Eur. Phys. J. C 82, 1033 (2022) The CUPID-Mo experiment to search for 0$\nu\beta\beta$ decay in $^{100}$Mo
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 0$\nu\beta\beta$ decay experiment. CUPID-Mo was comprised of 20
enriched Li$_2$$^{100}$MoO$_4$ scintillating calorimeters, each with a mass of
$\sim$ 0.2 kg, operated at $\sim$20 mK. We present here the final analysis with
the full exposure of CUPID-Mo ($^{100}$Mo exposure of 1.47 kg$\times$yr) used
to search for lepton number violation via 0$\nu\beta\beta$ 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 $^{100}$Mo 0$\nu\beta\beta$ decay
half-life of $T^{0\nu}_{1/2} > 1.8 \times 10^{24}$ year (stat.+syst.) at 90%
CI. Under the light Majorana neutrino exchange mechanism this corresponds to an
effective Majorana neutrino mass of $\left<m_{\beta \beta}\right> <
(0.28$--$0.49)$ eV, dependent upon the nuclear matrix element utilized.
Neutrinoless double beta (\(0\nu\beta\beta\)) decay is a hypothetical rare nuclear transition (\(T_{1/2}>10^{26}\) y). Its observation would provide an important insight about the nature of neutrinos ...(Dirac or Majorana particle) demonstrating that the lepton number is not conserved. This decay can be investigated with bolometers embedding the double beta decay isotope (\(^{76}\)Ge, \(^{82}\)Se, \(^{100}\)Mo, \(^{116}\)Cd, \(^{130}\)Te...), which perform as low temperature calorimeters (10 mK) detecting particle interactions via a small temperature rise read out by a dedicated thermometer. CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity) aims at the development of bolometric detectors (Li\(_{2}\)MoO\(_{4}\) and TeO\(_{2}\)) capable of discriminating surface \(\alpha\) and \(\beta\) interactions by exploiting superconducting properties of Al film deposited on the crystal surface. We report in this paper the results of tests on prototypes performed at CSNSM (Orsay, France) that showed the capability of a-few-\(\mu\)m-thick superconducting Al film deposited on crystal surface to discriminate surface \(\alpha\) from bulk events, thus providing the detector with the required surface sensitivity capability. The CROSS technology would further improve the background suppression and simplify the detector construction with a view to future competitive double beta decay searches.
The CUPID-Mo experiment at the Laboratoire Souterrain de Modane (France) is a demonstrator for CUPID, the next-generation ton-scale cryogenic \(0\nu\beta\beta\) experiment. It consists of a 4.2 kg ...array of 20 enriched Li$_{2}$$^{100}\(MoO\)_4\( scintillating bolometers to search for the lepton number violating process of \)0\nu\beta\beta\( decay in \)^{100}\(Mo. With more than one year of operation (2.16 kg\)\times\(yr of physics data), no event in the region of interest and hence no evidence for \)0\nu\beta\beta\( is observed. We report a new limit on the half-life of \)0\nu\beta\beta\( decay in \)^{100}\(Mo of \)T_{1/2} > 1.5 \times 10^{24}\,\(yr at 90 % C.I. The limit corresponds to an effective Majorana neutrino mass \)\langle m_{\beta\beta} \rangle\( \)<\( (0.31--0.54)\)\,$eV, dependent on the nuclear matrix element in the light Majorana neutrino exchange interpretation.
CUPID-Mo is a cryogenic detector array designed to search for neutrinoless double-beta decay (\(0\nu\beta\beta\)) of \(^{100}\)Mo. It uses 20 scintillating \(^{100}\)Mo-enriched Li\(_2\)MoO\(_4\) ...bolometers instrumented with Ge light detectors to perform active suppression of \(\alpha\) backgrounds, drastically reducing the expected background in the \(0\nu\beta\beta\) signal region. As a result, pileup events and small detector instabilities that mimic normal signals become non-negligible potential backgrounds. These types of events can in principle be eliminated based on their signal shapes, which are different from those of regular bolometric pulses. We show that a purely data-driven principal component analysis based approach is able to filter out these anomalous events, without the aid of detector response simulations.
Random coincidences of events could be one of the main sources of background in the search for neutrino-less double-beta decay of \(^{100}\)Mo with macro-bolometers, due to their modest time ...resolution. Scintillating bolometers as those based on Li\(_2\)MoO\(_4\) crystals and employed in the CROSS and CUPID experiments can eventually exploit the coincident fast signal detected in a light detector to reduce this background. However, the scintillation provides a modest signal-to-noise ratio, making difficult a pile-up pulse-shape recognition and rejection at timescales shorter than a few ms. Neganov-Trofimov-Luke assisted light detectors (NTL-LDs) offer the possibility to effectively increase the signal-to-noise ratio, preserving a fast time-response, and enhance the capability of pile-up rejection via pulse shape analysis. In this article we present: a) an experimental work performed with a Li\(_2\)MoO\(_4\) scintillating bolometer, studied in the framework of the CROSS experiment, and utilizing a NTL-LD; b) a simulation method to reproduce, synthetically, randomly coincident two-neutrino double-beta decay events; c) a new analysis method based on a pulse-shape discrimination algorithm capable of providing high pile-up rejection efficiencies. We finally show how the NTL-LDs offer a balanced solution between performance and complexity to reach background index $\sim$$10^{-4}\( counts/keV/kg/year with 280~g Li\)_2\(MoO\)_4\( (\)^{100}$Mo enriched) bolometers at 3034 keV, the Q-value of the double-beta decay, and target the goal of a next generation experiment like CUPID.
In preparation to the CROSS \(2\beta\) decay experiment, we installed a new detector suspension with magnetic dumping inside a pulse-tube cryostat of a dedicated low-background facility at the LSC ...(Spain). The suspension was tested with two scintillating bolometers based on large-volume 116CdWO4 (CWO-enr) and Li2MoO4 (LMO) crystals. The former, a reference device, was used for testing new noise conditions and for comparing bolometric performance of an advanced Li2MoO4 crystal developed in the framework of the CLYMENE project, in view of next-generation double-beta decay experiments like CUPID. We cooled down detectors to 15 mK and achieved high performance for all tested devices. In particular both CWO-enr and LMO bolometers demonstrated the energy resolution of 6 keV FWHM for the 2.6 MeV gamma quanta, among the best for thermal detectors based on such compounds. The baseline noise resolution (FWHM) of the CWO-enr detector was improved by 2 keV, compared to the best previous measurement of this detector in the CROSS facility, while the noise of the Ge-based optical bolometer was improved by a factor 2, to 100 eV FWHM. Despite of the evident progress in the improving of noise conditions of the set-up, we see high-frequency harmonics of a pulse-tube induced noise, suggesting a noise pick-up by cabling. Another Ge light detector was assisted with the signal amplification exploiting the Neganov-Trofimov-Luke effect, which allowed to reach 20 eV FWHM noise resolution by applying 60 V electrode bias. Highly-efficient particle identification was achieved with both detectors, despite a low scintillation efficiency of the LMO material. The radiopurity level of the LMO crystal is rather high; only traces of 210Po and 226Ra were detected (0.1 mBq/kg each), while the 228Th activity is expected to be at least an order of magnitude lower, as well as a 40K activity is found to be < 6 mBq/kg.
J. High Energ. Phys. (2020) 2020: 18 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 $\alpha$ 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 $\sim 1$ 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 $\sim 3\times 10^{25}$ 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.
The current experiments searching for neutrinoless double-\(\beta\) (\(0\nu\beta\beta\)) decay also collect large statistics of Standard Model allowed two-neutrino double-\(\beta\) ...(\(2\nu\beta\beta\)) decay events. These can be used to search for Beyond Standard Model (BSM) physics via \(2\nu\beta\beta\) decay spectral distortions. \(^{100}\)Mo has a natural advantage due to its relatively short half-life, allowing higher \(2\nu\beta\beta\) 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 \(\times\) y, acquired in the CUPID-Mo experiment at the Modane underground laboratory (France). We set limits on \(0\nu\beta\beta\) decays with the emission of one or more Majorons, on \(2\nu\beta\beta\) decay with Lorentz violation, and \(2\nu\beta\beta\) decay with a sterile neutrino emission. In this analysis, we investigate the systematic uncertainty induced by modeling the \(2\nu\beta\beta\) 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 \(2\nu\beta\beta\) decay events among the next-generation experiments.
We report on the development of scintillating bolometers based on lithium molybdate crystals containing molybdenum depleted in the double-\(\beta\) active isotope \(^{100}\)Mo ...(Li$_2$$^{100\textrm{depl}}\(MoO\)_4\(). We used two Li\)_2$$^{100\textrm{depl}}\(MoO\)_4\( cubic samples, 45 mm side and 0.28 kg each, produced following purification and crystallization protocols developed for double-\)\beta\( search experiments with \)^{100}\(Mo-enriched Li\)_2\(MoO\)_4\( crystals. Bolometric Ge detectors were utilized to register scintillation photons emitted by the Li\)_2$$^{100\textrm{depl}}\(MoO\)_4\( crystal scintillators. The measurements were performed in the CROSS cryogenic set-up at the Canfranc underground laboratory (Spain). We observed that the Li\)_2$$^{100\textrm{depl}}\(MoO\)_4\( scintillating bolometers are characterized by excellent spectrometric performance (\)\sim\(3--6 keV FWHM at 0.24--2.6 MeV \)\gamma\('s), moderate scintillation signal (\)\sim\(0.3--0.6 keV/MeV depending on light collection conditions) and high radiopurity (\)^{228}\(Th and \)^{226}\(Ra activities are below a few \)\mu\(Bq/kg), comparable to the best reported results of low-temperature detectors based on Li\)_2\(MoO\)_4\( with natural or \)^{100}\(Mo-enriched molybdenum content. Prospects of Li\)_2$$^{100\textrm{depl}}\(MoO\)_4$ bolometers for use in rare-event search experiments are briefly discussed.
We report the measurement of the two-neutrino double-beta ($2\nu\beta\beta$)
decay of $^{100}$Mo to the ground state of $^{100}$Ru using lithium molybdate
(\crystal) scintillating bolometers. The ...detectors were developed for the
CUPID-Mo program and operated at the EDELWEISS-III low background facility in
the Modane underground laboratory. From a total exposure of $42.235$
kg$\times$d, the half-life of $^{100}$Mo is determined to be
$T_{1/2}^{2\nu}=7.12^{+0.18}_{-0.14}\,\mathrm{(stat.)}\pm0.10\,\mathrm{(syst.)}\times10^{18}$
years. This is the most accurate determination of the $2\nu\beta\beta$
half-life of $^{100}$Mo to date. We also confirm, with the statistical
significance of $>3\sigma$, that the single-state dominance model of the
$2\nu\beta\beta$ decay of $^{100}$Mo is favored over the high-state dominance
model.
