Search for Neutrinoless Double-Beta Decay of (130)Te with CUORE-0

A Short Review on the Latest Neutrinos Mass and Number Constraints from Cosmological Observables

We review the neutrino science, focusing on its impact on cosmology along with the latest constraints on its mass and number of species. We also discuss its status as a possible solution to some of the recent cosmological tensions, such as the Hubble constant or the matter fluctuation parameter. We end by showing forecasts from next-generation planned or candidate surveys, highlighting their constraining power, alone or in combination, but also the limitations in determining neutrino mass distribution among its species.

Bolometric Double Beta Decay Experiments: Review and Prospects

This review aims to cover the history and recent developments on cryogenic bolometers for neutrinoless double beta decay (0ν2β) searches. A 0ν2β decay observation would confirm the total lepton charge non-conservation, which is related to a global U(1)LC symmetry. This discovery would also provide essential information on neutrino masses and nature, opening the door to new physics beyond the Standard Model. The bolometric technology shows good prospects for future ton-scale experiments that aim to fully investigate the inverted ordering region of neutrino masses. The big advantage of bolometers is the high energy resolution and the possibility of particle identification, as well as various methods of additional background rejection. The CUORE experiment has proved the feasibility of ton-scale cryogenic experiments, setting the most stringent limit on 130Te 0ν2β decay. Two CUPID demonstrators (CUPID-0 and CUPID-Mo) have set the most stringent limits on 82Se and 100Mo isotopes, respectively, with compatibly low exposures. Several experiments are developing new methods to improve the background in the region of interest with bolometric detectors. CUPID and AMoRE experiments aim to cover the inverted hierarchy region, using scintillating bolometers with hundreds of kg of 100Mo. We review all of these efforts here, with a focus on the different types of radioactive background and the measures put in place to mitigate them.

Status and Perspectives on Rare Decay Searches in Tellurium Isotopes

Neutrinoless double beta decay (0νββ) is a posited lepton number violating decay whose search is an increasingly active field in modern astroparticle physics. A discovery would imply neutrinos are Majorana particles and inform neutrino physics, cosmology and beyond-standard-model theories. Among the few nuclei where double beta decay (ββ) is allowed, tellurium isotopes stand for their high natural abundance and are currently employed in multiple experiments. The search for 0νββ will provide large exposure data sets in the coming years, paving the way for unprecedented sensitivities. We review the latest rare decay searches in tellurium isotopes and compare past results with theories and prospects from running experiments.

Development of Cryogenic Detectors for Neutrinoless Double Beta Decay Searches with CUORE and CUPID

Searching for neutrinoless double beta decay is a top priority in particle and astroparticle physics, being the most sensitive test of lepton number violation and the only suitable process to probe the Majorana nature of neutrinos. In order to increase the experimental sensitivity for this particular search, ton-scale detectors operated at nearly zero-background conditions with a low keV energy resolution at the expected signal peak are required. In this scenario, cryogenic detectors have been proven effective in addressing many of these issues simultaneously. After long technical developments, the Cryogenic Underground Observatory for Rare Events (CUORE) experiment established the possibility to operate large-scale detectors based on this technology. Parallel studies pointed out that scintillating cryogenic detectors represent a suitable upgrade for the CUORE design, directed towards higher sensitivities. In this work, we review the recent development of cryogenic detectors, starting from the state-of-the-art and outlying the path toward next-generation experiments.

Double Beta Decay to Excited States of Daughter Nuclei

In this paper we review results obtained in the searches of double beta decays to excited states of the daughter nuclei and illustrate the related experimental techniques. In particular, we describe in some detail the only two cases in which the transition has been observed; that is the 2β−(0+→01+) decay of 100Mo and 150Nd nuclides. Moreover, the most significant results in terms of lower limits on the half-life are also summarized.

Cosmogenic Activation in Double Beta Decay Experiments

Double beta decay is a very rare nuclear process and, therefore, experiments intended to detect it must be operated deep underground and in ultra-low background conditions. Long-lived radioisotopes produced by the previous exposure of materials to cosmic rays on the Earth’s surface or even underground can become problematic for the required sensitivity. Here, the studies developed to quantify and reduce the activation yields in detectors and materials used in the set-up of these experiments will be reviewed, considering target materials like germanium, tellurium and xenon together with other ones commonly used like copper, lead, stainless steel or argon. Calculations following very different approaches and measurements from irradiation experiments using beams or directly cosmic rays will be considered for relevant radioisotopes. The effect of cosmogenic activation in present and future double beta decay projects based on different types of detectors will be analyzed too.

Improved Limit on Neutrinoless Double-Beta Decay in ^{130} Te with CUORE

We report new results from the search for neutrinoless double-beta decay in ^{130} Te with the CUORE detector. This search benefits from a fourfold increase in exposure, lower trigger thresholds, and analysis improvements relative to our previous results. We observe a background of (1.38±0.07)×10^{-2}  counts/(keV kg yr)) in the 0νββ decay region of interest and, with a total exposure of 372.5 kg yr, we attain a median exclusion sensitivity of 1.7×10^{25}  yr. We find no evidence for 0νββ decay and set a 90% credibility interval Bayesian lower limit of 3.2×10^{25}  yr on the ^{130} Te half-life for this process. In the hypothesis that 0νββ decay is mediated by light Majorana neutrinos, this results in an upper limit on the effective Majorana mass of 75-350 meV, depending on the nuclear matrix elements used.

Realistic shell model and nuclei around 132Sn

This contribution reports on a shell-model study of nuclei in the 132Sn region employing a realistic effective interaction derived from the CD-Bonn nucleon-nucleon potential renormalized through the use of the Vlow−k approach. We shall focus on some selected results for nuclei with a few valence particles and/or holes with respect to 132Sn, namely Sn isotopes with N > 82 and 130Te, which have, in part, been discussed in previous papers. Results are compared with experiments, and predictions that may provide guidance to future experiments are also discussed. It is the aim of this contribution to underline the importance of studying 132Sn neighbours to acquire a deep understanding of nuclear structure, that may be very useful also in other physics fields, and to show that the realistic shell model is a very effective tool to conduct these studies.

Final Result of CUPID-0 Phase-I in the Search for the ^{82}Se Neutrinoless Double-β Decay

CUPID-0 is the first pilot experiment of CUPID, a next-generation project for the measurement of neutrinoless double beta decay (0νDBD) with scintillating bolometers. The detector, consisting of 24 enriched and 2 natural ZnSe crystals, has been taking data at Laboratori Nazionali del Gran Sasso from June 2017 to December 2018, collecting a ^{82}Se exposure of 5.29  kg×yr. In this Letter we present the phase-I results in the search for 0νDBD. We demonstrate that the technology implemented by CUPID-0 allows us to reach the lowest background for calorimetric experiments: (3.5_{-0.9}^{+1.0})×10^{-3}  counts/(keV kg yr). Monitoring 3.88×10^{25} ^{82}Se nuclei×yr we reach a 90% credible interval median sensitivity of T_{1/2}^{0ν}>5.0×10^{24}  yr and set the most stringent limit on the half-life of ^{82}Se 0νDBD: T_{1/2}^{0ν}>3.5×10^{24}  yr (90% credible interval), corresponding to m_{ββ}<(311-638)  meV depending on the nuclear matrix element calculations.

Precision Analysis of the ^{136}Xe Two-Neutrino ββ Spectrum in KamLAND-Zen and Its Impact on the Quenching of Nuclear Matrix Elements

We present a precision analysis of the ^{136}Xe two-neutrino ββ electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino ββ rate allows us to measure the ratio of the leading and subleading 2νββ nuclear matrix elements (NMEs), ξ_{31}^{2ν}=-0.26_{-0.25}^{+0.31}. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the ξ_{31}^{2ν} range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted ξ_{31}^{2ν} values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Because these aspects are also at play in neutrinoless ββ decay, ξ_{31}^{2ν} provides new insights toward reliable neutrinoless ββ NMEs.

Results from the Cuore Experiment †

The Cryogenic Underground Observatory for Rare Events (CUORE) is the first bolometric experiment searching for neutrinoless double beta decay that has been able to reach the 1-ton scale. The detector consists of an array of 988 TeO 2 crystals arranged in a cylindrical compact structure of 19 towers, each of them made of 52 crystals. The construction of the experiment was completed in August 2016 and the data taking started in spring 2017 after a period of commissioning and tests. In this work we present the neutrinoless double beta decay results of CUORE from examining a total TeO 2 exposure of 86.3 kg yr , characterized by an effective energy resolution of 7.7 keV FWHM and a background in the region of interest of 0.014 counts / ( keV kg yr ) . In this physics run, CUORE placed a lower limit on the decay half-life of neutrinoless double beta decay of 130 Te > 1.3 · 10 25 yr (90% C.L.). Moreover, an analysis of the background of the experiment is presented as well as the measurement of the 130 Te 2 ν β β decay with a resulting half-life of T 1 / 2 2 ν = [ 7.9 ± 0.1 ( stat . ) ± 0.2 ( syst . ) ] × 10 20 yr which is the most precise measurement of the half-life and compatible with previous results.

Contributed Review: The saga of neutrinoless double beta decay search with TeO2 thermal detectors

Neutrinoless double beta decay (0νββ) is a direct probe of physics beyond the standard model. Its discovery would demonstrate that the lepton number is not a symmetry of nature and would provide us with unique information on the nature and mass of the neutrinos. Among the experimental techniques employed in the investigation of this rare process, thermal detectors fulfill the requirements for a powerful search, showing an excellent energy resolution and the possibility of scaling to very large masses. In this work, we review the long chain of bolometric experiments based on TeO₂ crystals that were and continue to be carried out at the Laboratori Nazionali del Gran Sasso (Italy), searching for 0νββ of ¹³⁰Te. We illustrate the progress and improvements achieved in almost thirty years of measurements and compare the various performance and results. We describe the several steps that led to the CUORE detector, the latest of this series and presently in data taking, and we highlight the challenges that a next bolometric experiment will face in order to further improve the sensitivity, especially concerning the background abatement. Finally, we emphasize the advantages of ¹³⁰Te in the search for 0νββ with a further future experiment.

Two neutrino double-beta decay and effective axial-vector coupling constant

The theoretical and experimental study of the two-neutrino double-beta decay (2νββ-decay) is of crucial importance for reliable calculation of matrix elements governing the neutrinoless double-beta decay (0νββ-decay). That will allow to determine masses of neutrinos once the 0νββ-decay, which occurs if the neutrino is a massive Majorana particle and the total lepton number is not onserved, will be observed. Experiments studying the 2νββ-decay are currently approaching a qualitatively new level, where high-precision measurements are performed not only for half-lives but for all other observables of the process. In this context an improved formula for the 2νββ-decay is presented. Further, a novel approach for determining the effective axial-vector coupling constant is proposed.

Search of the neutrino-less double beta decay of 82 Se into the excited states of 82 Kr with CUPID-0

The CUPID-0 experiment searches for double beta decay using cryogenic calorimeters with double (heat and light) read-out. The detector, consisting of 24 ZnSe crystals 95 % enriched in 82 Se and two natural ZnSe crystals, started data-taking in 2017 at Laboratori Nazionali del Gran Sasso. We present the search for the neutrino-less double beta decay of 82 Se into the 0 1 + , 2 1 + and 2 2 + excited states of 82 Kr with an exposure of 5.74 kg · yr (2.24 × 10 25  emitters · yr). We found no evidence of the decays and set the most stringent limits on the widths of these processes: Γ ( 82 Se → 82 Kr 0 1 + )8.55 × 10 - 24  yr - 1 , Γ ( 82 Se → 82 Kr 2 1 + ) < 6.25 × 10 - 24  yr - 1 , Γ ( 82 Se → 82 Kr 2 2 + )8.25 × 10 - 24  yr - 1 (90 % credible interval).

Heavy Physics Contributions to Neutrinoless Double Beta Decay from QCD

Observation of neutrinoless double beta decay, a lepton number violating process that has been proposed to clarify the nature of neutrino masses, has spawned an enormous world-wide experimental effort. Relating nuclear decay rates to high-energy, beyond the standard model (BSM) physics requires detailed knowledge of nonperturbative QCD effects. Using lattice QCD, we compute the necessary matrix elements of short-range operators, which arise due to heavy BSM mediators, that contribute to this decay via the leading order π^{-}→π^{+} exchange diagrams. Utilizing our result and taking advantage of effective field theory methods will allow for model-independent calculations of the relevant two-nucleon decay, which may then be used as input for nuclear many-body calculations of the relevant experimental decays. Contributions from short-range operators may prove to be equally important to, or even more important than, those from long-range Majorana neutrino exchange.

Analysis of cryogenic calorimeters with light and heat read-out for double beta decay searches

The suppression of spurious events in the region of interest for neutrinoless double beta decay will play a major role in next generation experiments. The background of detectors based on the technology of cryogenic calorimeters is expected to be dominated by α particles, that could be disentangled from double beta decay signals by exploiting the difference in the emission of the scintillation light. CUPID-0, an array of enriched Zn 82 Se scintillating calorimeters, is the first large mass demonstrator of this technology. The detector started data-taking in 2017 at the Laboratori Nazionali del Gran Sasso with the aim of proving that dual read-out of light and heat allows for an efficient suppression of the α background. In this paper we describe the software tools we developed for the analysis of scintillating calorimeters and we demonstrate that this technology allows to reach an unprecedented background for cryogenic calorimeters.

First Result on the Neutrinoless Double-β Decay of ^{82}Se with CUPID-0

We report the result of the search for neutrinoless double beta decay of ^{82}Se obtained with CUPID-0, the first large array of scintillating Zn^{82}Se cryogenic calorimeters implementing particle identification. We observe no signal in a 1.83 kg yr ^{82}Se exposure, and we set the most stringent lower limit on the 0νββ ^{82}Se half-life T_{1/2}^{0ν}>2.4×10^{24}  yr (90% credible interval), which corresponds to an effective Majorana neutrino mass m_{ββ}<(376-770)  meV depending on the nuclear matrix element calculations. The heat-light readout provides a powerful tool for the rejection of α particles and allows us to suppress the background in the region of interest down to (3.6_{-1.4}^{+1.9})×10^{-3}  counts/(keV kg yr), an unprecedented level for this technique.

CUPID-0: the first array of enriched scintillating bolometers for 0 ν β β decay investigations

The CUPID-0 detector hosted at the Laboratori Nazionali del Gran Sasso, Italy, is the first large array of enriched scintillating cryogenic detectors for the investigation of82 Se neutrinoless double-beta decay ( 0 ν β β ). CUPID-0 aims at measuring a background index in the region of interest (RoI) for 0 ν β β at the level of 10- 3  counts/(keV kg years), the lowest value ever measured using cryogenic detectors. CUPID-0 operates an array of Zn82 Se scintillating bolometers coupled with bolometric light detectors, with a state of the art technology for background suppression and thorough protocols and procedures for the detector preparation and construction. In this paper, the different phases of the detector design and construction will be presented, from the material selection (for the absorber production) to the new and innovative detector structure. The successful construction of the detector lead to promising preliminary detector performance which is discussed here.

Double Gamow-Teller Transitions and its Relation to Neutrinoless ββ Decay

We study the double Gamow-Teller (DGT) strength distribution of ^{48}Ca with state-of-the-art large-scale nuclear shell model calculations. Our analysis shows that the centroid energy of the DGT giant resonance depends mostly on the isovector pairing interaction, while the resonance width is more sensitive to isoscalar pairing. Pairing correlations are also key in neutrinoless ββ (0νββ) decay. We find a simple relation between the centroid energy of the ^{48}Ca DGT giant resonance and the 0νββ decay nuclear matrix element. More generally, we observe a very good linear correlation between the DGT transition to the ground state of the final nucleus and the 0νββ decay matrix element. The correlation, which originates on the dominant short-range character of both transitions, extends to heavier systems including several ββ emitters and also holds in energy-density functional results. Our findings suggest that DGT experiments can be a very valuable tool to obtain information on the value of 0νββ decay nuclear matrix elements.

First Results from CUORE: A Search for Lepton Number Violation via 0νββ Decay of ^{130}Te

The CUORE experiment, a ton-scale cryogenic bolometer array, recently began operation at the Laboratori Nazionali del Gran Sasso in Italy. The array represents a significant advancement in this technology, and in this work we apply it for the first time to a high-sensitivity search for a lepton-number-violating process: ^{130}Te neutrinoless double-beta decay. Examining a total TeO_{2} exposure of 86.3 kg yr, characterized by an effective energy resolution of (7.7±0.5)  keV FWHM and a background in the region of interest of (0.014±0.002)  counts/(keV kg yr), we find no evidence for neutrinoless double-beta decay. Including systematic uncertainties, we place a lower limit on the decay half-life of T_{1/2}^{0ν}(^{130}Te)>1.3×10^{25}  yr (90% C.L.); the median statistical sensitivity of this search is 7.0×10^{24}  yr. Combining this result with those of two earlier experiments, Cuoricino and CUORE-0, we find T_{1/2}^{0ν}(^{130}Te)>1.5×10^{25}  yr (90% C.L.), which is the most stringent limit to date on this decay. Interpreting this result as a limit on the effective Majorana neutrino mass, we find m_{ββ}<(110-520)  meV, where the range reflects the nuclear matrix element estimates employed.

Synthesis, crystal growth and characterization of Zn0.5Mn0.5Te single crystal grown via the Bridgman technique

Single crystal of Zn_0.5Mn_0.5Te has been grown by the vertical Bridgman method.

Development of 100 Mo -containing scintillating bolometers for a high-sensitivity neutrinoless double-beta decay search

This paper reports on the development of a technology involving100 Mo -enriched scintillating bolometers, compatible with the goals of CUPID, a proposed next-generation bolometric experiment to search for neutrinoless double-beta decay. Large mass ( ∼ 1 kg ), high optical quality, radiopure100 Mo -containing zinc and lithium molybdate crystals have been produced and used to develop high performance single detector modules based on 0.2-0.4 kg scintillating bolometers. In particular, the energy resolution of the lithium molybdate detectors near the Q-value of the double-beta transition of100 Mo (3034 keV) is 4-6 keV FWHM. The rejection of the α -induced dominant background above 2.6 MeV is better than 8 σ . Less than 10 μ Bq/kg activity of232 Th ( 228 Th ) and226 Ra in the crystals is ensured by boule recrystallization. The potential of100 Mo -enriched scintillating bolometers to perform high sensitivity double-beta decay searches has been demonstrated with only 10 kg × d exposure: the two neutrino double-beta decay half-life of100 Mo has been measured with the up-to-date highest accuracy as T 1 / 2 = [6.90 ± 0.15(stat.) ± 0.37(syst.)] × 10 18 years . Both crystallization and detector technologies favor lithium molybdate, which has been selected for the ongoing construction of the CUPID-0/Mo demonstrator, containing several kg of100 Mo .

Search for Neutrinoless Quadruple-β Decay of ^{150}Nd with the NEMO-3 Detector

We report the results of a first experimental search for lepton number violation by four units in the neutrinoless quadruple-β decay of ^{150}Nd using a total exposure of 0.19 kg yr recorded with the NEMO-3 detector at the Modane Underground Laboratory. We find no evidence of this decay and set lower limits on the half-life in the range T_{1/2}>(1.1-3.2)×10^{21}  yr at the 90% C.L., depending on the model used for the kinematic distributions of the emitted electrons.

Status and future of nuclear matrix elements for neutrinoless double-beta decay: a review

The nuclear matrix elements that govern the rate of neutrinoless double beta decay must be accurately calculated if experiments are to reach their full potential. Theorists have been working on the problem for a long time but have recently stepped up their efforts as ton-scale experiments have begun to look feasible. Here we review past and recent work on the matrix elements in a wide variety of nuclear models and discuss work that will be done in the near future. Ab initio nuclear-structure theory, which is developing rapidly, holds out hope of more accurate matrix elements with quantifiable error bars.

Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen

We present an improved search for neutrinoless double-beta (0νββ) decay of ^{136}Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the ^{110m}Ag contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0νββ decay half-life of T_{1/2}^{0ν}>1.07×10^{26}  yr at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 61-165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasidegenerate neutrino mass region.

First array of enriched Zn[Formula: see text]Se bolometers to search for double beta decay

The R&D activity performed during the last years proved the potential of ZnSe scintillating bolometers to the search for neutrino-less double beta decay, motivating the realization of the first large-mass experiment based on this technology: CUPID-0. The isotopic enrichment in [Formula: see text]Se, the Zn[Formula: see text]Se crystals growth, as well as the light detectors production have been accomplished, and the experiment is now in construction at Laboratori Nazionali del Gran Sasso (Italy). In this paper we present the results obtained testing the first three Zn[Formula: see text]Se crystals operated as scintillating bolometers, and we prove that their performance in terms of energy resolution, background rejection capability and intrinsic radio-purity complies with the requirements of CUPID-0.

Large-Scale Shell-Model Analysis of the Neutrinoless ββ Decay of ^{48}Ca

We present the nuclear matrix element for the neutrinoless double-beta decay of ^{48}Ca based on large-scale shell-model calculations including two harmonic oscillator shells (sd and pf shells). The excitation spectra of ^{48}Ca and ^{48}Ti, and the two-neutrino double-beta decay of ^{48}Ca are reproduced in good agreement to the experimental data. We find that the neutrinoless double-beta decay nuclear matrix element is enhanced by about 30% compared to pf-shell calculations. This reduces the decay lifetime by almost a factor of 2. The matrix-element increase is mostly due to pairing correlations associated with cross-shell sd-pf excitations. We also investigate possible implications for heavier neutrinoless double-beta decay candidates.