# Development of Cryogenic Detectors for Neutrinoless Double Beta Decay Searches with CUORE and CUPID
## 4 Beyond the CUORE Sensitivity  
The technical results obtained by CUORE showed that it is possible to run a ton-scale bolometric experiment, overcoming the mass scalability limitation related to the use of a dilution refrigerator. The next-generation experiments aim to explore the whole inverted hierarchy of neutrino masses and a relevant fraction of the normal one, reaching a sensitivity of \(>\)10\({}^{27}\) yr on the \(0\
u\beta\beta\) half-life [26; 42; 43]. To achieve this goal with cryogenic calorimeters, the CUORE background level must be reduced by two orders of magnitude, exploiting a particle identification technique to discriminate degraded \(\alpha\) particles from electron signals. In the search for a novel approach, the scintillation-based detectors have proven to be effective in addressing this issue. The combination of these two techniques, resulting in scintillating bolometric detectors, was proposed in 1989 for solar neutrino experiments [44]. The first application to \(0\
u\beta\beta\) searches with \(\alpha\) background suppression were performed with crystal facing standard light detectors (LD), such as photodiodes [45], and were limited by the technical difficulties in using standard LDs at cryogenic temperatures. An innovation in this field was the proposal to use cryogenic calorimeters as custom low-temperature LDs [46]. This idea led to the development of new possible detector designs [47]. The main advantages of this approach is the sensitivity shown by these calorimeters. With respect to standard LDs, these devices have higher photon detection efficiencies in a wide region of photon wavelength. On the other hand, they cannot be applied in single-photon counting, due to their low native gain, and are characterized by slower response time with respect to standard photo-detectors (ms versus ns or even less). Different studies have been performed exploring the possibilities of this technique [48; 49; 50], proving the background rejection capabilities and the possibility to investigate different isotopes. This success paved the way for the construction of two demonstrators, which allowed to test different crystals and to set limits of \(0\
u\beta\beta\) half-lives for two isotopes: CUPID-0 (\({}^{82}\)Se) and CUPID-Mo (\({}^{100}\)Mo ). The excellent performance of CUORE, CUPID-0 [51; 52] and CUPID-Mo [53; 54] laid the foundation for CUORE Upgrade with Particle Identification, i.e., CUPID [55; 56]. The main goal of the CUPID project is the design and operation of a ton-scale detector to reach a sensitivity of 10\({}^{27}\) yr on \(0\
u\beta\beta\)[26]. The progress in the development of cryogenic calorimeters is summarized in Figure 1, where we show for each experiment the exposure and the ROI background integrated in a detector FWHM, both corrected for the efficiency and expressed in unit of moles of the isotope under study [17]. These variables are calculated using the data in Table 1, extracted from the latest published data from the experiments. The evolution towards better performances of the CUORE precursors shows the results of the technical developments performed during the CUORE design optimization. The subsequent experiment kept increasing their mass, while continuously lowering the intrinsic background levels. Unfortunately, both the \({}^{130}\)Te characteristics and the unavoidable degraded \(\alpha\) background prevented this technical approach from reaching the zero-background regime. Upgrading the CUORE design using scintillating calorimeters with dual heat-light readout will allow reaching this goal. The pilot experiments (CUPID-0 and CUPID-Mo) were able to cross this boundary, reaching world-leading performances even with small exposures. The ongoing CUPID program is the next step towards the scale-up of the scintillating calorimetric technique to the CUORE scale. Figure 1: Comparison of the performances of the different \(0\
u\beta\beta\) experiments based on cryogenic calorimeters described in this work.