Final Results of GERDA on the Two-Neutrino Double-β Decay Half-Life of ^{76}Ge

We present the measurement of the two-neutrino double-β decay rate of ^{76}Ge performed with the GERDA Phase II experiment. With a subset of the entire GERDA exposure, 11.8 kg yr, the half-life of the process has been determined: T_{1/2}^{2ν}=(2.022±0.018_{stat}±0.038_{syst})×10^{21}  yr. This is the most precise determination of the ^{76}Ge two-neutrino double-β decay half-life and one of the most precise measurements of a double-β decay process. The relevant nuclear matrix element can be extracted: M_{eff}^{2ν}=(0.101±0.001).

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

CUPID-0, an array of Zn^{82}Se cryogenic calorimeters, was the first medium-scale demonstrator of the scintillating bolometers' technology. The first project phase (March 2017-December 2018) allowed the most stringent limit on the neutrinoless double beta decay half-life of the isotope of interest, ^{82}Se, to be set. After a six month long detector upgrade, CUPID-0 began its second and last phase (June 2019-February 2020). In this Letter, we describe the search for neutrinoless double beta decay of ^{82}Se with a total exposure (phase I+II) of 8.82  kg yr^{-1} of isotope. We set a limit on the half-life of ^{82}Se to the ground state of ^{82}Kr of T_{1/2}^{0ν}(^{82}Se)>4.6×10^{24}  yr (90% credible interval), corresponding to an effective Majorana neutrino mass m_{ββ}<(263-545)  meV. We also set the most stringent lower limits on the neutrinoless decays of ^{82}Se to the 0_{1}^{+}, 2_{1}^{+}, and 2_{2}^{+} excited states of ^{82}Kr, finding 1.8×10^{23}  yr, 3.0×10^{23}  yr, and 3.2×10^{23}  yr (90% credible interval) respectively.

Investigation on Rare Nuclear Processes in Hf Nuclides

In this work, a review of recent studies concerning rare nuclear processes in Hf isotopes is presented. In particular, the investigations using HP-Ge spectrometry and Hf-based crystal scintillators are focused; the potentiality and the results of the “source = detector” approach are underlined. In addition, a short introduction concerning the impact of such kind of research in the context of astroparticle and nuclear physics is pointed out. In particular, the study of α decay and double beta decay of 174Hf, 176Hf, 177Hf, 178Hf, 179Hf, 180Hf isotopes either to the ground state or to the lower bounded levels have been discussed. The observation of α decay of 174Hf isotope to the ground state with a T1/2=7.0(1.2)×1016 y is reported and discussed. No decay was detected for α decay of 174Hf isotope at the first excited level of daughter and of 176Hf, 177Hf, 178Hf, 179Hf, 180Hf isotopes either to the ground state or to the lower bounded levels. The T1/2 lower limits for these decays are at the level of 1016–1020 y. Nevertheless, the T1/2 lower limits for the transitions of 176Hf→172Yb (0+→0+) and 177Hf→173Yb (7/2−→5/2−) are near to the theoretical predictions, giving hope to their observation in the near future. All the other experimental limits (∼1016–1020 y) are absolutely far from the theoretical expectations. The experiments investigating the 2ϵ and ϵβ+ processes in 174Hf are also reported; the obtained half-life limits are set at the level of 1016–1018 y. Moreover, we estimate the T1/2 of 2ν2ϵ of 174Hf decay at the level of (0.3–6) × 1021 y (at now the related measured lower limit is 7.1×1016 y).

Astrophysical Neutrinos in Testing Lorentz Symmetry

An overview of searches related to neutrinos of astronomical and astrophysical origin performed within the framework of the Standard-Model Extension is provided. For this effective field theory, key definitions, intriguing physical consequences, and the mathematical formalism are summarized within the neutrino sector to search for effects from a background that could lead to small deviations from Lorentz symmetry. After an introduction to the fundamental theory, examples of various experiments within the astronomical and astrophysical context are provided. Order-of-magnitude bounds of SME coefficients are shown illustratively for the tight constraints that this sector allows us to place on such violations.

Scintillation in Low-Temperature Particle Detectors

Inorganic crystal scintillators play a crucial role in particle detection for various applications in fundamental physics and applied science. The use of such materials as scintillating bolometers, which operate at temperatures as low as 10 mK and detect both heat (phonon) and scintillation signals, significantly extends detectors performance compared to the conventional scintillation counters. In particular, such low-temperature devices offer a high energy resolution in a wide energy interval thanks to a phonon signal detection, while a simultaneous registration of scintillation emitted provides an efficient particle identification tool. This feature is of great importance for a background identification and rejection. Combined with a large variety of elements of interest, which can be embedded in crystal scintillators, scintillating bolometers represent powerful particle detectors for rare-event searches (e.g., rare alpha and beta decays, double-beta decay, dark matter particles, neutrino detection). Here, we review the features and results of low-temperature scintillation detection achieved over a 30-year history of developments of scintillating bolometers and their use in rare-event search experiments.

Measurement of the 2νββ Decay Half-Life of ^{130}Te with CUORE

We measured two-neutrino double beta decay of ^{130}Te using an exposure of 300.7 kg yr accumulated with the CUORE detector. Using a Bayesian analysis to fit simulated spectra to experimental data, it was possible to disentangle all the major background sources and precisely measure the two-neutrino contribution. The half-life is in agreement with past measurements with a strongly reduced uncertainty: T_{1/2}^{2ν}=7.71_{-0.06}^{+0.08}(stat)_{-0.15}^{+0.12}(syst)×10^{20}  yr. This measurement is the most precise determination of the ^{130}Te 2νββ decay half-life to date.

Techniques for Background Identification in the Search for Rare Processes with Crystal Scintillators

In astroparticle, nuclear and subnuclear physics, low-counting experiments play an increasingly important role in the investigation of rare processes such as dark matter, double beta decay, some neutrino processes and low-background spectrometry. Extremely low-background features are more and more required to produce detectors and apparata of suitable sensitivity. Over time, a great deal of interest and attention in developing experimental techniques suitable to improve, verify and maintain the radiopurity of these detectors has arisen. In this paper, the characterization of inorganic crystal scintillators (such as, e.g., NaI(Tl), ZnWO4 and CdWO4) using α, β and γ radioactive sources and the main experimental techniques applied in the field to quantitatively identify the radioactive contaminants are highlighted; in particular, we focus on inorganic crystal scintillators, widely used in rare processes investigation, considering their applications at noncryogenic temperatures in the framework of the DAMA experiment activities at the Gran Sasso National Laboratory of the INFN (National Institute for Nuclear Physics, INFN).

Enriched Crystal Scintillators for 2β Experiments

The investigation of 2β decay is an important issue in modern physics, allowing the test of the Standard Model of elementary particles and the study of the nature and properties of neutrinos. The crystal scintillators, especially made of isotopically-enriched materials, are powerful detectors for 2β decay experiments thanks to the high radiopurity level and the possibility to realize the calorimetric “source = detector” approach with a high detection efficiency. For the moment, the 2ν2β processes have been observed at the level of 1019–1024 years with enriched crystals; the sensitivity to the 0ν mode have reached the level of 1024–1026 years in some decay channels for different nuclides allowing one to calculate the upper limits on the effective mass of the Majorana neutrino at the level of 0.1–0.6 eV. The paper is intended to be a review on the latest results to investigate 2β processes with crystal scintillators enriched in 48Ca, 106Cd, and 116Cd.

Initial State Interaction for the 20Ne + 130Te and 18O + 116Sn Systems at 15.3 AMeV from Elastic and Inelastic Scattering Measurements

Double charge exchange (DCE) reactions could provide experimentally driven information about nuclear matrix elements of interest in the context of neutrinoless double-β decay. To achieve this goal, a detailed description of the reaction mechanism is mandatory. This requires the full characterization of the initial and final-state interactions, which are poorly known for many of the projectile-target systems involved in future DCE studies. Among these, we intend to study the 20Ne + 130Te and 18O + 116Sn systems at 15.3 AMeV, which are particularly relevant due to their connection with the 130Te→130Xe and 116Cd→116Sn double-β decays. We measure the elastic and inelastic scattering cross-section angular distributions and compare them with theoretical calculations performed in the optical model, one-step distorted wave Born approximation, and coupled-channel approaches using the São Paulo double-folding optical potential. A good description of the experimental data in the whole explored range of transferred momenta is obtained provided that couplings with the 21+ states of the projectile and target are explicitly included within the coupled-channel approach. These results are relevant also in the analysis of other quasi-elastic reaction channels in these systems, in which the same couplings should be included.

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.

Search for Double Beta Decay of 106Cd with an Enriched 106CdWO4 Crystal Scintillator in Coincidence with CdWO4 Scintillation Counters

Studies on double beta decay processes in 106Cd were performed by using a cadmium tungstate scintillator enriched in 106Cd at 66% (106CdWO4) with two CdWO4 scintillation counters (with natural Cd composition). No effect was observed in the data that accumulated over 26,033 h. New improved half-life limits were set on the different channels and modes of the 106Cd double beta decay at level of limT1/2∼1020−1022 yr. The limit for the two neutrino electron capture with positron emission in 106Cd to the ground state of 106Pd, T1/22νECβ+≥2.1×1021 yr, was set by the analysis of the 106CdWO4 data in coincidence with the energy release 511 keV in both CdWO4 counters. The sensitivity approaches the theoretical predictions for the decay half-life that are in the range T1/2∼1021−1022 yr. The resonant neutrinoless double-electron capture to the 2718 keV excited state of 106Pd is restricted at the level of T1/20ν2K≥2.9×1021 yr.

Precise Half-Life Values for Two-Neutrino Double-β Decay: 2020 Review

All existing positive results on two-neutrino double beta decay and two-neutrino double electron capture in different nuclei have been analyzed. Weighted average and recommended half-life values for 48Ca, 76Ge, 82Se, 96Zr, 100Mo, 100Mo - 100Ru (01+), 116Cd, 128Te, 130Te, 136Xe, 150Nd, 150Nd - 150Sm (01+), 238U, 78Kr, 124Xe and 130Ba have been obtained. Given the measured half-life values, effective nuclear matrix elements for all these transitions were calculated.

Recent Developments and Results on Double Beta Decays with Crystal Scintillators and HPGe Spectrometry

Recent developments, results, and perspectives arising from double beta decay experiments at the Gran Sasso National Laboratory (LNGS) of the INFN by using HPGe detectors and crystal scintillators and by exploiting various approaches and different isotopes are summarized. The measurements here presented have been performed in the experimental set-ups of the DAMA collaboration. These setups are optimized for low-background studies and operate deep underground at LNGS. The presented results are of significant value to the field, and the sensitivity achieved for some of the considered isotopes is one of the best available to date.