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COSINUS is a new cryogenic observatory for rare event searches located in the Laboratori Nazionali del Gran Sasso in Italy. COSINUS’s first goal is to clarify whether the signal detected by the DAMA/LIBRA experiment originates from dark matter particle interactions or has a different nature. To this aim, sodium iodide (NaI) cryogenic scintillating calorimeters read out by transition edge sensors (TESs) are developed. To preserve the NaI crystal from the TES fabrication process, COSINUS implemented a novel design, the remoTES, where the TES is deposited on a separate wafer and coupled to the absorber through a Au-bonding wire and a Au-phonon collector. This design has reached baseline resolutions below 100 eV for Si, 200 eV for TeO 2 and 400 eV for NaI absorbers. These results show that the remoTES not only brings COSINUS close to its performance goal of 1 keV energy threshold, but also offers the possibility to employ delicate crystals previously excluded for cryogenic applications as absorbers and to avoid the exposure of the absorbers to the TES fabrication process. It therefore extends the choice of target materials of the rare event searches using TES. In this work, we will provide a detailed description of the remoTES design and present the results of the latest prototypes.

In the last years, the COSINUS (Cryogenic Observatory for SIgnals seen in Next generation Underground Searches) experiment has made significant progress both in the construction of its facility and in pursuing its physics goals: At Laboratori Nazionali del Gran Sasso (LNGS) in Italy, an underground facility was constructed, which will house experimental detectors for dark matter direct detection in a dry dilution cryostat. Construction of the main structures at the COSINUS site is finished, including the control building, the cryostat access level, and the water tank which will serve as a Cherenkov muon veto around the cryostat. With a nuclear recoil threshold of 4 keV, the latest COSINUS detector prototype approaches the design goal of 1 keV, and particle discrimination on event-by-event basis has been demonstrated. This contribution gives a brief overview on the status of COSINUS.

COSINUS will be among the first underground experiments to operate Transition Edge Sensors in a dry dilution refrigerator, measuring temperature changes on the order of μ K. A pulse tube cryocooler is used to cool down to 3K, trading simplified handling, by not using liquid noble gases, for an increased vibration noise level in the acoustic frequency range. As the signals measured with a TES are in the same frequency region, it is necessary to decouple the detectors from all possible noise sources. In COSINUS, a two-level passive decoupling system was developed and tested using piezo-based accelerometers. At the first level, the refrigerator is mechanically isolated from all external noise sources. For the second level an internal spring-based system was developed and tested on a mockup system. On the first level a reduction of the vibrational background up to a factor 4 below 10 Hz could be measured. On the second level a resonance frequency of 1.2 Hz with damping of higher frequencies was achieved.

COSINUS is a dark matter (DM) direct search experiment that uses sodium iodide (NaI) crystals as cryogenic calorimeters. Thanks to the low nuclear recoil energy threshold and event-by-event discrimination capability, COSINUS will address the long-standing DM claim made by the DAMA/LIBRA collaboration. The experiment is currently under construction at the Laboratori Nazionali del Gran Sasso, Italy, and employs a large cylindrical water tank as a passive shield to meet the required background rate. However, muon-induced neutrons can mimic a DM signal therefore requiring an active veto system, which is achieved by instrumenting the water tank with an array of photomultiplier tubes (PMTs). This study optimizes the number, arrangement, and trigger conditions of the PMTs as well as the size of an optically invisible region. The objective was to maximize the muon veto efficiency while minimizing the accidental trigger rate due to the ambient and instrumental background. The final configuration predicts a veto efficiency of 99.63 ± 0.16% and 44.4 ± 5.6% in the tagging of muon events and showers of secondary particles, respectively. The active veto will reduce the cosmogenic neutron background rate to 0.11 ± 0.02 cts · kg - 1 · year - 1 , corresponding to less than one background event in the region of interest for the whole COSINUS-1 π exposure of 1000 kg · days.

COSINUS is a dark matter (DM) direct search experiment that uses sodium iodide (NaI) crystals as cryogenic calorimeters. Thanks to the low nuclear recoil energy threshold and event-by-event discrimination capability, COSINUS will address the long-standing DM claim made by the DAMA/LIBRA collaboration. The experiment is currently under construction at the Laboratori Nazionali del Gran Sasso, Italy, and employs a large cylindrical water tank as a passive shield to meet the required background rate. However, muon-induced neutrons can mimic a DM signal therefore requiring an active veto system, which is achieved by instrumenting the water tank with an array of photomultiplier tubes (PMTs). This study optimizes the number, arrangement, and trigger conditions of the PMTs as well as the size of an optically invisible region. The objective was to maximize the muon veto efficiency while minimizing the accidental trigger rate due to the ambient and instrumental background. The final configuration predicts a veto efficiency of 99.63 \\(\\pm\\) 0.16 \\(\\%\\) and 44.4 \\(\\pm\\) \\(5.6\\%\\) in the tagging of muon events and showers of secondary particles, respectively. The active veto will reduce the cosmogenic neutron background rate to 0.11 \\(\\pm\\) 0.02 cts\\(\\cdot\\)kg$^{-1}$$\\cdot\\(year\\)^{-1}\\(, corresponding to less than one background event in the region of interest for the whole COSINUS-1\\)\\pi\\( exposure of 1000 kg\\)\\cdot$days.

We present results from a 3.25 kg-day target exposure of two silicon charge-coupled devices (CCDs), each with 24 megapixels and skipper readout, deployed in the DAMIC setup at SNOLAB. With a reduction in pixel readout noise of a factor of 10 relative to the previous detector, we investigate the excess population of low-energy events in the CCD bulk previously observed above expected backgrounds. We address the dominant systematic uncertainty of the previous analysis through a depth fiducialization designed to reject surface backgrounds on the CCDs. The measured bulk ionization spectrum confirms the presence of an excess population of low-energy events in the CCD target with characteristic rate of \\({\\sim}7\\) events per kg-day and electron-equivalent energies of \\({\\sim}80~\\)eV, whose origin remains unknown.

While neutrinos are often treated as a background for many dark matter experiments, these particles offer a new avenue for physics: the detection of core-collapse supernovae. Supernovae are extremely energetic, violent and complex events that mark the death of massive stars. During their collapse stars emit a large number of neutrinos in a short burst. These neutrinos carry 99\\% of the emitted energy which makes their detection fundamental in understanding supernovae. This paper illustrates how COSINUS (Cryogenic Observatory for SIgnatures seen in Next-generation Underground Searches), a sodium iodide (NaI) based dark matter search, will be sensitive to the next galactic core-collapse supernova. The experiment is composed of two separate detectors which will be sensitive to far and nearby supernovae. The inner core of the experiment will consist of NaI crystals operating as scintillating calorimeters, mainly sensitive to the Coherent Elastic Scattering of Neutrinos (CE\\(\\nu\\)NS) against the Na and I nuclei. The low mass of the cryogenic detectors gives the experiment a sensitivity to close supernovae below 1kpc without pileup. They will see up to hundreds of CE\\(\\nu\\)NS events from a supernova happening at 200pc. The crystals reside at the center of a cylindrical 230T water tank, instrumented with 30 photomultipliers. This tank acts as a passive and active shield able to detect the Cherenkov radiation induced by impinging charged particles from ambient and cosmogenic radioactivity. A supernova near the Milky Way Center (10kpc) will be easily detected inducing \\(\\sim\\)60 measurable events, and the water tank will have a 3\\(\\sigma\\) sensitivity to supernovae up to 22kpc, seeing \\(\\sim\\)10 events. This paper shows how, even without dedicated optimization, modern dark matter experiments will also play their part in the multi-messenger effort to detect the next galactic core-collapse supernova.

Fully-depleted thick silicon Skipper-charge-coupled devices (Skipper-CCDs) are an important technology to probe neutrino and light-dark-matter interactions due to their sub-electron read-out noise. However, the successful search for rare neutrino or dark-matter events requires the signal and all backgrounds to be fully characterized. In particular, a measurement of the electron-hole pair creation energy below 150 eV and the Fano factor are necessary for characterizing the dark matter and neutrino signals. Moreover, photons from background radiation may Compton scatter in the silicon bulk, producing events that can mimic a dark matter or neutrino signal. We present a measurement of the Compton spectrum using a Skipper-CCD and a \\(^{241}\\)Am source. With these data, we estimate the electron-hole pair-creation energy to be \\(\\left(3.71 \\pm 0.08\\right)\\) eV at 130 K in the energy range between 99.3 eV and 150 eV. By measuring the widths of the steps at 99.3 eV and 150 eV in the Compton spectrum, we introduce a novel technique to measure the Fano factor, setting an upper limit of 0.31 at 90% C.L. These results prove the potential of Skipper-CCDs to characterize the Compton spectrum and to measure precisely the Fano factor and electron-hole pair creation energy below 150 eV.