase II, is an experiment searching for Dark Matter. In the LNGS facility in Gran Sasso, Italy, a cryogenic detector setup is operated in order to detect WIMPs by elastic scattering off nuclei, generating phononic lattice excitations and scintillation light. The thermometers used in the experiment consist of a tungsten thin‐film structure evaporated onto the CaWO4 absorber crystal. The process of evaporation causes a decrease in the scintillation light output. This, together with the need of a big‐scale detector production for the upcoming EURECA experiment lead to investigations for producing thermometers on smaller crystals which are glued onto the absorber crystal. In our Run 31 we tested composite detectors for the first time in the Gran Sasso setup. They seem to produce higher light yields as hoped and could provide an additional time based discrimination mechanism for low light yield clamp events.

Latest results from the CRESST-II Dark Matter Search

The CRESST-II experiment is searching for Dark Matter particles in the form of WIMPs via their elastic scattering off nuclei in a target material. The CRESST target consists of scintillating CaWO4 crystals which are operated as cryogenic calorimeters at millikelvin temperatures and read out by transition edge sensors. Each interaction in CaWO4 produces a phonon signal in the target crystal and also a light signal that is measured by a secondary cryogenic calorimeter, allowing a very efficient discrimination between electron recoils from radioactive e/γ background and nuclear recoils. Moreover, to some extent, the different types of recoiling nuclei (O, Ca, W) can be distinguished. In the paper we present the latest results of the experiment, obtained from a net exposure of 730kg days acquired with 8 detectors between July 2009 and March 2011. The data has shown a considerable number of events in our signal region. Since this large number of events is not consistent with the known sources of background in our experiment, we discuss the compatibility of this excess of events with a possible WIMP-signal.

The CRESST dark matter search

CRESST is a WIMP dark matter search using scintillating CaWO4 cryogenic detectors with active background suppression. First results obtained in the commissioning run are presented.

THE CRESST DARK MATTER SEARCH

We discuss the short and long term perspectives of the CRESST (Cryogenic Rare Event Search using Superconducting Thermometers) project and present the current status of the experiment and new results concerning detector development. In the search for elementary particle dark matter, CRESST is presently the most advanced deep underground, low background, cryogenic facility. The basic technique involved is to search for WIMPS (Weakly Interacting Massive Particles) by the measurement of non-thermal phonons, as created by WIMP-induced nuclear recoils. Combined with our newly developed method for the simultaneous measurement of scintillation light, strong background discrimination is possible, resulting in a substantial increase in WIMP detection sensitivity. This will allow a test of the reported positive evidence for a WIMP signal by the DAMA collaboration in the near future. In the long term, the present CRESST set-up permits the installation of a detector mass up to 100 kg. In contrast to other projects, CRESST technology allows the employment of a large variety of detection materials. This offers a powerful tool in establishing a WIMP signal and in investigating WIMP properties in the event of a positive signal. PACS: 95.35+d, 29.40

Results and status of the CRESST experiment

CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) employs cryogenic detectors for the direct search for weakly interacting massive dark matter particles (WIMPs). In the second phase of the experiment scintillating calcium tungstate crystals are used to discriminate background by means of different light yield for background and WIMP signals. After first results with this novel technique have been obtained, the experimental setup is being upgraded for further background reduction and larger target mass. The results and present status of the experiment will be presented.