The COSINUS project: Development of new NaI-based cryogenic detectors for direct dark matter search

The current results of direct dark matter searches are controversial. The long-standing dark-matter claim from the DAMA/LIBRA collaboration is excluded by null-results of several other experiments. However, a comparison of the results by experiments with different detector materials introduces model dependencies. The R&D project COSINUS (Cryogenic Observatory for SIgnatures seen in Next-generation Underground Searches) aims to develop cryogenic detectors based on (hygroscopic) sodium iodide (NaI). If successful, such detectors could be used in future experiments to investigate the origin of the annual modulation signal seen by the NaI-based scintillation detectors of the DAMA/LIBRA experiment. COSINUS detectors should be able to simultaneously detect phonons and scintillation light produced by a particle interaction inside the NaI crystal. This technique allows for an active suppression of β/γ backgrounds as well as detailed studies of a large variety of dark-matter models predicting nuclear interactions. For such kind of studies only moderate exposures of ≲100kg-days are needed. In addition to the projected sensitivities of COSINUS detectors, we also show the result of first tests using (only mildly hygroscopic) caesium iodide (CsI) crystals as target material. For this measurement we achieved an energy threshold of ∼4.7 keV for nuclear recoils.