Deep-underground dark matter search with a COSINUS detector prototype

Sodium iodide (NaI)-based cryogenic scintillating calorimeters using quantum sensors for signal readout have shown promising first results toward a model-independent test of the annually modulating signal detected by the DAMA/LIBRA dark matter experiment. The COSINUS Collaboration has previously reported on the first above-ground measurements using a dual-channel readout of phonons and light based on transition edge sensors (TESs) that allows for particle discrimination on an event-by-event basis. In this article, we outline the first underground measurement of a NaI cryogenic calorimeter readout via the novel remoTES scheme. A 3.67 g NaI absorber with an improved silicon light detector design was operated at the Laboratori Nazionali del Gran Sasso, Italy. A significant improvement in the discrimination power of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msup><a:mrow><a:mi>e</a:mi></a:mrow><a:mrow><a:mo>−</a:mo></a:mrow></a:msup><a:mo>/</a:mo><a:mi>γ</a:mi></a:mrow></a:math> events to nuclear recoils was observed with a fivefold improvement in the nuclear recoil baseline resolution, achieving <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>σ</c:mi><c:mo>=</c:mo><c:mn>441</c:mn><c:mtext> </c:mtext><c:mtext> </c:mtext><c:mi>eV</c:mi></c:math>. Furthermore, we present a limit on the spin-independent dark matter nucleon elastic scattering cross section, achieving a sensitivity of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi mathvariant="script">O</e:mi><e:mo stretchy="false">(</e:mo><e:mi>pb</e:mi><e:mo stretchy="false">)</e:mo></e:math> with an exposure of only 11.6 g d. Published by the American Physical Society 2024