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Direct dark matter searches aim to measure interactions of the until today undiscovered dark matter particles in their detector. The crucial challenge to overcome is to distinguish potential interactions of dark matter particles from those induced by ordinary, already known particles. Two main approaches exist, the first one are experiments with a dual-channel readout, one channel to measure the energy deposited in the detector and another channel for particle identification. The second approach is to search for an annual modulation of the interaction rate, which is expected for dark matter due to the earth orbiting around the sun, thus changing its velocity with respect to the dark matter particles throughout the year. The DAMA/LIBRA experiment exploits the second approach and, interestingly, observes exactly such a predicted annual modulation. Up to now, no other experiment could confirm their observation, instead numerous other direct dark matter searches rule-out the DAMA/LIBRA signal claim. However, none of those searches also uses NaI as target material like DAMA/LIBRA which makes the comparison subject to potential target-material dependencies.
The COSINUS experiment (Cryogenic Observatory for SIgnals seen in Next-generation Underground Searches) aims to remove this uncertainty and to arrive at a fully model-independent clarification of the DAMA/LIBRA claim by combining the DAMA/LIBRA target material (NaI) with a dual-channel readout (first approach), a unique feature in the field of NaI-based direct dark matter detection. Two channels are obtained by operating NaI as a cryogenic calorimeter at mK temperatures, simultaneously reading the heat signal (energy information) and the scintillation light (particle identification) created upon a particle interaction in the NaI crystal. CRESST - which is the leading experiment for low-mass dark matter - uses the same technology, but a different target material: Calcium tungstate (CaWO4) for CRESST, sodium iodide (NaI) for COSINUS.
The main hurdles to face for COSINUS lie in the unfortunate properties of NaI, in particular its vulnerability to humidity requiring a totally new handling procedure, from detector production to final installation in the experiment. In the years 2016 and 2017, COSINUS has successfully measured two prototypes, the first one constituting the world-wide first successful measurement of NaI as cryogenic calorimeter, the second one serving as proof-of-principle test for the final detector design. Currently, we are improving the detector performance, the basic requirement for COSINUS to illuminate one of the most puzzling mysteries of present-day particle physics in the near future.