The NEMO-3 experiment and the SuperNEMO project

The NEMO experiment investigates the neutrinoless double beta decay. The NEMO-3 detector is collecting data in the Frejus Underground Laboratory.The goal of the SuperNEMO detector is to reach a sensitivity of the order of 10²⁶ year on the half-life of the ββ0ν process. The chosen isotopes for the future detector are ⁸²Se and ¹⁵⁰Nd, because of the reduced background. The collaboration has started a 3-year R& D development on all components: tracking detector, calorimeter, source enrichment and purification and radiopurity measurements.     

Pinning down the mechanism of neutrinoless double beta decay with measurements in different nuclei

A measurement of neutrinoless double beta decay in one isotope does not allow us to determine the underlying physics mechanism. We discuss the discrimination of mechanisms for neutrinoless double beta decay by comparing ratios of half-life measurements for different isotopes. Six prominent examples for specific new physics contributions to neutrinoless double beta decay are analyzed. We find that the change in corresponding ratios of half lives varies from 60% for supersymmetric models up to a factor of 5-20 for extra-dimensional and left-right-symmetric mechanisms.     

Neutrinoless Double Beta Decay-A New Formalism

A new method-the operator expansion method is used in neutrinoless double beta decay processes. Both the neutrino mass and mixing of right-handed leptonic current are included. It is shown that for nuclear neutrinoless double beta decay in 2n mechanism, there appear some new terms besides the terms given by the conventional treatment based on closure approximation. The ββ with a Majoron emission is also discussed.     

Neutrinoless Double Beta Decay: Status of Evidence

The present experimental status in the search for neutrinoless double beta decay is reviewed, with emphasis on the first indication for neutrinoless double beta decay found in the HEIDELBERG-MOSCOW experiment, giving first evidence for lepton number violation and a Majorana nature of the neutrinos. Future perspectives of the field are briefly outlined.     

A Limit of Neutrinoless Double Beta Decay of 48Ca

A search for the neutrinoless double beta decay of ⁴⁸Ca is carried out in a coal mine which is located 512m underground near Beijing.Large scintillation crystals of natural CaF₂ were used as both the detector and the double beta decay source.Result obtained after a total of 7588.5 hours of data taking gives 1.1×10²² years (68%C.L.) as the lower limit of the half-life of the neutrinoless double beta decay of ⁶⁸Ca.     

Hiding neutrinoless double beta decay in the minimal seesaw model with the TM1 or μ–τ reflection symmetry

In Asaka et al (2021 Phys. Rev. D 103, 015014), Asaka, Ishida and Tanaka put forward an interesting possibility that the neutrinoless double beta decay can be hidden in the minimal seesaw model with the two right-handed neutrinos having a hierarchical mass structure: the lighter one is lighter enough than the typical Fermi-momentum scale of nuclei while the heavier one is sufficiently heavy to decouple from the neutrinoless double beta decay. Then, in the basis where the mass matrices of the charged leptons and right-handed neutrinos are diagonal, for some particular texture of the Dirac neutrino mass matrix ${M}_{{\rm{D}}}^{}$, the neutrinoless double beta decay can be hidden. In this paper, on top of this specified model, we study the interesting scenario that ${M}_{{\rm{D}}}^{}$ further obeys the TM1 symmetry or μ–τ reflection symmetry which are well motivated by the experimental results for the neutrino mixing parameters.     

The Two-Nucleon Mechanism for the Neutrinoless Double Beta Decay and the BCS Nuclear Wave Function

A method of using the Bardeen-Cooper-Schrieffer (BCS) nuclear wave function to treat the two-nucleon mechanism for neutiinoless double beta decay process 0⁺ → 0⁺ is proposed.The neutrinoless decay mode and the neutrinoless decay accompanied by a Majoron emission mode of ⁸²Se are studikd. Our cdculated results show that to reproduce the experimental value of γ^(ov) > 1.8 × 10²² yr for neutrinoless double beta decai of ⁸²Se the Majorana neutrino mass m_v < 6.2 eV and the mixing parameter of right-handed current η < 7.0 × 10^-6 In the emission with a Majoron mode the effective Majoron coupling to neutrino is deduced from the experimental value of γ^(ov,H) > 4.4 × 10²⁰ yr for ⁸²Se with the result H0> < 6.2 × 10^-4.     

A highly efficient HPGE gamma-ray spectrometer for investigating ββ decay to excited states

A new low-background spectrometer based on a HPGe detector with a sensitive volume of 600 cm³ is developed to investigate rare nuclear processes such as resonance neutrinoless double electron capture and two-neutrino double beta decay to excited states of daughter nuclei. The spectrometer is installed at the Modane underground laboratory (France) at a depth of 4800 m w.e. The spectrometer background is measured and the spectrometer sensitivity is determined for measuring sources of double beta decay.     

Cerenkov emission as a positive tag of double beta decays in bolometric experiments

Active background reduction in high resolution calorimeters is a promising approach to achieve ultimate sensitivity in neutrinoless double beta decay experiments. We propose Cerenkov emission from beta rays in bolometric crystals as a viable alternative to scintillation. This novel approach could broaden the range of materials of interest for calorimetric searches of the double beta decay. We discuss the optical properties of TeO₂ crystals, as a show case.     

Neutrinoless double beta decay in the LHC era

Once neutrinoless double beta decay is discovered, the question which mechanism triggers the decay becomes crucial for drawing any conclusion about the concrete physics underlying the process, like the neutrino Majorana mass. For example, in the minimal supersymmetric extension with R-parity violation both neutrino Majorana masses and superpartners can trigger the decay. We show that in this case, if the decay is triggered by superpartners, there exist good prospects to observe single slepton production at the LHC. Resonant single slepton production at the LHC can therefore discriminate between the neutrinoless double beta decay mechanism and others.     

New limits on double beta decay of 76Ge

New results are presented from an experiment on double beta decay of ⁷⁶Ge carried out with two Ge Li detectors in the Mont Blanc tunnel. No evidence has been found for neutrinoless double beta decay, and the obtained lower limit of the half lifetime (1.2 × 10²³ yr) is discussed in terms of the mass of Majorana neutrinos and of the possible presence of right-handed currents.     

A large Hilbert space QRPA and RQRPA calculation of neutrinoless double beta decay

A large Hilbert space is used for the calculation of the nuclear matrix elements governing the light neutrino mass mediated mode of neutrinoless double beta decay (Ovββ-decay) of⁷⁶ Ge,¹⁰⁰ Mo,¹¹⁶ Cd,¹²⁸ Te, and¹³⁶ Xe within the proton-neutron quasiparticle random phase approximation (pn-QRPA) and the renormalized QRPA with proton-neutron pairing (full-RQRPA) methods. We have found that the nuclear matrix elements obtained with the standard pn-QRPA for several nuclear transitions are extremely sensitive to the renormalization of the particle-particle component of the residual interaction of the nuclear hamiltonian. Therefore the standard pn-QRPA does not guarantee the necessary accuracy to allow us to extract a reliable limit on the effective neutrino mass. This behavior already known from the calculation of the two-neutrino double beta decay matrix elements, manifests itself in the neutrinoless double-beta decay but only if a large model space is used. The full-RQRPA, which takes into account proton-neutron pairing and considers the Pauli principle in an approximate way, offers a stable solution in the physically acceptable region of the particle-particle strength. In this way more accurate values on the effective neutrino mass have been deduced from the experimental lower limits of the half-lifes of neutrinoless double beta decay.     

Phenomenology of neutrino oscillations

The phenomenology of solar, atmospheric, supernova and laboratory neutrino oscillations is described. Analytical formulae for matter effects are reviewed. The results from oscillations are confronted with neutrinoless double beta decay.     

Non-collapsing QRPA for neutrinoless double beta decay

The standard quasiparticle random phase approximation(QRPA) is widely used to describe the neutrinoless double beta decay process. Although it has been quite successful in many cases of interest, it has some shortcomings. The most important one is that its solutions collapse for physical values of the particle-particle strength. We shall show that modifications can be done which can extend the validity of this standard QRPA beyond the point of collapse. Such modifications are: The introduction of proton-neutron pairing, the inclusion of the Pauli principle and the extension of the Hilbert space. If all these modifications are introduced into the standard QRPA then the collapse does not occur for physical values of the particle-particle strengths. Thus, one might be able to extract more accurate values on the effective neutrino mass by using the best available experimental limits on the half life of neutrinoless double beta decay. Presented by G. Pantis at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’97), Prague, May 27–31, 1997.     

Neutrinoless double beta decay with small and hierarchical neutrino mass

We construct a model where neutrino Majorana masses are small and hierarchical but where neutrinoless double beta decay occurs at an observable rate potentially detectable by present day experiments.     

Neutrinoless double beta decay and a new limit on the lepton number violation

We have calculated the neutrinoless double beta decay rate of ⁷⁶Ge. We take into account for the first time a relativistic correction to the nuclear current including weak magnetism. Its effect is to cancel a considerable part of the decay amplitude and we obtain less stringent upper limits on the neutrino Majorana mass and the right-handed weak leptonic current compared with previous calculations.     

Measurements of events simulating 2β-decay using a nuclear photoemulsion with molybdenum filling

The possibility of neutrinoless double beta decay is studied in the photoemulsion experiment with Mo-100 isotope. The results of measurements of positron-nucleus collisions for the purpose of estimating the background events simulating 2β-decay in the nuclear photoemulsion region adjoining molybdenum conglomerates are presented.     

Spectroscopy of double-beta and inverse-beta decays from 100Mo for neutrinos

Spectroscopic studies of two beta rays from 100Mo are shown to be of potential interest for investigating both the Majorana nu mass by neutrinoless double beta decay (0nubetabeta) and low energy solar nu's by inverse beta decay. With a multiton 100Mo detector, coincidence studies of correlated betabeta from 0nubetabeta, together with the large Q value ( Q(betabeta)), permit identification of the nu-mass term with a sensitivity of approximately 0.03 eV. Correlation studies of the inverse beta decay and the successive beta decay of 100Tc, together with the large capture rates for low energy solar nu's, make it possible to detect, in real time, individual low energy solar nu in the same detector.     

The BiPo low-background detector project

A low-background detector designed to search for weak radioactive pollution by ²⁰⁸Tl and ²¹⁴Bi in thin samples of a large area is described. The samples are 12-m² source foils made of ⁸²Se or ¹⁵⁰Nd enriched isotopes. Such samples are planned for use in investigating neutrinoless double β decay in the SuperNEMO experiment. The principle of the detector operation is based on registering the delayed β-α coincidence from the uranium and thorium radioactive chains. The sensitivity of the detector is planned to be at the level of ²⁰⁸Tl < 2 μBq/kg and ²¹⁴Bi < 10 μBq/kg. Alternate versions of the detector are described. The first results obtained by the prototype detector, operated in the Frejus Underground Laboratory in France, are presented.     

Calculation of double beta decay rates and the neutrino mass

Predictions for 2v and 0v double beta decay rates are given for all nuclei with A ⩾ 70, for which double beta decay is energetically allowed. These predictions are based on detailed nuclear structure studies of the beta strength distribution and replace earlier estimates basing mostly on phase space considerations. New and more stringent limits on the Majorana neutrino mass are deduced from existing double beta decay experiments. Since the collective effects arising from spin-isospin as well as quadrupole-quadrupole forces are found to lead to a strong reduction of the nuclear matrix elements for two-neutrino double beta decay, but to have only minor influence on the matrix elements M^0v for the neutrinoless decay mode, the smaller limits for m_v result mainly from the fact that the widely used scaling procedure underestimates the 0_v matrix elements. It is further discussed to what extent interference between different neutrinos affects the obtained mass limits.