Narrow Results

The data of the Heidelberg–Moscow double beta decay experiment for the measuring period August 1990–May 2000 (54.9813 kg y or 723.44 molyears), published recently, are analyzed using the potential of the Bayesian method for low counting rates. First evidence for neutrinoless double beta decay is observed giving first evidence for lepton number violation. The evidence for this decay mode is 97% (2.2σ) with the Bayesian method, and 99.8% c.l. (3.1σ) with the method recommended by the Particle Data Group. The half-life of the process is found with the Bayesian method to be (95% c.l.) with a best value of 1.5 × 10²⁵y. The deduced value of the effective neutrino mass is, with the nuclear matrix elements from Ref. 1, <m> = (0.11–0.56) eV (95% c.l.), with a best value of 0.39 eV. Uncertainties in the nuclear matrix elements may widen the range given for the effective neutrino mass by at most a factor 2. Our observation which at the same time means evidence that the neutrino is a Majorana particle, will be of fundamental importance for neutrino physics.

For the first time the expected pulse shapes to be observed for neutrinoless double beta events in a big ⁷⁶Ge detector have been calculated starting from their Monte Carlo calculated time history and spatial energy distribution. It is shown that with the spatial resolution of a large size Ge detector for the majority of 0νββ events it is not possible to differentiate between the contributions of different particle physics parameters entering into the 0νββ decay process — in the mass mechanism the effective neutrino mass and the right-handed weak current parameters λ, η. It is shown that on the other hand it is possible in a ⁷⁶Ge double beta decay experiment to reject a background of larger sizes (high multiplicity) gamma events by selecting low size (low multiplicity) events. First comparison of the theoretical ββ pulses to events from the line observed at^3,4Q_ββ shows very good agreement. It is shown further that a rather good radial position determination of ββ events in the detector is possible. By the same type of calculation it is shown that use of the pulse shapes of the 1592 keV double escape line of the 2614 keV γ-transition from ²²⁸Th for calibrating a neuronal net for search of events of neutrinoless double beta decay should be helpful.

In this brief review, a description of the observed evidence for neutrinoless double beta^3–5 in the ⁷⁶Ge experiment in Gran Sasso (Heidelberg–Moscow experiment) which has been operated with 11 kg enriched ⁷⁶Ge detectors in the period 1990–2003, is provided. Two different methods of pulse shape analysis have been used to select potential 0νββ events from the γ background of the measured spectrum — a selection by a neuronal net approach,^3,4,16 and a selection by a new method comparing measured pulses with a library of pulse shapes of point-like events calculated from simulation of the electric field distribution in the detectors (see Refs. 6–8 and 37). The latter method also allows spatial localization of measured events. Both methods lead to selections of events at Q_ββ with almost no γ-background. The observed line at Q_ββ is identified as a 0νββ signal. It has a confidence level of more than 6σ.

Averaged neutrino masses defined by are reanalyzed using up-to-date observed MNS parameters and neutrino masses by the neutrino oscillation experiments together with the cosmological constraint on neutrino masses. The values of 〈m_ν〉_ab are model-independently evaluated in terms of effective neutrino mass defined by which is observable in the single beta decay. We obtain lower bound for 〈m_ν〉_ee in the inverted hierarchy (IH) case, 17 meV ≤〈m_ν〉_ee and one for 〈m_ν〉_τμ in the normal hierarchy (NH) case, 5 meV≤〈m_ν〉_τμ. We also obtain that all the averaged masses 〈m_ν〉_ab have upper bounds which are at most 80 meV.

The Gran Sasso National Laboratory of INFN (LNGS) is the largest underground laboratory for astroparticle physics in the world. Located in Italy between the cities of L'Aquila and Teramo, 120 km far from Rome, is a research infrastructure mainly dedicated to astroparticle and neutrino physics. It offers the most advanced underground facility in terms of dimensions, complexity and completeness of its infrastructures. LNGS is one of the four national laboratories run by the Istituto Nazionale di Fisica Nucleare (INFN).

The scientific program at LNGS is mainly focused on astroparticle, particle and nuclear physics. The laboratory presently hosts many experiments as well as R&D activities, including world-leading research in the fields of solar neutrinos, accelerator neutrinos (CNGS neutrino beam from CERN to Gran Sasso), dark matter (DM), neutrinoless double beta decay (2β0ν) and nuclear cross-section of astrophysical interest. Associate sciences like earth physics, biology and fundamental physics complement the activities. The laboratory is operated as an international science facility and hosts experiments whose scientific merit is assessed by an international advisory Scientific Committee.

A review of the main experiments carried out at LNGS will be given, together with the most recent and relevant scientific results achieved.

We describe an approach to the study of neutrino masses that combines quantum optics techniques with radiation detectors to obtain unprecedented sensitivity. With our approach, the search for Majorana neutrino masses down to ~10 meV will become feasible. The experimental technique uses the detection of individual Ba⁺-ions in the final states of ¹³⁶Xe double-beta decay via optical excitation to suppress radioactive backgrounds.

Double beta decay is indispensable to solve the question of the neutrino mass matrix together with ν oscillation experiments. Recent analysis of the most sensitive experiment since nine years - the HEIDELBERG-MOSCOW experiment in Gran-Sasso - yields a first indication for the neutrinoless decay mode. This result is the first evidence for lepton number violation and proves the neutrino to be a Majorana particle. We give the present status of the analysis in this report. It excludes several of the neutrino mass scenarios allowed from present neutrino oscillation experiments - only degenerate scenarios and those with inverse mass hierarchy survive. This result allows neutrinos to still play an important role as dark matter in the Universe. To improve the accuracy of the present result, considerably enlarged experiments are required, such as GENIUS. A GENIUS Test Facility has been funded and will come into operation by early 2003.

In this review for the first time a theoretical description of the tracks of events of nuclear double beta decay in a large Ge detector is presented. It is obvious that in principle the shapes and sizes of these tracks — and the corresponding time structure of pulses — depend on particle physics and nuclear physics parameters such as neutrino mass, right-handed current parameters, and nuclear matrix elements. The knowledge of this dependence is of importance, since the key to probe the existence of 0νββ decay beyond observation of a signal at the Q value of the process, Q_ββ, is the discrimination of ββ events from background γ events (or other events), in almost any double beta decay experiment (see Refs. 2 and 3). In this review Monte-Carlo simulations of tracks of neutrino-accompanied (2νββ) and neutrinoless double beta decay (0νββ) events, and of various kinds of background processes such as multiple and other γ interactions are reported for a large Ge detector. The time history of the evolution of the individual events is followed and the sizes of the events (volumes in the detector inside which the energy of the event is released) are investigated. Effects of the angular correlations of the two electrons in ββ decay, which again depend on the above nuclear and (for 0νββ decay) particle physics parameters, are taken into account and have been calculated for this purpose for the first time on basis of the experimental half-life of ⁷⁶Ge and of realistic nuclear matrix elements.

It is shown for ββ decay of ⁷⁶Ge, that 0νββ events are to a large extent separable from Compton scattering of γ events of the same energy, while double escape peaks of γ-lines show very similar behavior as 0νββ events, and in that sense can be useful for corresponding "calibration" of pulse shapes of the detector. The possibility to distinguish 0νββ events from γ events is found to be essentially independent of the particle physics parameters of the 0νββ process. A brief outlook is given on the potential of future experiments with respect to determination of the particle physics parameters 〈m_ν〉, 〈λ〉, 〈η〉. It is suggested, that the strategy in future 0νββ research should be, to combine confirmation of the HEIDELBERG-MOSCOW result with determination of the mechanism of the dominating decay, instead of repeating earlier experiments or ideas. The future experiment thus should not use ⁷⁶Ge or ¹³⁶Xe, but instead ¹²⁴Xe.

The main purpose of the Cryogenic Underground Observatory for Rare Events (CUORE) experiment is the search for the Neutrinoless Double Beta Decay (0νDBD) of ¹³⁰Te reaching a sensitivity on Majorana mass better than 50 meV. Cuoricino represents not only the first stage of CUORE, but also the most massive 0νDBD experiment presently running. Present results and future planning of these experiments will be described in the paper.

In recent years, there have been impressive advances in the technology of cameras using charged coupled devices (CCD's) and electron multiplying charged coupled devices (EMCCD's) that make possible a number of applications for the detection of ionizing radiation. The new cameras have quantum efficiencies exceeding 90%, effective noise levels less than one electron per pixel, and can be made to detect light ranging from the ultraviolet to the infrared. When combined with photomultiplier tubes (PMT's), and when used with Time-Projection-Chambers (TPC's) that contain narrow gap mesh charge amplification stages and scintillating gas compositions, these cameras can be used to provide three-dimensional images of particle tracks. There are many applications for such devices, including direction sensitive searches for dark matter, measurements of thermal and fast neutrons, and searches for double-beta-decay. I will describe the operation of optical TPC's and their various applications in this review article.

Development of low radioactive technique allows to investigate many other rare nuclear and subnuclear processes: double $\beta$ decay, rare $\beta$ and $\alpha$ decays, and to search for hypothetical particles and processes like axions, charge nonconserving decays, the nucleon, di-nucleon and tri-nucleon decays into invisible channels, to test the Pauli principle. Here, we present results of the rare processes searches and development of instrumentation for low counting experiments.

Crystal scintillators are very promising detectors to investigate double beta decay of atomic nuclei. Recent achievements in development and application of tungstate and molybdate crystal scintillators, as well as prospects for the next generation double beta decay experiments are discussed.

The current situation in double beta decay experiments, the characteristics of modern detectors and the possibility of increasing the sensitivity to neutrino mass in future experiments are discussed. The issue of the production and use of enriched isotopes in double beta decay experiments is discussed in addition.

TeO₂ bolometers have been used for many years to search for neutrinoless double beta decay in ${}^{130}$Te. CUORE, a tonne-scale TeO₂ detector array, recently published the most sensitive limit on the half-life, $T_{1/2}^{0\nu }>1.5\times 10^{25}$ yr, which corresponds to an upper bound of 140–400 meV on the effective Majorana mass of the neutrino. While it makes CUORE a world-leading experiment looking for neutrinoless double beta decay, it is not the only study that CUORE will contribute to in the field of nuclear and particle physics. As already done over the years with many small-scale experiments, CUORE will investigate both rare decays (such as the two-neutrino double beta decay of ${}^{130}$Te and the hypothesized electron capture in ${}^{123}$Te), and rare processes (e.g. dark matter and axion interactions). This paper describes some of the achievements of past experiments that used TeO₂ bolometers, and perspectives for CUORE.

Bolometers are cryogenic calorimeters which feature excellent energy resolution, low energy threshold, high detection efficiency, flexibility in choice of materials, particle identification capability if operated as hybrid devices. After 30 years of rapid progresses, they represent nowadays a leading technology in several fields: particle and nuclear physics, X-ray astrophysics, cosmology. However, further and substantial developments are required to increase the sensitivity to the levels envisioned by future researches. A review of the challenges to be addressed and potentialities of bolometers in the search for rare nuclear decays is given, with particular emphasis to the neutrinoless double beta decay physics case.

The DAMA project has obtained many competitive or new results in the search for various rare nuclear processes. Most of them have been obtained with the help of many different high purity crystal scintillators which have been measured in the low-background DAMA set-ups located in the Gran Sasso underground laboratory of INFN In this paper, the main results will be summarized.

The neutrinoless $\beta \beta$ decay of atomic nuclei continues to attract fervent interest due to its potential to confirm the possible Majorana nature of the neutrino, and thus the nonconservation of the lepton number. At the same time, the direct dark matter experiments are looking for weakly interacting massive particles (WIMPs) through their scattering on nuclei. The neutrino-oscillation experiments on reactor antineutrinos base their analyses on speculations of $\beta$-spectrum shapes of nuclear decays, thus leading to the notorious “reactor antineutrino anomaly.” In all these experimental efforts, one encounters the problem of $\beta$-spectrum shapes of forbidden $\beta$ decays, either as unwanted backgrounds or unknown components in the analyses of data. In this work, the problem of spectrum shapes is discussed and illustrated with a set of selected examples. The relation of the $\beta$-spectrum shapes to the problem of the effective value of the weak axial-vector coupling strength $g_{\mathrm{\text{A}}}$ and the enhancement of the axial-charge matrix element is also pointed out.

Nuclear double beta decay provides an extraordinarily broad potential to search for beyond-standard-model physics. The occurrence of the neutrinoless decay(0νββ) mode has fundamental consequences: first, the total lepton number is not conserved, and second, the neutrino is a Majorana particle. Furthermore, the measured effective mass provides an absolute scale of the neutrino mass spectrum. In addition, double beta experiments yield sharp restrictions for other beyond-standard-model physics. These include SUSY models (R-parity breaking and conserving), leptoquarks (leptoquark-Higgs coupling), compositeness, left-right symmetric models (right-handed W boson mass), test of special relativity and of the equivalence principle in the neutrino sector and others. First evidence for neutrinoless double beta decay was reported by the HEIDELBERG–MOSCOW experiment in 2001. The HEIDELBERG–MOSCOW experiment is by far the most sensitive0νββ experiment since more than 10 years. It is operating 11 kg of enriched ⁷⁶Ge in the GRAN SASSO Underground Laboratory. The analysis of the data taken from 2 August 1990–20 May 2003 is presented here. The collected statistics is 71.7 kg y. The background achieved in the energy region of the Q value for double beta decay is 0.11 events/kg y keV. The two-neutrino accompanied half-life is determined on the basis of more than 100,000 events to be years. The confidence level for the neutrinoless signal has been improved to a 4.2σ level. The half-life is years. The effective neutrino mass deduced is (0.2–0.6) eV (99.73% C.L.), with the consequence that neutrinos have degenerate masses. The sharp boundaries for other beyond SM physics, mentioned above, are comfortably competitive to the corresponding results from high-energy accelerators like TEVATRON, HERA, etc.

This paper describes the lessons we have to draw after the observation of neutrinoless ββ decay by the enriched ⁷⁶Ge experiment, for present and future experiments so as (a) to fulfill the task to confirm the present result (b) to deliver additional information on the main contributions of effective neutrino mass and right-handed weak currents etc. to the 0νββ amplitude. It is shown that presently running and planned experiments are probably not sensitive enough to check the evidence on a reasonable time scale. It is further demonstrated that, the only way to get more information on the individual contributions of m, η, λ etc. to the 0νββ amplitude is to go to completely different types of experiments, rather than those under construction and preparation at present, e.g. to mixed-mode β⁺/EC decay experiments, such as ¹²⁴Xe decay. It is pointed out that the sometimes observed "tension" between the result of 0νββ decay and cosmological experiments like WMAP, SDSS etc. does not exist and is an artificial product of improper analysis of the latter.

Neutrino-less double beta decay (0νββ) is currently known to be only experiment to verify whether lepton number conservation is violated. The violation is the key to create matter dominated universe by the so-called Leptogenesys. We have been studying double beta decay of ⁴⁸Ca. First stage of the experiment was carried out by using the ELEGANT VI detector system. We gave the best lower limit of the half life of 0νββ of ⁴⁸Ca. We have been developing CANDLES detector system to sense much longer life-time region. We have developed techniques to reduce further backgrounds. One of the techniques has been used in the ELEGANTS VI system and improved further the half life limit. The CADLES detector system will be installed Kamioka underground laboratory next year. I describe these studies.