Results of Quenching Factor Measurements of CaWO$_4$ at mK Temperatures for the Direct Dark Matter Search Experiment CRESST

Abstract

The CRESST experiment aims at a direct detection of WIMP dark matter (DM) using scintillating CaWO$_4$ crystals operated as phonon detectors at mK temperatures. An important feature of the experiment is the active background discrimination technique exploiting the different light outputs depending on the kind of particle interaction. The reduced light yield of nuclear recoils compared to electron recoils is quantified by quenching factors (QFs). The precise measurement of the QFs and thus the identification of the individual recoiling nucleus in the multi-target material CaWO$_4$ is crucial for neutron background discrimination and assuming a positive DM signal would allow to a certain extent WIMP-mass spectroscopy. At the Munich tandem accelerator a dedicated neutron scattering facility has been set up to measure the QFs of CaWO$_4$, in particular that of tungsten, at mK temperatures. Monoenergetic neutrons (11 MeV) produced by the accelerator are scattered off a CRESST-like detector module that is operated in a dilution refrigerator. In this setup, the recoiling nucleus (O, Ca and W) is identified by time-of-flight measurement in liquid-scintillator detectors placed at fixed scattering angles. The QF of W could be determined with unprecedented accuracy at mK temperatures and under realistic measurement conditions: $QF_W=0.0196\pm 0.0022$ (preliminary value).