0νββ Decay Research Articles

Physics of Nuclear Processes Triggered by the Interplay of Strong and Weak Interactions

Neutrinoless double beta (0νββ) decay of nuclei is a process that requires the neutrino to be a massive Majorana particle and thus cannot proceed in the standard model of electro-weak interactions. Recent results of the neutrino-oscillation experiments have produced accurate information on the mixing of neutrinos and their squared mass differences. The 0νββ decay takes place in atomic nuclei where it can be observed, at least in principle, by underground neutrino experiments. The information about the weak-interaction observables, like the neutrino mass, has to be filtered from the data through the nuclear matrix elements (NMEs). In this article recent work of the Jyväskylä group on the NMEs related to double beta decays is reviewed. This work concerns (1) the relevance to neutrinoless double beta decay of occupancies of single-particle orbitals close to the Fermi surface and (2) an example of the resonant neutrinoless double electron-capture decay of an atomic nucleus.

Double Beta Decay, Nuclear Structure and Physics beyond the Standard Model

The Neutrinoless Double Beta Decay (0νββ) allows to determine the absolute scale of the neutrino masses. For this one must assume, that the light Majorana neutrino exchange is the leading mechanism for 0νββ and that the matrix element of this transition can ba calculated reliably. The different methods: Quasi-particle Random Phase Approximation (QRPA), Shell Model (SM), Projected Hartree-Fock-Bogoliubov (PHFB) and Interacting Boson Model (IBM2) to calculate these matrix elements are reviewed. The question, how one can determine the leading mechanism or mechanisms from the data of the 0νββ decay in different nuclei is studied in the later part of this article. Possible interference terms allow to test CP (Charge and Parity conjugation) violation.

FRpnQRPA approach with the gauge symmetry restored. Application for the 2νββdecay

A many body Hamiltonian involving the mean field for a projected spherical single particle basis, the pairing interactions for alike nucleons, a repulsive dipole-dipole proton-neutron interaction in the particle-hole (ph ) channel and an attractive dipole-pairing interaction is treated by a gauge restored and fully renormalized proton-neutron quasiparticle random phase approximation formalism. Application to the 2νββ decay rate show a good agreement with the corresponding data. The Ikeda sum rule is obeyed.

Phenomenology of neutrinoless double beta decay

Neutrinoless double beta decay (0νββ) violates lepton number by two units, a positive observation therefore necessarily implies physics beyond the standard model. Here, three possible contributions to 0νββ decay are briefly reviewed: (a) The mass mechanism and its connection to neutrino oscillations; (b) Left-right symmetric models and the lower limit on the right-handed W boson mass; and (c) R-parity violating supersymmetry. In addition, the recently published “extended black box” theorem is briefly discussed. Combined with data from oscillation experiments this theorem provides proof that the 0νββ decay amplitude must receive a non-zero contribution from the mass mechanism, if neutrinos are indeed Majorana particles.

NEUTRINO MASSES, COSMOLOGICAL BOUND AND FOUR ZERO YUKAWA TEXTURES

Four zero neutrino Yukawa textures in a specified weak basis, combined with μτ symmetry and type-I seesaw, yield a highly constrained and predictive scheme. Two alternately viable 3×3 light neutrino Majorana mass matrices mνA/mνBresult with inverted/normal mass ordering. Neutrino masses, Majorana in character and predicted within definite ranges with laboratory and cosmological inputs, will have their sum probed cosmologically. The rate for 0νββ decay, though generally below the reach of planned experiments, could approach it in some parameter region. Departure from μτ symmetry due to RG evolution from a high scale and consequent CP violation, with a Jarlskog invariant whose magnitude could almost reach 6×10-3, are explored.

Advances in the theory of 0 νββ decay

Recent advances in the theory of 0 νββ decay are briefly discussed. New (2010) results for the nuclear matrix elements within the framework of the Interacting Boson Model (IBM-2) are presented.

In-trap decay spectroscopy for 2νββ decay experiments

Knowledge of 2νββ nuclear matrix elements is essential to probe the theoretical framework of 0νββ decays. At TITAN, TRIUMF’s Ion Trap for Atomic and Nuclear science, a novel technique has been developed to measure electron capture branches of virtual intermediate nuclei in ββ decays. During two experiments with radioactive 124, 126Cs isotopes the feasibility of this new method was proven.

Description of the 2ννββ decay within a gauge restored of a fully renormalized pnQRPA approach

A many-body Hamiltonian involving the mean field for a projected spherical single particle basis, the pairing interactions for alike nucleons, the dipole–dipole proton–neutron interactions in the particle–hole (ph) channel and the ph dipole-pairing potential is treated by the projected gauge of fully renormalized proton–neutron quasiparticle random phase approximation (PGFRpnQRPA) approach. The resulting wavefunctions and energies for the mother and the daughter nuclei are used to calculate the 2νββ decay rate and the process half life. For illustration, the formalism is applied for the decays 100Mo→ 100Ru and 116Cd→ 116Sn. The results are in good agreement with the corresponding experimental data. The Ikeda sum rule is obeyed.

NUCLEAR-STRUCTURE EFFECTS ON DOUBLE BETA DECAYS TO 0+ STATES IN 76Ge

Neutrinoless double beta (0νββ) decay of 76 Ge to the ground state and first excited 0+ state in 76 Se is discussed in terms of the associated nuclear matrix elements. The effects arizing from the size of the single-particle model space and the occupancies of the individual orbits are discussed in the framework of the (higher) quasiparticle random-phase approximation with effective, G -matrix-derived nuclear forces. It is found that the orbital occupancies play a role for the size of the nuclear matrix element. Contrary to the ground-state transition the transition to the first excited 0+ state does not depend sensitively on the size of the model space.

The development of a cryogenic detector with CaMoO 4 crystals for neutrinoless double beta decay search

The search for neutrinoless double beta decay (0 νββ) is a key experiment for evaluating the mass of the neutrino and for determining the Majorana/Dirac nature of neutrinos. A scintillating CaMoO 4 crystal is a good material for use in investigating 0 νββ with cryogenic detectors. The high transition energy ( Q = 3034 keV) and nearly 10% natural abundance of 100Mo, together with the scintillating property of the crystal, provide favorable conditions to search for 0 νββ of 100Mo. We report a prototype experiment operating at low temperatures using a CaMoO 4 crystal as a particle absorber. Prior to testing of isotopically-enriched 40Ca 100MoO 4 crystals, a CaMoO 4 crystal with Mo of natural isotopic composition was tested. Using a metallic magnetic calorimeter as a temperature sensor, we achieved high energy resolutions for alpha particles and low-energy gamma rays. This experiment shows the feasibility of scaling up the crystal size to perform a high sensitivity search for 0 νββ decay of 100Mo.

Quasi-Degenerate Neutrino Masses with Normal and Inverted Hierarchy

The effects of CP-phases on the three absolute quasi-degenerate Majorana neutrino (QDN) masses are studied with neutrino mass matrices obeying µ – τ symmetry for normal as well as inverted hierarchical mass patterns. We have made further investigations on 1) the prediction of solar mixing angle which lies below tri-bimaximal mixing value in consistent with neutrino oscillation observational data, 2) the prediction on absolute neutrino mass parameter (mee) in 0νββ decay, and 3) cosmological bound on the sum of the three absolute neutrino masses . The numerical analysis is carried out through the parameterization of neutrino mass matrices using only two unknown parameters (ε, η) within µ – τ symmetry. The results show the validity of QDN mass models in both normal and inverted hierarchical patterns. These models are far from discrimination and hence not yet ruled out. The results presented in this article are new and have subtle ef- fects in the discrimination of neutrino mass models.

A Pulse Shape Analysis technique for the MAJORANA experiment

In order to achieve background count rates sufficiently low so as to allow the observation of rare events such as neutrinoless double beta (0νββ) decay, background suppression techniques are routinely employed. In this paper we present details of a novel Pulse Shape Analysis algorithm, which allows single-site events such as 0νββ decay to be distinguished from multi-site background events in germanium detectors. The algorithm, which is based on the event-by-event χ 2 fitting of experimental signals to a basis data set of unique single-site pulse shapes, has been developed through simulation studies and tested experimentally using a Broad Energy Germanium detector. It is found experimentally that the technique is able to successfully identify and reject 99% of multi-site events in the single escape peak associated with the gamma decay of 208Tl, whilst maintaining a survival probability of 98% for neutrinoless double-beta-decay-like double escape peak events.

Contributions of different neutron pairs in different approaches for neutrinoless double-beta decay

The methods used till now to calculate the neutrinoless double-beta decay (0νββ decay) matrix elements are the quasiparticle random phase approximation (QRPA), the shell model (SM), the angular momentum projected Hartee–Fock–Bogoliubov approach (HFB) and the interacting boson model (IBM). The different approaches are compared specifically concerning the angular momenta and parities of the neutron pairs, which are changed into two protons by the 0νββ decay. The QRPA and SM involve about the same angular momentum and parity neutron pairs, while the HFB is restricted to 0+, 2+, 4+, … and the IBM to 0+ and 2+ nucleon pairs.

TeV scale see-saw mechanisms of neutrino mass generation, the Majorana nature of the heavy singlet neutrinos and (ββ)0ν-decay

It is shown that the Majorana nature of the heavy neutrinos N j having masses in the range of M j ~ (100 - 1000) GeV and present in the TeV scale type I and inverse see-saw scenarios of neutrino mass generation, is unlikely to be observable in the currently operating and future planned accelerator experiments (including LHC) due to the existence of very strong constraints on the parameters and couplings responsible for the corresponding ∣ΔL∣= 2 processes, L being the total lepton charge. If the heavy Majorana neutrinos N j are observed and they are associated only with the type I or inverse see-saw mechanisms and no additional TeV scale “new physics”, they will behave like Dirac fermions to a relatively high level of precision, being actually pseudo-Dirac particles. The observation of effects proving the Majorana nature of N j would imply that these heavy neutrinos have additional relatively strong couplings to the Standard Model particles (as, e.g. in the type III see-saw scenario), or that light neutrino masses compatible with the observations are generated by a mechanism other than see-saw (e.g., radiatively at one or two loop level) in which the heavy Majorana neutrinos N j are nevertheless involved.

Influence of pairing in double beta decay of 48Ca

Two-neutrino ββ decay matrix elements and half-life of 48Ca are calculated after including neutron-proton pairing correlations in projected Hartree-Fock-Bogoliubov (PHFB) formalism. The GT matrix elements in 2νββ decay are reduced due to broader smearing of Fermi surfaces. Half-life results for 2νββ decay of 48Ca with np pairing are better than without pairing.

Study of 48Ca double beta decay with CANDLES

CANDLES is the project to search for neutrinoless double beta (Oμββ) decay of 48Ca by using CaF2 scintillators. The observation of 0νββ decay will prove the existence of massive Majorana neutrinos. Expected performances and current status of the CANDLES system are described.

Description of the two-neutrino ββ decay of 100Mo by pnMAVA

The microscopic anharmonic vibrator approach (MAVA) is a scheme where the one- and two-phonon states of an even–even nucleus are treated consistently by using a realistic microscopic nuclear Hamiltonian. This model has recently been extended to describe odd–odd nuclei by adding proton–neutron phonons in a scheme called the proton–neutron MAVA (pnMAVA). In this paper, we apply pnMAVA to compute the nuclear matrix elements corresponding to the two-neutrino double beta (2νββ) decay of 100Mo to the ground state and the first excited 0+ state of 100Ru in a realistic single-particle space. We also compute the GT− and GT+ Gamow–Teller strength functions and compare them with the plain pnQRPA (proton–neutron QRPA) and available data. The redistribution of strength to four-quasiparticle degrees of freedom can be clearly seen in the GT+ function. The more striking effect is seen in the 2νββ matrix element corresponding to the ground-state transition where the incoherence of individual contributions is stronger for the pnMAVA than for the pnQRPA, and a 15% reduction in the magnitude of the matrix element is obtained for the pnMAVA. The 2νββ transition rate to the excited 0+ state is zero in a pnQRPA calculation, whereas the pnMAVA result is not far from the measured decay rate.

Measurement of the double-βdecay half-life ofNd150and search for neutrinoless decay modes with the NEMO-3 detector

The half-life for double-β decay of Nd150 has been measured by the NEMO-3 experiment at the Modane Underground Laboratory. Using 924.7 days of data recorded with 36.55 g of Nd150, we measured the half-life for 2νββ decay to be T1/22ν=(9.11−0.22+0.25(stat.)±0.63(syst.))×1018 yr. The observed limit on the half-life for neutrinoless double-β decay is found to be T1/20ν>1.8×1022 yr at 90% confidence level. This translates into a limit on the effective Majorana neutrino mass of 〈mν〉<4.0−6.3 eV if the nuclear deformation is taken into account. We also set limits on models involving Majoron emission, right-handed currents, and transitions to excited states.Received 2 October 2008DOI:https://doi.org/10.1103/PhysRevC.80.032501©2009 American Physical Society

Low-energy limits on heavy Majorana neutrino masses from the neutrinoless double-beta decay and non-unitary neutrino mixing

In the type-I seesaw mechanism, both the light Majorana neutrinos (ν1,ν2,ν3) and the heavy Majorana neutrinos (N1,…,Nn) can mediate the neutrinoless double-beta (0νββ) decay. We point out that the contribution of νi to this 0νββ process is also dependent on the masses Mk and the mixing parameters Rek of Nk as a direct consequence of the exact seesaw relation, and the effective mass term of νi is in most cases dominant over that of Ni. We obtain a new bound |∑Rek2Mk|<0.23 eV (or <0.85 eV as a more conservative limit) at the 2σ level, which is much stronger than |∑Rek2Mk−1|<5×10−8 GeV−1 used in some literature, from current experimental constraints on the 0νββ decay. Taking the minimal type-I seesaw scenario for example, we illustrate the possibility of determining or constraining two heavy Majorana neutrino masses by using more accurate low-energy data on lepton number violation and non-unitarity of neutrino mixing.

Two-neutrino double-beta decay of 76Ge in an anharmonic vibrator approach

We have calculated the nuclear matrix element of two-neutrino double-beta (2νββ) decay of 76Ge by using the proton–neutron microscopic anharmonic vibrator approach (pnMAVA). In the pnMAVA the wavefunctions of the intermediate 1+ states in 76As have a one-phonon part containing the proton–neutron QRPA (pnQRPA) phonons and a two-phonon part built by coupling the pnQRPA and charge-conserving QRPA phonons. We compare the measured GT− and GT+ Gamow–Teller strength functions with the measured ones. The two-phonon components of the pnMAVA wavefunctions cause the pnQRPA strength to redistribute over a finite energy range. This has only a small effect on the value 2νββ nuclear matrix element. This shows that at the spherical limit the pnQRPA is an appropriate tool to deal with the double-beta transitions in nuclear systems with only slight deviations from spherical symmetry.