The Cryogenic Underground Observatory for Rare Events (CUORE) is the most sensitive experiment searching for neutrinoless double-beta decay (0νββ) in 130Te. CUORE uses a cryogenic array of 988 TeO2 calorimeters operated at ∼10 mK with a total mass of 741 kg. To further increase the sensitivity, the detector response must be well understood. Here, we present a non-linear thermal model for the CUORE experiment on a detector-by-detector basis. We have examined both equilibrium and dynamic electro-thermal models of detectors by numerically fitting non-linear differential equations to the detector data of a subset of CUORE channels which are well characterized and representative of all channels. We demonstrate that the hot-electron effect and electric-field dependence of resistance in NTD-Ge thermistors alone are inadequate to describe our detectors' energy-dependent pulse shapes. We introduce an empirical second-order correction factor in the exponential temperature dependence of the thermistor, which produces excellent agreement with energy-dependent pulse shape data up to 6 MeV. We also present a noise analysis using the fitted thermal parameters and show that the intrinsic thermal noise is negligible compared to the observed noise for our detectors.

The possibility that neutrinos may be their own antiparticles, unique among the known fundamental particles, arises from the symmetric theory of fermions proposed by Ettore Majorana in 1937. Given the profound consequences of such Majorana neutrinos, among which is a potential explanation for the matter–antimatter asymmetry of the universe via leptogenesis, the Majorana nature of neutrinos commands intense experimental scrutiny globally; one of the primary experimental probes is neutrinoless double beta (0νββ) decay. Here we show results from the search for 0νββ decay of ¹³⁰Te, using the latest advanced cryogenic calorimeters with the CUORE experiment. CUORE, operating just 10  millikelvin above absolute zero, has pushed the state of the art on three frontiers: the sheer mass held at such ultralow temperatures, operational longevity, and the low levels of ionizing radiation emanating from the cryogenic infrastructure. We find no evidence for 0νββ decay and set a lower bound of the process half-life as 2.2 × 10²⁵  years at a 90 per cent credibility interval. We discuss potential applications of the advances made with CUORE to other fields such as direct dark matter, neutrino and nuclear physics searches and large-scale quantum computing, which can benefit from sustained operation of large payloads in a low-radioactivity, ultralow-temperature cryogenic environment.

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The CUORE experiment is a large bolometric array searching for the lepton number violating neutrino-less double beta decay (0νββ) in the isotope ¹³⁰Te. In this work we present the latest results on two searches for the double beta decay (DBD) of ¹³⁰Te to the first 0⁺₂ excited state of ¹³⁰Xe: the 0νββ decay and the Standard Model-allowed two-neutrinos double beta decay (2νββ). Both searches are based on a 372.5 kg×yr TeO₂ exposure. The de-excitation gamma rays emitted by the excited Xe nucleus in the final state yield a unique signature, which can be searched for with low background by studying coincident events in two or more bolometers. The closely packed arrangement of the CUORE crystals constitutes a significant advantage in this regard. The median limit setting sensitivities at 90% Credible Interval (C.I.) of the given searches were estimated as S^0ν_1/2 = 5.6×10²⁴ yr for the 0νββ decay and S^2ν_1/2 = 2.1×10²⁴ yr for the 2νββ decay. No significant evidence for either of the decay modes was observed and a Bayesian lower bound at 90% C.I. on the decay half lives is obtained as: (T_1/2)^0ν_0⁺2 > 5.9×10²⁴ yr for the 0νββ mode and (T_1/2)^2ν_0⁺2 > 1.9×10²⁴ yr for the 2νββ mode. These represent the most stringent limits on the DBD of ¹³⁰Te to excited states and improve by a factor ~5 the previous results on this process.

The cryogenic underground observatory for rare events (CUORE) is a cryogenic experiment searching for neutrinoless double beta decay (0νββ) of ¹³⁰Te. The detector consists of an array of 988 TeO₂ crystals arranged in a compact cylindrical structure of 19 towers. We report the CUORE initial operations and optimization campaigns. We then present the CUORE results on 0νββ and 2νββ decay of ¹³⁰Te obtained from the analysis of the physics data acquired in 2017.

We report on a search for double beta decay of ¹³⁰Te to the first 0⁺ excited state of ¹³⁰Xe using a 9.8 kg yr exposure of ¹³⁰Te collected with the CUORE-0 experiment. In this work we exploit different topologies of coincident events to search for both the neutrinoless and two-neutrino double beta decay modes. We find no evidence for either mode and place lower bounds on the half-lives: T^0ν_0⁺1 > 7.9×10²³ yr and T^2ν_0⁺1 > 2.4×10²³ (90% CL). Combining our results with those obtained by the CUORICINO experiment, we achieve the most stringent constraints available for these processes: T^0ν_0⁺1 > 1.4×10²⁴ yr and T^2ν_0⁺1 > 2.5×10²³ yr (90% CL).

We have performed a search for neutrinoless β⁺EC decay of ¹²⁰Te using the final CUORE-0 data release. We describe a new analysis method for the simultaneous fit of signatures with different event topology, and of data subsets with different signal efficiency, obtaining a limit on the half-life of the decay of T_1/2 > 1.6 × 10²¹ yr at 90% CI. Combining this with results from Cuoricino, a predecessor experiment, we obtain the strongest limit to date, corresponding to T_1/2 > 2.7 × 10²¹ yr at 90% CI.

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: ¹³⁰Te neutrinoless double-beta decay. Examining a total TeO₂ 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. The median statistical sensitivity of this search is 7.0 × 10²⁴ yr. Including systematic uncertainties, we place a lower limit on the decay half-life of T_1∕2^0ν > 1.3 × 10²⁵ yr (90% C.L.). Combining this result with those of two earlier experiments, Cuoricino and CUORE-0, we find T_1∕2^0ν > 1.5 × 10²⁵ 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 < 140–400 meV, where the range reflects the nuclear matrix element estimates employed.

The Cryogenic Underground Observatory for Rare Events (CUORE) is designed to search for neutrinoless double beta decay of 130Te with an array of 988 TeO₂ bolometers operating at temperatures around 10 mK. The experiment is currently being commissioned in Hall A of Laboratori Nazionali del Gran Sasso, Italy. The goal of CUORE is to reach a 90% C.L. exclusion sensitivity on the ¹³⁰Te decay half-life of 9 × 10²⁵ years after 5 years of data taking. The main issue to be addressed to accomplish this aim is the rate of background events in the region of interest, which must not be higher than 10⁻² counts/keV/kg/y. We developed a detailed Monte Carlo simulation, based on results from a campaign of material screening, radioassays, and bolometric measurements, to evaluate the expected background. This was used over the years to guide the construction strategies of the experiment and we use it here to project a background model for CUORE. In this paper we report the results of our study and our expectations for the background rate in the energy region where the peak signature of neutrinoless double beta decay of ¹³⁰Te is expected.

We report on the measurement of the two-neutrino double beta decay half-life of ¹³⁰Te with the CUORE-0 detector. From an exposure of 33.4 kg⋅y of TeO₂, the half-life is determined to be T^2ν_1/2 = [8.2 ± 0.2 (stat.) ± 0.6 (syst.)] × 10²⁰ y. This result is obtained after a detailed reconstruction of the sources responsible for the CUORE-0 counting rate, with a specific study of those contributing to the ¹³⁰Te neutrinoless double beta decay region of interest.

We describe in detail the methods used to obtain the lower bound on the lifetime of neutrinoless double-beta (0νββ) decay in ¹³⁰Te and the associated limit on the effective Majorana mass of the neutrino using the CUORE-0 detector. CUORE-0 is a bolometric detector array located at the Laboratori Nazionali del Gran Sasso that was designed to validate the background reduction techniques developed for CUORE, a next-generation experiment scheduled to come online in 2016. CUORE-0 is also a competitive 0νββ decay search in its own right and functions as a platform to further develop the analysis tools and procedures to be used in CUORE. These include data collection, event selection and processing, as well as an evaluation of signal efficiency. In particular, we describe the amplitude evaluation, thermal gain stabilization, energy calibration methods, and the analysis event selection used to create our final 0νββ decay search spectrum. We define our high level analysis procedures, with emphasis on the new insights gained and challenges encountered. We outline in detail our fitting methods near the hypothesized 0νββ decay peak and catalog the main sources of systematic uncertainty. Finally, we derive the 0νββ decay half-life limits previously reported for CUORE-0, T_1∕2^0ν > 2.7 × 10²⁴ yr, and in combination with the Cuoricino limit, T_1∕2^0ν > 4.0 × 10²⁴ yr.

Neutrinoless double beta decay (0νββ) is one of the most sensitive probes for physics beyond the Standard Model, providing unique information on the nature of neutrinos. In this paper we review the status and outlook for bolometric 0νββ decay searches. We summarize recent advances in background suppression demonstrated using bolometers with simultaneous readout of heat and light signals. We simulate several configurations of a future CUORE-like bolometer array which would utilize these improvements and present the sensitivity reach of a hypothetical next-generation bolometric 0νββ experiment. We demonstrate that a bolometric experiment with the isotope mass of about 1 ton is capable of reaching the sensitivity to the effective Majorana neutrino mass (|m_ee|) of order 10–20 meV, thus completely exploring the so-called inverted neutrino mass hierarchy region. We highlight the main challenges and identify priorities for an R&D program addressing them.

Neutrinoless double-beta (0νββ) decay is a hypothesized lepton-number-violating process that offers the only known means of asserting the possible Majorana nature of neutrino mass. The Cryogenic Underground Observatory for Rare Events (CUORE) is an upcoming experiment designed to search for 0νββ decay of ¹³⁰Te using an array of 988 TeO₂ crystal bolometers operated at 10  mK. The detector will contain 206  kg of ¹³⁰Te and have an average energy resolution of 5  keV; the projected 0νββ decay half-life sensitivity after five years of livetime is 1.6  × 10²⁶  y at 1σ (9.5  × 10²⁵ y at the 90% confidence level), which corresponds to an upper limit on the effective Majorana mass in the range 40–100  meV (50–130  meV). In this paper, we review the experimental techniques used in CUORE as well as its current status and anticipated physics reach.

In this article we describe the background challenges for the CUORE experiment posed by surface contamination of inert detector materials such as copper, and present three techniques explored to mitigate these backgrounds. Using data from a dedicated test apparatus constructed to validate and compare these techniques we demonstrate that copper surface contamination levels better than 10⁻⁷-10⁻⁸ Bq/cm² are achieved for ²³⁸U and ²³²Th. If these levels are reproduced in the final CUORE apparatus the projected 90% C.L. upper limit on the number of background counts in the region of interest is 0.02–0.03 counts/keV/kg/y depending on the adopted mitigation technique.

The CUORICINO experiment was an array of 62 TeO₂ single-crystal bolometers with a total ¹³⁰Te mass of 11.3 kg. The experiment finished in 2008 after more than 3 yr of active operating time. Searches for both 0ν and 2ν double-β decay to the first excited 0⁺ state in ¹³⁰Xe were performed by studying different coincidence scenarios. The analysis was based on data representing a total exposure of N(¹³⁰Te)⋅t = 9.5×10²⁵ yr. No evidence for a signal was found. The resulting lower limits on the half-lives are T^2ν_1/2(¹³⁰Te→¹³⁰Xe*) > 1.3×10²³ yr (90% C.L.), and T^0ν_1/2(¹³⁰Te→¹³⁰Xe*) > 9.4×10²³ yr (90% CL).

We report the final result of the CUORICINO experiment. Operated between 2003 and 2008, with a total exposure of 19.75 kg y of ¹³⁰Te, CUORICINO was able to set a lower bound on the ¹³⁰Te 0νββ half-life of 2.8 × 10²⁴ years at 90% C.L. The limit here reported includes the effects of systematic uncertainties that are examined in detail in the paper. The corresponding upper bound on the neutrino Majorana mass is in the range 300–710 meV, depending on the adopted nuclear matrix element evaluation.

High purity TeO₂ crystals are produced to be used for the search for the neutrinoless double beta decay of ¹³⁰Te. Dedicated production lines for raw material synthesis, crystal growth, and surface processing were built compliant with radio-purity constraints specific to rare event physics experiments. High sensitivity measurements of radio-isotope concentrations in raw materials, reactants, consumables, ancillaries, and intermediary products used for TeO₂ crystals production are reported. Indications are given on the crystals perfection and how it is achieved and maintained in a large scale production process. Production and certification protocols are presented and resulting ready-to-use TeO₂ crystals are described.

A detailed description of the CUORICINO ¹³⁰Te neutrinoless double-beta (0νββ) decay experiment is given and recent results are reported. CUORICINO is an array of 62 tellurium oxide (TeO₂) bolometers with an active mass of 40.7 kg. It is cooled to ~8−10 mK by a dilution refrigerator shielded from environmental radioactivity and energetic neutrons. It is running in the Laboratori Nazionali del Gran Sasso (LNGS) in Assergi, Italy. These data represent an exposure of 11.83 kg yr or 91 mole-years of ¹³⁰Te. No evidence for 0νββ-decay was observed and a limit of T0ν1/2(¹³⁰Te) ≥ 3.0 × 10²⁴ y (90% CL) is set. This corresponds to an upper limit on the effective mass, <m_ν>, between 0.19 and 0.68 eV when analyzed with the many published nuclear structure calculations. In the context of these nuclear models, the values fall within the range corresponding to the claim of evidence of 0νββ-decay by H. V. Klapdor-Kleingrothaus et al. The experiment continues to acquire data.

Recently, neutrino oscillation experiments have unequivocally demonstrated that neutrinos have mass and mix. These experiments have yielded valuable information on the mixing angles and on the mass differences of the three eigenstates but cannot determine the scale of the neutrino mass, which is fixed by the lightest neutrino mass eigenvalue. This can only be directly determined by kinematic measurements or by the observation and measurement of the neutrinoless double-beta decay (NDBD) half-life. The CUORE experiment is designed with a sensitivity capable of probing the range indicated by oscillation experiments. It consists of an array of 988, 750 g, TeO₂ bolometers operating at 8 to 10 mK and arranged in a cylindrical geometry of 19 towers hanging on the bottom of the mixing chamber of a dilution refrigerator. Each tower consists of 13 4-detector modules for a total of 52 bolometers. One such tower has been successfully constructed and is now operated in the Gran Sasso Laboratory as a test experiment, and also as an independent NDBD experiment called CUORICINO. In fact, the 3 year half-life sensitivity of CUORICINO for NDBD decay of ¹³⁰Te is 6.1 × 10²⁴ years. Thus far the CUORICINO data demonstrate the technical feasibility of the expanded CUORE array while showing what and where the sources of background are and how to reduce them. A background reduction to 0.01 counts/keV/kg/y in the NDBD energy region is therefore possible. This would allow the full CUORE array to achieve a 5-year sensitivity of the order of 30 meV on the effective neutrino mass. Another order of magnitude reduction (corresponding to a sensitivity of about 15 meV) will present a real challenge, but is also possible. The CUORE experiment will be installed in the Laboratori Nazionali del Gran Sasso (LNGS) in Assergi, Italy.

The first results are reported on the limit for neutrinoless double decay of 130Te obtained with the new bolometric experiment CUORICINO. The set-up consists of 44 cubic crystals of natural TeO₂, 5 cm on the side and 18 crystals of 3 × 3 × 6 cm³. Four of these latter crystals are made with isotopically enriched materials: two in ¹²⁸Te and two others in ¹³⁰Te. With a sensitive mass of ~40 kg, our array is by far the most massive running cryogenic detector to search for rare events. The array is operated at a temperature of ~10 mK in a dilution refrigerator under a heavy shield in the Gran Sasso Underground Laboratory at a depth of about 3500 m.w.e. The counting rate in the region of neutrinoless double beta decay is ~0.2 counts keV⁻¹ kg⁻¹ y⁻¹ , among the lowest in this type of experiment. No evidence for neutrinoless double beta decay is found with the present statistics obtained in about three months with a live time of 72%. The corresponding lower limit for the lifetime of this process is of 5.5 × 10²³ years at 90% C.L. The corresponding limit for the effective neutrino mass ranges between 0.37 to 1.9 eV depending on the theoretically calculated nuclear matrix elements used. This constraint is the most restrictive one except those obtained with Ge diodes, and is comparable to them.