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.

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.

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.

The search for neutrinoless double beta decay (0 νββ ) is fundamental for investigating lepton-number violation, probing new physics beyond the Standard Model, and determining whether neutrinos are Majorana particles. CUORE (Cryogenic Underground Observatory of Rare Events), a cryogenic bolometric experiment at LNGS, studies 0 νββ in ¹³⁰ Te using 988 TeO ₂ crystals. It is a milestone of cryogenic detector arrays with a tonne-scale detector operated for more than 7 years below 15 mK. Since 2017, CUORE has accumulated over 2.9 tonne-years of exposure. Thanks to a detailed background reconstruction across a broad energy range, CUORE has achieved one of the leading 0 νββ limits and one of the most precise two-neutrino double beta decay (2 νββ ) half-life measurements. Building on CUORE's success, CUPID (CUORE Upgrade with Particle ID) aims to significantly enhance its 0 νββ discovery sensitivity to 10 ²⁷ yr in ¹⁰⁰ Mo, covering the Inverted Hierarchy of neutrino masses. It will employ lithium molybdate (Li ₂ MoO ₄ ) crystals enriched in ¹⁰⁰ Mo, alongside germanium light detectors with Neganov-Trofimov-Luke amplification. The simultaneous readout of heat and light will benefit to enhance background rejection, particularly against alpha contamination and 2 νββ pileup. CUPID will reuse CUORE's cryostat and infrastructure. Current efforts focus on detector performance validation, sensitivity studies, and finalizing the experimental design to maximize physics reach. This work presents the latest CUORE results and outlines the key milestones toward CUPID's realization.