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The CUPID-Mo experiment to search for 0νββ decay in 100Mo has been recently completed after about 1.5 years of operation at Laboratoire Souterrain de Modane (France). It served as a demonstrator for CUPID, a next generation 0νββ decay experiment. CUPID-Mo was comprised of 20 enriched Li2100MoO4 scintillating calorimeters, each with a mass of ∼0.2 kg, operated at ∼20 mK. We present here the final analysis with the full exposure of CUPID-Mo (100Mo exposure of 1.47 kg×year) used to search for lepton number violation via 0νββ decay. We report on various analysis improvements since the previous result on a subset of data, reprocessing all data with these new techniques. We observe zero events in the region of interest and set a new limit on the 100Mo 0νββ decay half-life of T1/20ν>1.8×1024 year (stat. + syst.) at 90% CI. Under the light Majorana neutrino exchange mechanism this corresponds to an effective Majorana neutrino mass of mββ <(0.28-0.49) eV, dependent upon the nuclear matrix element utilized.

The full data set of the NEMO-3 experiment has been used to measure the half-life of the two-neutrino double beta decay of \\[^{100}\\]Mo to the ground state of \\[^{100}\\]Ru, \\[T_{1/2} = \\left[ 6.81 \\pm 0.01\\,\\left( \\text{ stat }\\right) ^{+0.38}_{-0.40}\\,\\left( \\text{ syst }\\right) \\right] \\times 10^{18}\\] year. The two-electron energy sum, single electron energy spectra and distribution of the angle between the electrons are presented with an unprecedented statistics of \\[5\\times 10^5\\] events and a signal-to-background ratio of \\[\\sim \\] 80. Clear evidence for the Single State Dominance model is found for this nuclear transition. Limits on Majoron emitting neutrinoless double beta decay modes with spectral indices of \\[\\mathrm{n}=2,3,7\\], as well as constraints on Lorentz invariance violation and on the bosonic neutrino contribution to the two-neutrino double beta decay mode are obtained.

CUPID will be a next generation experiment searching for the neutrinoless double β decay, whose discovery would establish the Majorana nature of the neutrino. Based on the experience achieved with the CUORE experiment, presently taking data at LNGS, CUPID aims to reach a background free environment by means of scintillating Li2100MoO4 crystals coupled to light detectors. Indeed, the simultaneous heat and light detection allows us to reject the dominant background of α particles, as proven by the CUPID-0 and CUPID-Mo demonstrators. In this work we present the results of the first test of the CUPID baseline module. In particular, we propose a new optimized detector structure and light sensors design to enhance the engineering and the light collection, respectively. We characterized the heat detectors, achieving an energy resolution of (5.9 ± 0.2) keV FWHM at the Q-value of 100Mo (about 3034 keV). We studied the light collection of the baseline CUPID design with respect to an alternative configuration which features gravity-assisted light detectors’ mounting. In both cases we obtained an improvement in the light collection with respect to past measures and we validated the particle identification capability of the detector, which ensures an α particle rejection higher than 99.9%, fully satisfying the requirements for CUPID.

The$$2\\nu 2\\beta $$2 ν 2 β decay of$$^{150}\\hbox {Nd}$$150 Nd to the first excited 740.5 keV$$0^{+}_{1}$$0 1 + level of$$^{150}\\hbox {Sm}$$150 Sm was measured over 5.845 years with the help of a four-crystal low-background HPGe$$\\gamma $$γ spectrometry system in the underground low-background laboratory STELLA of LNGS-INFN. A 2.381 kg highly purified Nd-containing sample was employed as the decay source. The expected de-excitation gamma-quanta of the$$0^{+}_{1}$$0 1 + level with energies 334.0 keV and 406.5 keV were observed both in one-dimensional spectrum and in coincidence data resulting in the half-life$$T_{1/2}=[0.83^{+0.18}_{-0.13}\\mathrm {(stat)}^{+0.16}_{-0.19}\\mathrm {(syst)}]\\times 10^{20}$$T 1 / 2 = [ 0 . 83 - 0.13 + 0.18 ( stat ) - 0.19 + 0.16 ( syst ) ] × 10 20 year. Interpreting an excess of the 334.0-keV peak area as an indication of the$$2\\beta $$2 β decay of$$^{150}\\hbox {Nd}$$150 Nd to the 334.0 keV$$2^+_1$$2 1 + excited level of$$^{150}\\hbox {Sm}$$150 Sm with a half-life of$$T_{1/2}=[1.5^{+2.3}_{-0.6}\\mathrm {(stat)}\\pm 0.4\\mathrm {(syst)}]\\times 10^{20}$$T 1 / 2 = [ 1 . 5 - 0.6 + 2.3 ( stat ) ± 0.4 ( syst ) ] × 10 20 year, the$$2\\nu 2\\beta $$2 ν 2 β half-life of$$^{150}\\hbox {Nd}$$150 Nd for the transition to the 0$$^{+}_{1}$$1 + level is$$T_{1/2}=[1.03^{+0.35}_{-0.22}\\mathrm {(stat)}^{+0.16}_{-0.19}\\mathrm {(syst)}]\\times 10^{20}$$T 1 / 2 = [ 1 . 03 - 0.22 + 0.35 ( stat ) - 0.19 + 0.16 ( syst ) ] × 10 20 year, in agreement with the previous experiments. Both half-life values reasonably agree with the theoretical calculations in the framework of proton-neutron QRPA with isospin restoration combined with like nucleon QRPA for description of excited states in the final nuclei. For$$2\\nu 2\\beta $$2 ν 2 β and$$0\\nu 2\\beta $$0 ν 2 β transitions of$$^{150}\\hbox {Nd}$$150 Nd and$$^{148}\\hbox {Nd}$$148 Nd to several excited levels of$$^{150}\\hbox {Sm}$$150 Sm and$$^{148}\\hbox {Sm}$$148 Sm , limits were set at level of$$T_{1/2}>10^{20}-10^{21}$$T 1 / 2 > 10 20 - 10 21 year.

The 2 ν 2 β decay of 150 Nd to the first excited 740.5 keV 0 1 + level of 150 Sm was measured over 5.845 years with the help of a four-crystal low-background HPGe γ spectrometry system in the underground low-background laboratory STELLA of LNGS-INFN. A 2.381 kg highly purified Nd-containing sample was employed as the decay source. The expected de-excitation gamma-quanta of the 0 1 + level with energies 334.0 keV and 406.5 keV were observed both in one-dimensional spectrum and in coincidence data resulting in the half-life T 1 / 2 = [ 0 . 83 - 0.13 + 0.18 ( stat ) - 0.19 + 0.16 ( syst ) ] × 10 20 year. Interpreting an excess of the 334.0-keV peak area as an indication of the 2 β decay of 150 Nd to the 334.0 keV 2 1 + excited level of 150 Sm with a half-life of T 1 / 2 = [ 1 . 5 - 0.6 + 2.3 ( stat ) ± 0.4 ( syst ) ] × 10 20 year, the 2 ν 2 β half-life of 150 Nd for the transition to the 0 1 + level is T 1 / 2 = [ 1 . 03 - 0.22 + 0.35 ( stat ) - 0.19 + 0.16 ( syst ) ] × 10 20 year, in agreement with the previous experiments. Both half-life values reasonably agree with the theoretical calculations in the framework of proton-neutron QRPA with isospin restoration combined with like nucleon QRPA for description of excited states in the final nuclei. For 2 ν 2 β and 0 ν 2 β transitions of 150 Nd and 148 Nd to several excited levels of 150 Sm and 148 Sm , limits were set at level of T 1 / 2 > 10 20 - 10 21 year.

CUPID-Mo, located in the Laboratoire Souterrain de Modane (France), was a demonstrator for the next generation 0 ν β β decay experiment, CUPID. It consisted of an array of 20 enriched Li 2 100 MoO 4 bolometers and 20 Ge light detectors and has demonstrated that the technology of scintillating bolometers with particle identification capabilities is mature. Furthermore, CUPID-Mo can inform and validate the background prediction for CUPID. In this paper, we present a detailed model of the CUPID-Mo backgrounds. This model is able to describe well the features of the experimental data and enables studies of the 2 ν β β decay and other processes with high precision. We also measure the radio-purity of the Li 2 100 MoO 4 crystals which are found to be sufficient for the CUPID goals. Finally, we also obtain a background index in the region of interest of 3.7  - 0.8 + 0.9  (stat) - 0.7 + 1.5  (syst)  × 10 - 3  counts/ Δ E FWHM / mol iso / year , the lowest in a bolometric 0 ν β β decay experiment.

Cryogenic calorimeters are among the leading technologies for searching for rare events. The CUPID experiment is exploiting this technology to deploy a tonne-scale detector to search for neutrinoless double-beta decay of$$^{100}$$100 Mo. The CUPID collaboration proposed an innovative approach to assembling cryogenic calorimeters in a stacked configuration, held in position solely by gravity. This gravity-based assembly method is unprecedented in the field of cryogenic calorimeters and offers several advantages, including relaxed mechanical tolerances and simplified construction. To assess and optimize its performance, we constructed a medium-scale prototype hosting 28  Li$$_2$$2 MoO$$_4$$4 crystals and 30 Ge light detectors, both operated as cryogenic calorimeters at the Laboratori Nazionali del Gran Sasso (Italy). Despite an unexpected excess of noise in the light detectors, the results of this test proved (i) a thermal stability better than ±0.5 mK at 10 mK, (ii) a good energy resolution of Li$$_2$$2 MoO$$_4$$4 cryogenic calorimeters, (6.6 ± 2.2) keV FWHM at 2615 keV, and (iii) a Li$$_2$$2 MoO$$_4$$4 light yield measured by the closest light detector of 0.36 keV/MeV, sufficient to guarantee the particle identification requested by CUPID.

For the first time, a cadmium tungstate crystal scintillator enriched in$^{116}$ Cd has been succesfully tested as a scintillating bolometer. The measurement was performed above ground at a temperature of 18 mK. The crystal mass was 34.5 g and the enrichment level $\\sim $ 82 %. Despite a substantial pile-up effect due to above-ground operation, the detector demonstrated high energy resolution (2–7 keV FWHM in 0.2–2.6 MeV $\\gamma $ energy range and 7.5 keV FWHM at the$^{116}$ Cd double-beta decay transition energy of 2813 keV), a powerful particle identification capability and a high level of internal radio-purity. These results prove that cadmium tungstate is a promising detector material for a next-generation neutrinoless double-beta decay bolometric experiment, like that proposed in the CUPID project (CUORE Upgrade with Particle IDentification).

The current experiments searching for neutrinoless double- β ( 0 ν β β ) decay also collect large statistics of Standard Model allowed two-neutrino double- β ( 2 ν β β ) decay events. These can be used to search for Beyond Standard Model (BSM) physics via 2 ν β β decay spectral distortions. 100 Mo has a natural advantage due to its relatively short half-life, allowing higher 2 ν β β decay statistics at equal exposures compared to the other isotopes. We demonstrate the potential of the dual read-out bolometric technique exploiting a 100 Mo exposure of 1.47 kg  ×  years, acquired in the CUPID-Mo experiment at the Modane underground laboratory (France). We set limits on 0 ν β β decays with the emission of one or more Majorons, on 2 ν β β decay with Lorentz violation, and 2 ν β β decay with a sterile neutrino emission. In this analysis, we investigate the systematic uncertainty induced by modeling the 2 ν β β decay spectral shape parameterized through an improved model, an effect never considered before. This work motivates searches for BSM processes in the upcoming CUPID experiment, which will collect the largest amount of 2 ν β β decay events among the next-generation experiments.

The current experiments searching for neutrinoless double-$$\\beta $$β ($$0\\nu \\beta \\beta $$0 ν β β ) decay also collect large statistics of Standard Model allowed two-neutrino double-$$\\beta $$β ($$2\\nu \\beta \\beta $$2 ν β β ) decay events. These can be used to search for Beyond Standard Model (BSM) physics via$$2\\nu \\beta \\beta $$2 ν β β decay spectral distortions. 100 Mo has a natural advantage due to its relatively short half-life, allowing higher$$2\\nu \\beta \\beta $$2 ν β β decay statistics at equal exposures compared to the other isotopes. We demonstrate the potential of the dual read-out bolometric technique exploiting a 100 Mo exposure of 1.47 kg $$\\times $$×  years, acquired in the CUPID-Mo experiment at the Modane underground laboratory (France). We set limits on$$0\\nu \\beta \\beta $$0 ν β β decays with the emission of one or more Majorons, on$$2\\nu \\beta \\beta $$2 ν β β decay with Lorentz violation, and$$2\\nu \\beta \\beta $$2 ν β β decay with a sterile neutrino emission. In this analysis, we investigate the systematic uncertainty induced by modeling the$$2\\nu \\beta \\beta $$2 ν β β decay spectral shape parameterized through an improved model, an effect never considered before. This work motivates searches for BSM processes in the upcoming CUPID experiment, which will collect the largest amount of$$2\\nu \\beta \\beta $$2 ν β β decay events among the next-generation experiments.