Direct Dark Matter Search with the CRESST-III Experiment Michael Willers on behalf of the CRESST collaboration Physik-Department and Excellence Cluster Universe, Technische Universität München, 85748 Garching The CRESST-III Experiment The CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) [1, 2] experiment aims for the direct detection of Dark Matter (DM) particles via elastic scattering off nuclei in CaWO4. CRESST-III, the current stage of the experiment, searches for low-mass (M ≲10 GeV/c2) DM particles and ultimately aims to probe the parameter space down to the neutrino ﬂoor. The experiment is located in the Gran Sasso National Laboratory (LNGS), Italy, shielded against cosmic rays by ∼3600 m.w.e. of rock overburden. CRESST uses a multi-element target: scintillating CaWO4 single crystals operated as cryogenic detectors. Requirements for probing low-mass DM particles: • sub-keV energy threshold due to low momentum transfer to target crystal. • Low background rate in region of interest and good background suppression capabilities. CaWO4 target crystal: • Block-shaped CaWO4 crystal (20 × 20 × 10) mm3, Mass 25g. • Sensitivity gain due to reduced heat capacity and adapted TES design. Silicon-On-Sapphire light detector: simultaneous measurement of phonon- and light-signal enables particle identiﬁcation and background suppression. plain CaWO4 stick holding light detector Copper housing with reﬂective and scintillating inner surface (3M® Vikuiti foil [3]). Tungsten (W) TES Instrumented CaWO4 sticks (iStick) holding the CaWO4 target crystal: • events within the CaWO4 sticks are a potential back- ground for low-mass DM search. • instrumentation enables ef- ﬁcient veto of these events. • 3 iStick per module (1 read- out channel). Recent results from CRESST III phase 1 CRESST-III phase 1 started in summer of 2016 with an initial campaign to set-up and calibrate the detectors. The physics run of phase 1 started in September of 2016. • 10 detector modules installed. • 8 modules with TUM-grown crystals, 2 with commercial crystals. • 5 modules reached / exceeded design goal of 100eV nu- clear recoil energy threshold. • Detector A has lowest threshold in CRESST-III phase 1. First analysis of detector A • Data taking period: Oct. 2016 - May 2017 • Gross exposure (before cuts): 2.39 kg days • Blind analysis with non-blind training set (20% randomly selected, excluded from DM data set). Training set data used to deﬁne cuts. • Conservative analysis threshold of 100 eV. • Data quality (rate, stability, unphysical events) and coincidence (µ- veto and iStick) cuts applied. 79.5% survival probability at 100eV. • Limit on DM-nucleon cross-section calculated using Yellin’s optimal interval method [4] →sensitivity extended to lower DM masses (lim- ited by non-ﬂat background at low energies). The analysis of the full dataset as well as a low-threshold analysis are currently ongoing. Furthermore, the application of optimum ﬁltering to a stream of continuously acquired data is investigated with the goal of improving the trigger threshold of the detectors [5]. Data taken with detector A in the light yield - energy plane e−/γ-band recoils off oxygen recoils off tungsten dark matter acceptance region Low-energy spectrum of all events recorded with detector A CaWO4 crystal growth at TUM A high radio-purity of the CaWO4 crystals is es- sential for CRESST. In order to have full control of all crystal growth parameters, TUM has estab- lished a complete in-house CaWO4 crystal growth process [6]: • Powder production via solid state reaction or precipitation including chemical puriﬁcation (net reaction: CaCO3 + WO3 →CaWO4 + CO2). • Crystal growth in dedicated Czochralski fur- nace. • Annealing of grown ingots in pure O2 atmo- sphere. • Cutting and polishing of target crystals. In addition, recrystallisation of grown crystals is investigated as a measure to further improve the radio-purity of the CaWO4 crystals [7, 8]. Czochralski Furnace & raw