The possibility that neutrinos may be their own antiparticles, unique among the known fundamental particles, arises from the symmetric theory of fermions proposed by Ettore Majorana in 1937¹. Given the profound consequences of such Majorana neutrinos, among which is a potential explanation for the matter–antimatter asymmetry of the universe via leptogenesis², the Majorana nature of neutrinos commands intense experimental scrutiny globally; one of the primary experimental probes is neutrinoless double beta (0νββ) decay. Here we show results from the search for 0νββ decay of ¹³⁰Te, using the latest advanced cryogenic calorimeters with the CUORE experiment³. CUORE, operating just 10 millikelvin above absolute zero, has pushed the state of the art on three frontiers: the sheer mass held at such ultralow temperatures, operational longevity, and the low levels of ionizing radiation emanating from the cryogenic infrastructure. We find no evidence for 0νββ decay and set a lower bound of the process half-life as 2.2 × 10²⁵ years at a 90 per cent credibility interval. We discuss potential applications of the advances made with CUORE to other fields such as direct dark matter, neutrino and nuclear physics searches and large-scale quantum computing, which can benefit from sustained operation of large payloads in a low-radioactivity, ultralow-temperature cryogenic environment.

BULLKID is a cryogenic, solid-state detector designed for direct searches of particle Dark Matter candidates, with mass $$\lesssim 1$$ ≲ 1 GeV/c $$^2$$ 2 , and coherent neutrino-nucleus scattering. It is based on an array of dice carved in 5 mm thick silicon crystal, sensed by phonon-mediated Kinetic Inductance Detectors. In previous works, the array was calibrated with bursts of optical photons, which are absorbed in the first hundreds nanometers of the dice and give rise to surface events. In this work, we present the reconstruction of bulk events through the 59.5 keV $$\gamma $$ γ -ray generated by an $$^{241}$$ 241 Am source, which emulates more closely the interaction of Dark Matter and neutrinos. The peak resolution is $$5\%~(\sigma )$$ 5 % ( σ ) and its position is shifted by less than $$10\%$$ 10 % with respect to the optical calibration. We observe that the resolution is further improved by a factor 2 combining the signal from neighboring dice. These results confirm the performance of the detector in view of the physics goals of the BULLKID-DM experiment for dark matter search.

BULLKID-DM is a cryogenic experiment that will use phonon-sensing Kinetic Inductance Detectors (KIDs) to search for $\leq 1\ \text{GeV}/\text{c}^{2}$ WIMP dark matter particles. The experiment will be conducted in the low background environment of the underground Gran Sasso National Laboratories in Italy. In the experiment's energy region of interest (200 eV - 2 keV), it is crucial to minimise background signals from muons, $\gamma$-rays and neutrons. For this reason, the collaboration is developing a cryogenic anticoincidence detector (veto) composed of high density scintillating crystals read out by KID-based light detectors. We conducted a study to select the optimal veto material. Along with Bismuth Germanate B $_{4}$ G$_{3}$O $_{12}$ (BGO), we have identified Gadolinium Oxyorthosilicate Gd$_{2}$ SiO$_{5}$ (GSO) and Gadolinium Aluminum Gallium Garnet Gd $_{3}$ Al$_{2}$Ga $_{3}$ O$_{12}$ (GAGG), which contain Gadolinium and thus are ideal for absorption of thermal neutrons. We tested the scintillating properties of the three crystals at cryogenic temperatures down to 20 K. GAGG exhibited the highest light yield and sub-microsecond scintillation times. Furthermore, we validated the effectiveness of the veto through simulation of the backgrounds originating from photon and neutron fluxes in the full BULLKID-DM experimental setup. A benchmark detector composed of a large (3x3 inch cylinder) BGO crystal coupled to a Si wafer equipped with with a KID was tested. We successfully operated this device by measuring the energy spectrum of a $^{137}$Cs gamma source. We then estimated the light yield, energy resolution, and expected energy threshold. We interpreted the results with a simulation that takes into account crystal scintillation, optical photon transport, and KID response. Our finding allow for the optimization of the coupling between a KID and a scintillating crystal to further enhance the detection performance of the BULLKID-DM veto.

The BULLKID-DM experiment aims to detect WIMP-like potential Dark Matter particles with masses below 1 GeV/c$^{2}$. Sensing these particles is challenging, as it requires nuclear recoil detectors characterized by high exposure and an energy threshold in the order of 100 eV, achieved with cryogenic phonon detectors. BULLKID-DM introduces a detector that combines the low energy threshold with a new scalable technology, reaching a total target mass of 800 g. The core detector of the BULLKID-DM experiment is composed of 16 100mm silicon wafers, each segmented in 145 dice for a total of more than 2000 units, each sensed by a Microwave Kinetic Inductance Detector (MKID). The MKIDs on each wafer are coupled to a single transmission line and read using a frequency division multiplexing approach by the room-temperature data acquisition. In this contribution, we describe and assess the design of the room-temperature readout electronics system, including the selected hardware components and the FPGA firmware which contains the real-time signal processing stages for tone generation, frequency demultiplexing, and event triggering. We evaluate the system on the ZCU216 board, a commercial evaluation card built around a Radio-Frequency System-on-Chip (RFSoC) with integrated high-speed DACs and ADCs, and connected it to a custom-designed analog front-end for signal conditioning.

We present the operation with moderate radiation shield in a surface laboratory of BULky and Low-threshold Kinetic Inductance Detector) (BULLKID), a cryogenic detector for searches of light dark matter or coherent elastic neutrino-nucleus scattering. The detector consists of an array of 60 cubic silicon particle absorbers of 0.34 g each, sensed by cryogenic kinetic inductance detectors. The analysis presented focuses on data from 15 elements of the array, with two central units used to evaluate the background and with their surrounding elements used as veto. The low energy spectrum resulting from an exposure of 290 hours to ambient backgrounds, acquired with the use of external and internal radiation shields, is compatible with the simulations at the level of ( 6.8 ± 0.4 stat . ± 0.1 syst . ) × 10 4 counts / keV kg days from 2 keV down to an energy of 600 eV. The region between 225 eV and 600 eV shows a rise in background in disagreement with the simulations, while not sharing some of the key traits of the low energy excess observed in other cryogenic experiments. The high energy spectrum shape is in overall agreement with the simulations and displays the typical particle-induced x-ray emission of the surrounding lead.

In fundamental physics, sensors operating below liquid helium temperatures are highly vulnerable to vibrations, which can affect the sensitivity, for example, of high-performance particle detectors. Pulse-tube refrigerators, while generating vibrations lower than those of conventional systems, may still introduce several disturbances. Hence, flexible thermal connections are a commonly used mechanical solution to mitigate these undesirable effects. Among the materials that can be used, ultra-high-purity aluminum (UHP-Al) has attracted the attention for low-amplitude-vibration cryogenic applications, including gravitational wave interferometry, quantum information systems, precision space instrumentation, and cryogenic resonators. Thus, the aim of the paper is the characterization of the mechanical and microstructure properties of three UHP-Als (i.e., 5N—99.999 wt%, 5N5—99.9995 wt% and 6N—99.9999 wt%) intended for the production of thermal flexible connections with low stiffness, specifically designed to reduce vibration transmission in cryogenic environments. Mechanical properties were evaluated through standard tensile tests from room (+25 °C) to low temperature (i.e., −150 °C), providing insights into yield strength, ultimate tensile strength, elongation and elastic modulus. In addition, the dynamic elastic modulus of material loads, at cryogenic conditions (i.e., about −180 °C), was determined by measuring the natural resonance frequency, thereby assessing the material’s response to vibrational. Moreover, an extensive microstructural analysis was conducted using electron backscatter diffraction and x-ray diffraction. The correlation between the observed microstructure and the elastic properties was systematically examined. The results underscore the pivotal role of microstructural characteristics in dictating the elastic behavior of UHP Als. Eventually, the analysis provides valuable guidelines for the materials employment inside cryogenic systems, where severe vibration control is critical to maintain high operational performance.