Energy-dependent light quenching in CaWO$$_4$$4 crystals at mK temperatures

Scintillating CaWO$$_4$$4 single crystals are a promising multi-element target for rare-event searches and are currently used in the direct dark matter experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers). The relative light output of different particle interactions in CaWO$$_4$$4 is quantified by quenching factors (QFs). These are essential for an active background discrimination and the identification of a possible signal induced by weakly interacting massive particles (WIMPs). We present the first precise measurements of the QFs of O, Ca and W at mK temperatures by irradiating a cryogenic detector with a fast neutron beam. A clear energy dependence of the QF of O and, less pronounced, of Ca was observed for the first time. Furthermore, in CRESST neutron-calibration data a variation of the QFs among different CaWO$$_4$$4 single crystals was found. For typical CRESST detectors the QFs in the region-of-interest (10–40 keV) are $$\hbox {QF}_\mathrm{O}^\mathrm{ROI}=(11.2\pm 0.5)$$QFOROI=(11.2±0.5) %, $$\hbox {QF}_\mathrm{Ca}^\mathrm{ROI}=(5.94\pm 0.49)$$QFCaROI=(5.94±0.49) % and $$\hbox {QF}_\mathrm{W}^\mathrm{ROI}=(1.72\pm 0.21)$$QFWROI=(1.72±0.21) %. The latest CRESST data (run32) is reanalyzed using these fundamentally new results on light quenching in CaWO$$_4$$4 having moderate influence on the WIMP analysis. Their relevance for future CRESST runs and for the clarification of previously published results of direct dark matter experiments is emphasised.