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CUPID-Mo is a bolometric experiment to search for neutrinoless double-beta decay ( 0 ν β β ) of 100 Mo . In this article, we detail the CUPID-Mo detector concept, assembly and installation in the Modane underground laboratory, providing results from the first datasets. The CUPID-Mo detector consists of an array of 20 100 Mo -enriched 0.2 kg Li 2 MoO 4 crystals operated as scintillating bolometers at ∼ 20 mK . The Li 2 MoO 4 crystals are complemented by 20 thin Ge optical bolometers to reject α events by the simultaneous detection of heat and scintillation light. We observe a good detector uniformity and an excellent energy resolution of 5.3 keV (6.5 keV) FWHM at 2615 keV, in calibration (physics) data. Light collection ensures the rejection of α particles at a level much higher than 99.9% – with equally high acceptance for γ / β events – in the region of interest for 100 Mo 0 ν β β . We present limits on the crystals’ radiopurity: ≤ 3 μ Bq/kg of 226 Ra and ≤ 2 μ Bq/kg of 232 Th . We discuss the science reach of CUPID-Mo, which can set the most stringent half-life limit on the 100 Mo 0 ν β β decay in half-a-year’s livetime. The achieved results show that CUPID-Mo is a successful demonstrator of the technology developed by the LUMINEU project and subsequently selected for the CUPID experiment, a proposed follow-up of CUORE, the currently running first tonne-scale bolometric 0 ν β β experiment.

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.

The $2\\nu2\\beta$ decay of $^{150}$Nd to the first excited 740.5 keV $0^{+}_{1}$ level of $^{150}$Sm was measured over 5.845 yr with the help of a four-crystal low-background HPGe $\\gamma$ 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 $T_{1/2}=[0.83^{+0.18}_{-0.13}\\mathrm{(stat)}^{+0.16}_{-0.19}\\mathrm{(syst)}]\\times 10^{20}$ yr. Interpreting an excess of the 334.0-keV peak area as an indication of the $2\\beta$ decay of $^{150}$Nd to the 334.0 keV $2^+_1$ excited level of $^{150}$Sm with a half-life of $T_{1/2}=[1.5^{+2.3}_{-0.6}\\mathrm{(stat)}\\pm 0.4\\mathrm{(syst)}]\\times10^{20}$ yr, the $2\\nu2\\beta$ half-life of $^{150}$Nd for the transition to the 0$^{+}_{1}$ level is $T_{1/2}=[1.03^{+0.35}_{-0.22}\\mathrm{(stat)}^{+0.16}_{-0.19}\\mathrm{(syst)}]\\times 10^{20}$ yr, 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 $2\\nu2\\beta$ and $0\\nu2\\beta$ transitions of $^{150}$Nd and $^{148}$Nd to several excited levels of $^{150}$Sm and $^{148}$Sm, limits were set at level of $T_{1/2}>10^{20}-10^{21}$ yr.

Triple beta decay of 150 Nd to the ground and excited states of 150 Eu and quadruple beta decay of 150 Nd to the excited states of 150 Gd have been studied using a 400 cm 3 low-background HPGe detector. A half-life limit for the quadruple beta decay to the 0 1 + state of 150 Gd was found to be T 1/2 (0 v + 4 v ) > 8.7 × 10 20 yr (90% C.L.). For other (0 v +4 v ) transitions to the excited states limits for the first time have been obtained at the level of (6.1 − 9.5) × 10 20 yr (90% C.L.). We report here also the results for the first search for triple beta decay to the ground and excited final states of 150 Eu, T 1/2 (0 v + 3 v ) > (0.04 − 4.8) × 10 20 yr (90% C.L.).

Triple beta decay of 150Nd to the ground and excited states of 150Eu and quadruple beta decay of 150Nd to the excited states of 150Gd have been studied using a 400 cm3 low-background HPGe detector. A half-life limit for the quadruple beta decay to the 01+ state of 150Gd was found to be T1/2(0v + 4v) > 8.7 × 1020 yr (90% C.L.). For other (0v+4v) transitions to the excited states limits for the first time have been obtained at the level of (6.1 − 9.5) × 1020 yr (90% C.L.). We report here also the results for the first search for triple beta decay to the ground and excited final states of 150Eu, T1/2(0v + 3v) > (0.04 − 4.8) × 1020 yr (90% C.L.).

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 NEMO-3 results for the double-$\\beta $ decay of $^{150}$Nd to the 0$^+_1$ and 2$^+_1$ excited states of $^{150}$Sm are reported. The data recorded during 5.25 year with 36.6 g of the isotope $^{150}$Nd are used in the analysis. The signal of the $2\\nu \\beta \\beta $ transition to the 0$^+_1$ excited state is detected with a statistical significance exceeding 5$\\sigma $. The half-life is measured to be $T_{1/2}^{2\\nu \\beta \\beta }(0^+_1) = \\left[ 1.11 ^{+0.19}_{-0.14} \\,\\left( \\hbox {stat}\\right) ^{+0.17}_{-0.15}\\,\\left( \\hbox {syst}\\right) \\right] \\times 10^{20}$ year, which is the most precise value that has been measured to date. 90% confidence-level limits are set for the other decay modes. For the $2\\nu \\beta \\beta $ decay to the 2$^+_1$ level the limit is $T^{2\\nu \\beta \\beta }_{1/2}(2^+_1) > 2.42 \\times 10^{20}~\\hbox {year}$. The limits on the $0\\nu \\beta \\beta $ decay to the 0$^+_1$ and 2$^+_1$ levels of $^{150}$Sm are significantly improved to $T_{1/2}^{0\\nu \\beta \\beta }(0^+_1) > 1.36 \\times 10^{22}~\\hbox {year}$ and $T_{1/2}^{0\\nu \\beta \\beta }(2^+_1) > 1.26 \\times 10^{22}~\\hbox {year}$.

The CROSS experiment is proposing to use a new technology of surface sensitive bolometers for low-background neutrinoless double beta decay searches. Efficient rejection of surface α and β events will allow to reach background in the region of interest below than 10 −4 cnts/keV/kg/yr. The isotopes of interest, which are 130 Te and 100 Mo, are investigated with TeO 2 and Li 2 MoO 4 bolometers. The surface sensitivity is achieved thanks to the evaporation of thin metallic film on the crystal surface that modifies the pulse shape of near-surface events. An investigation of various pulse shape parameters was performed. The analysis shows that one of the best parameters for discrimination is the integrated area of the raw signal both for TeO 2 and Li 2 MoO 4 with Pd-Al (10 nm - 100 nm) bi-layer.

CUPID-Mo, located in the Laboratoire Souterrain de Modane (France), was a demonstrator for the next generation$$0\\nu \\beta \\beta $$0 ν β β decay experiment, CUPID. It consisted of an array of 20 enriched Li$$_{2}$$2$$^{100}$$100 MoO$$_4$$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$$2\\nu \\beta \\beta $$2 ν β β decay and other processes with high precision. We also measure the radio-purity of the Li$$_{2}$$2$$^{100}$$100 MoO$$_4$$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 $$^{+0.9}_{-0.8}$$- 0.8 + 0.9  (stat)$$^{+1.5}_{-0.7}$$- 0.7 + 1.5  (syst) $$\\times ~10 ^{-3}$$× 10 - 3  counts/$$\\Delta E_{\\text {FWHM}}/\\text {mol}_{\\text {iso}}/\\text {year},$$Δ E FWHM / mol iso / year , the lowest in a bolometric$$0\\nu \\beta \\beta $$0 ν β β decay experiment.

Neutrinoless double-beta decay is a key process in particle physics. Its experimental investigation is the only viable method that can establish the Majorana nature of neutrinos, providing at the same time a sensitive inclusive test of lepton number violation. CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity) aims at developing and testing a new bolometric technology to be applied to future large-scale experiments searching for neutrinoless double-beta decay of the promising nuclei $^{100}$Mo and $^{130}$Te. The limiting factor in large-scale bolometric searches for this rare process is the background induced by surface radioactive contamination, as shown by the results of the CUORE experiment. The basic concept of CROSS consists of rejecting this challenging background component by pulse-shape discrimination, assisted by a proper coating of the faces of the crystal containing the isotope of interest and serving as energy absorber of the bolometric detector. In this paper, we demonstrate that ultra-pure superconductive Al films deposited on the crystal surfaces act successfully as pulse-shape modifiers, both with fast and slow phonon sensors. Rejection factors higher than 99.9% of α surface radioactivity have been demonstrated in a series of prototypes based on crystals of Li$_{2}$MoO$_{4}$ and TeO$_{2}$. We have also shown that point-like energy depositions can be identified up to a distance of ∼ 1 mm from the coated surface. The present program envisions an intermediate experiment to be installed underground in the Canfranc laboratory (Spain) in a CROSS-dedicated facility. This experiment, comprising ∼ 3×10$^{25}$ nuclei of $^{100}$Mo, will be a general test of the CROSS technology as well as a worldwide competitive search for neutrinoless double-beta decay, with sensitivity to the effective Majorana mass down to 70 meV in the most favorable conditions.[graphic not available: see fulltext]