The CRESST-II cryogenic Dark Matter search, aiming at detection of WIMPs via elastic scattering off nuclei in CaWO 4 crystals, completed 730 kg days of data taking in 2011. We present the data collected with eight detector modules, each with a two-channel readout; one for a phonon signal and the other for coincidently produced scintillation light. The former provides a precise measure of the energy deposited by an interaction, and the ratio of scintillation light to deposited energy can be used to discriminate different types of interacting particles and thus to distinguish possible signal events from the dominant backgrounds.Sixty-seven events are found in the acceptance region where a WIMP signal in the form of low energy nuclear recoils would be expected. We estimate background contributions to this observation from four sources: (1) "leakage" from the e/γ -band (2) "leakage" from the α-particle band (3) neutrons and (4) 206 Pb recoils from 210 Po decay. Using a maximum likelihood analysis, we find, at a statistical significance of more than 4σ , that these sources alone are not sufficient to explain the data. The addition of a signal due to scattering of relatively light WIMPs could account for this discrepancy, and we determine the associated WIMP parameters.

The CRESST cryogenic direct dark matter search at Gran Sasso, searching for WIMPs via nuclear recoil, has been upgraded to CRESST-II by several changes and improvements. The upgrade includes a new detector support structure capable of accommodating 33 modules, the associated multichannel readout with 66 SQUID channels, a neutron shield, a calibration source lift, and the installation of a muon veto. We present the results of a commissioning run carried out in 2007.The basic element of CRESST-II is a detector module consisting of a large (∼300 g) CaWO4 crystal and a very sensitive smaller (∼ 2 g) light detector to detect the scintillation light from the CaWO4. The large crystal gives an accurate total energy measurement. The light detector permits a determination of the light yield for an event, allowing an effective separation of nuclear recoils from electron-photon backgrounds. Furthermore, information from lightquenching factor studies allows the definition of a region of the energy-light yield plane which corresponds to tungsten recoils. A neutron test is reported which supports the principle of using the light yield to identify the recoiling nucleus.Data obtained with two detector modules for a total exposure of 48 kg-days are presented. Judging by the rate of events in the "all nuclear recoils" acceptance region the apparatus shows a factor ∼ten improvement with respect to previous results, which we attribute principally to the presence of the neutron shield. In the "tungsten recoils" acceptance region three events are found, corresponding to a rate of 0.063 per kg-day. Standard assumptions on the dark matter flux, coherent or spin independent interactions, then yield a limit for WIMP-nucleon scattering of 4.8 × 10 −7 pb, at M WIMP ∼ 50 GeV.

In experiments for direct dark matter searches, neutrinos coherently scattering off nuclei can produce similar events as Weakly Interacting Massive Particles (WIMPs). The calculated count rate for solar neutrinos in such experiments is a few events per ton-year. This count rate strongly depends on the nuclear recoil energy threshold achieved in the experiments for the WIMP search. We show that solar neutrinos can be a serious background source for direct dark matter search experiments using Ge, Ar, Xe and CaWO 4 as target materials. To reach sensitivities better than ∼ 10 −10 pb for the elastic WIMP nucleon spin-independent cross section in the zero-background limit, energy thresholds for nuclear recoils should be 2.05 keV for CaWO 4 , 4.91 keV for Ge, 2.89 keV for Xe, and 8.62 keV for Ar as target material. Next-generation experiments should not only strive for a reduction of the present energy thresholds but mainly focus on an increase of the target mass. Atmospheric neutrinos limit the achievable sensitivity for the background-free direct dark matter search to 10 −12 pb.

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The alpha decay of 210 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 CaWO4 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.PACS numbers: 29.40. Mc,29.40.Vj,95.35.+d