Hans Kraus

Abstract:

CRESST is a multi-stage experiment directly searching for dark matter (DM) using cryogenic CaWO4 crystals. Previous stages established leading limits for the spin-independent DM-nucleon cross section down to DM-particle masses mDM below 1GeV/c2. Furthermore, CRESST performed a dedicated search for dark photons (DP) which excludes new parameter space between DP masses mDP of 300 eV/c2 to 700 eV/c2. In this contribution we will discuss the latest results based on the previous CRESST-II phase 2 and we will report on the status of the current CRESST-III phase 1: in this stage we have been operating 10 upgraded detectors with 24, g target mass each and enhanced detector performance since summer 2016. The improved detector design in terms of background suppression and reduction of the detection threshold will be discussed with respect to the previous stage. We will conclude with an outlook on the potential of the next stage, CRESST-III phase 2.

Abstract:

We report on a dark matter search for a Weakly Interacting Massive Particle (WIMP) in the mass range mx ∈ [4; 30] GeV/c^2 with the EDELWEISS-III experiment. A 2D profile likelihood analysis is performed on data from eight selected detectors with the lowest energy thresholds leading to a combined fiducial exposure of 496 kg-days. External backgrounds from γ-and β-radiation, recoils from 206Pb and neutrons as well as detector intrinsic backgrounds were modelled from data outside the region of interest and constrained in the analysis. The basic data selection and most of the background models are the same as those used in a previously published analysis based on Boosted Decision Trees (BDT) [1]. For the likelihood approach applied in the analysis presented here, a larger signal efficiency and a subtraction of the expected background lead to a higher sensitivity, especially for the lowest WIMP masses probed. No statistically significant signal was found and upper limits on the spin-independent WIMP-nucleon scattering cross section can be set with a hypothesis test based on the profile likelihood test statistics. The 90% C.L. exclusion limit set for WIMPs with mx = 4 GeV=c^2 is 1:6 X 10^-39 cm2, which is an improvement of a factor of seven with respect to the BDT-based analysis. For WIMP masses above 15 GeV/c^2 the exclusion limits found with both analyses are in good agreement.

Abstract:

The EDELWEISS-III experiment uses cryogenic HP-Ge detectors Fully covered with Inter-Digitized electrodes (FID). They are operated at low fields (< 1 V=cm), and as a consequence charge-carrier trapping significantly affects both the ionization and heat energy measurements. This paper describes an analytical model of the signals induced by trapped charges in FID detectors based on the Shockley-Ramo theorem. It is used to demonstrate that veto electrodes, initially designed for the sole purpose of surface event rejection, can be used to provide a sensitivity to the depth of the energy deposits, characterize the trapping in the crystals, perform heat and ionization energy corrections and improve the ionization baseline resolutions. These procedures are applied successfully to actual data.

Abstract:

We analyzed low-background data from the CRESST-II experiment with a total net exposure of 730 kg days to extract limits on double electron capture processes. We established new limits for 40Ca with T1/22v2K > 9.9 × 1021 y and T1/20v2EC > 1.4 × 1022 y and for 180W T1/22v/2K > 3.1 × 1019 with y and T1/20v2ED > 9.4 × 1018 y at 90% CL. Depending on the process, these values improve the currently best limits by a factor of ∼1.4-30.