Half-life Of Double Beta Decay Research Articles

Critical view on double-beta decay matrix elements within Quasi Random Phase Approximation-based methods

A systematic study of the two-neutrino and neutrinoless double-beta decay matrix elements (ME) for the nuclei with A=76, 82, 96, 100, 116, 128, 130 and 136 is done. The calculations are performed with four different quasi random phase approximation (QRPA)-based methods, i.e. the proton–neutron QRPA (pnQRPA), the renormalized proton–neutron QRPA (pnRQRPA), the full-RQRPA and the second-QRPA (SQRPA). First we checked the conservation of the Ikeda sum rule (ISR) and found that it is fulfilled with a good accuracy for the SQRPA, while for the pnRQRPA and full-RQRPA the deviations are up to 17%. Then, we studied the dependence of the ME on the single-particle (s.p.) basis. For that we performed the calculations using the same set of parameters and two different s.p. basis. For the two-neutrino decay mode the ME manifest generally the largest sensitivity to the choice of the basis when they are calculated with the pnQRPA, while the smallest sensitivity is got with the SQRPA. For all the methods the largest differences between the results were found for 128,130Te and 136Xe. For the neutrinoless decay mode the ME display generally a stronger dependence on the basis than for the two-neutrino decay mode, when they are calculated with the pnQRPA, RQRPA and full-RQRPA, while for SQRPA differences in the results are within 30%. A better stability against the change of the s.p. basis used and a good fulfillment of the ISR allow to reduce the uncertainties in the values of the neutrinoless ME predicted by the QRPA-based methods to about 50% from their magnitude. Further, we fixed the values of g pp from the two-neutrino calculations and according to recent experimental data, and then we used them to compute the ME for the neutrinoless decay mode. Taking the most recent experimental limits for the neutrinoless half-lives, we deduce new upper limits for the neutrino mass parameter. Finally, there are estimated, for each nucleus, scales for the neutrinoless double-beta decay half-lives that the experiments should reach for exploring neutrino masses around 0.1 eV. This might guide the experimentalists in planning their setups.

Neutrinoless double beta decay of76Ge,82Se,100Mo,and136Xeto excited0+states

The neutrinoless double beta decay transition to the first excited 0^+ collective final state is examined for A=76, 82, 100 and 136 nuclei by assuming light and heavy Majorana neutrino exchange mechanisms as well as the trilinear R-parity violating contributions. Realistic calculations of nuclear matrix elements have been performed within the renormalized quasiparticle random phase approximation. Transitions to the first excited two-quadrupole phonon 0^+ state are described within a boson expansion formalism and alternatively by using the operator recoupling method. We present the sensitivity parameters to different lepton number violating signals, which can be used in planning the neutrinoless double beta decay experiments. The half-life of neutrinoless double beta decay to the first excited state 0^+_1 is by a factor of 10 to 100 larger than that of the transition to the ground state.

Present status of MI-BETA cryogenic experiment and preliminary results for CUORICINO

Present results on neutrinoless double beta decay of 130Te obtained with an array of 20 cryogenic detectors are presented. The setup consists of 20 crystals of tellurium oxide of 340 g each, corresponding to the largest presently operating cryogenic mass. Combining the results of the few runs obtained with the same array, corresponding to 1.5 kg yr, a limit on neutrinoless double beta decay half-life of 9.5×10 22 yr (90% CL) has been obtained. On the basis of the results obtained with the MI-BETA experiment, we propose a construction of a 42 kg array of 56 TeO 2 bolometers (CUORICINO project) to extend the sensitivity of the present experiment, and as a first test for CUORE. Thanks to an innovative technique of vibration reduction and despite the high mass (750 g instead of the 340 g of the presently running detectors) we reached an energy resolution of 3.9 keV FWHM at 2615 keV decreasing to 1.4 keV at low energies.

Limitations of the number self-consistent random phase approximation

The Quasiparticle Random Phase Approximation equations are solved taking into account the Pauli Principle at the expectation value level, and allowing changes in the mean field occupation numbers to minimize the energy while having the correct number of particles in the correlated vacuum. The study of Fermi pn excitations in $^{76}$Ge using a realistic Hilbert space shows that the pairing energy gaps in the modified mean field are diminished up to one half of the experimental value when strong proton-neutron correlations are present. Additionally, the Ikeda sum rule for Fermi transitions is violated due to the lack of scattering terms in the phonon operators. These results call for a critical revision of the double beta decay half-lives estimated using the QRPA extensions when standard QRPA calculations collapse.

Limits on the Majorana Neutrino Mass in the 0.1 eV Range

The Heidelberg-Moscow experiment gives the most stringent limit on the Majorana neutrino mass. After 24 kg yr of data with pulse shape measurements, we set a lower limit on the half-life of the neutrinoless double beta decay in 76Ge of T_1/2 > 5.7 * 10^{25} yr at 90% C.L., thus excluding an effective Majorana neutrino mass greater than 0.2 eV. This allows to set strong constraints on degenerate neutrino mass models.

Neutrinoless double beta decay in the nonminimal supersymmetric standard model

We consider the neutrinoless double beta decay in the minimal supersymmetric standard model (MSSM) with a gauge singlet superfield and R-parity violating interactions. Constraints on R-parity violating coupling, in particular, λ ′ 111 are obtained from the current experimental lower limit on the neutrinoless double beta decay half-life. Owing to the extended neutralino sector, the constraints on λ 111 ′ can be more stringent than that of the MSSM.

Dominance of Pion Exchange inR-Parity-Violating Supersymmetric Contributions to Neutrinoless Double Beta Decay

We present a new contribution of the R-parity violating supersymmetry (SUSY) to neutrinoless double beta decay via the pion exchange between decaying neutrons. The pion coupling to the final state electrons is induced by the R-parity violating SUSY interactions. We have found this pion-exchange mechanism to dominate over the conventional two-nucleon one. The latter corresponds to direct interaction between quarks from two decaying neutrons without any light hadronic mediator like pion. The constraints on the certain R-parity violating SUSY parameters are extracted from the current experimental neutrinoless double beta decay half-life limit. These constraints are significantly stronger than those previously known or expected from the ongoing accelerator experiments.

A measurement of the double beta decay half-life of 48Ca

48 Ca is the lightest of the many double beta decay nuclei, and the only one simple enough to be treated exactly in the shell model without truncation. Thus a ββ 2 v measurement of this isotope provides a unique test of the nuclear physics involved in ββ matrix element calculations. Enriched 48 Ca sources of two different thicknesses have been exposed in a time projection chamber, and yield a preliminary T 1/2 2 v = (5.5 −1.5 +3.5 ) × 10 19 y, in agreement with shell model calculations.

Two-neutrino double-beta decay half-lives of 96Zr and 100Mo to excited states of 96Mo and 100Ru

Two-neutrino double-beta decay half-lives for the transitions to the first excited 2 + and 0 + states have been computed in the framework of a second quasi random phase aproximation (QRPA) in the cases of 100Mo and 96Zr. In addition, other single-beta decay rates and B(E2) transitions related to the same nuclei are evaluated and compared with experimental data. We have found a good agreement between theory and experiment for all the computed quantities except the B(E2) transitions in 100Mo and 100Ru. For the ββ decay transitions studied we predict half-lives of the order of 10 21–10 22 yr.

Large calorimetric devices for double beta decay and dark matter

The use of cryogenic thermal particle detectors permits the realization of detectors of various compositions, various sizes and very good energy resolution. In particular these characteristics are very promising for the realization of double beta decay and dark matter search experiments. Our group is mainly interested in the study of double beta decay of 130Te and 116Cd. For tellurium we have realized various detectors using TeO 2 crystals, the final one with a mass of 334 g. For cadmium a CdWO 4 crystal of 58 g has been used in various tests. The double beta decay measurement has been performed in the Gran Sasso Underground Laboratory. Measurements on the 334 g TeO 2 crystal have been performed for 3000 h. Detector resolution is around 10 keV FWHM and the internal contamination of 238U and 232Th in the crystal is of the order of 10 −13 g/g. A lower limit on the half-life of neutrinoless double beta decay for 130Te of 8.2 × 10 21 yr (90% CL) is measured. The test measurements of CdWO 4 reach an energy resolution of about 5 keV FWHM with a very high efficiency to gamma ray detection. A limit on the neutrinoless channel of 7 × 10 19 yr (90% CL) is evaluated in 340 h. In this test an end point energy of 318.8 ± 1.4 ± 5 keV and a half-life of (9.3 ± 0.5 ± 1) × 10 15 yr for the beta decay of 113Cd are also measured. The last part of the paper is dedicated to a brief discussion of our proposal for the solar neutrino flux measurement and for the search of dark matter using cryogenic detectors.

Half-lives for two neutrino double-beta-decay transitions to first 2(+) excited states.

Two neutrino double-beta decay half-lives for transitions to first ${2}_{1}^{+}$ excited states have been calculated in the framework of a second-quasirandom phase approximation procedure for the following nuclei: $^{76}\mathrm{Ge}$, $^{82}\mathrm{Se}$, $^{110}\mathrm{Pd}$, $^{116}\mathrm{Cd}$, $^{128,130}\mathrm{Te}$, and $^{136}\mathrm{Xe}$. It was found that the corresponding decay rates are only weakly dependent on the ${\mathit{g}}_{\mathit{p}\mathit{p}}$ parameter. Our calculations indicate as the most favorable cases $^{82}\mathrm{Se}$, $^{116}\mathrm{Cd}$, and $^{136}\mathrm{Xe}$ for which these half-lives are of the order of ${10}^{23}$ yr.

Estimation of bounds for left-right mixing from nuclear double beta decay processes

Nuclear matrix elements, extracted from a systematic calculation of neutrinoless double beta decay rates for 0 + g.s→o + g.s transitions in 76Ge, 82Se, 128Te, 130Te and 136Xe, have been used to compute bounds on the left-right mixing angle ζ of the weak currents. The parameters of the nuclear model have been fixed by calculating the corresponding two-neutrino double beta decay half lives. The present analysis favours a mixing of the order of |tanξ| < 10 −3 which is to be compared with those obtained in muon decay (|tan ξ| ⩽ 0.035) and in supernova (|tan ξ| ⩽ 10 −5) analysis. It shows that nuclear structure calculations yield estimates on ξ which complement the existing data.

Double beta decay and dark matter in the Gotthard germanium experiment

An array of eight high purity germanium crystals is operated in the Gotthard road tunnel. Based on a data sample of 6.2 kg years, limits on different modes of double beta decay in 76Ge are obtained. In particular, the half-life for the neutrinoless double beta decay to the ground state of 76Se is greater than 2.0*1023 (3.4*1023) years at the 90% (68%) confidence level. A low energy sample of 51.6 kg days was used to set limits on cold dark matter. In particular, Dirac neutrinos with masses between 10 and 2400 GeV are ruled out. A new lower limit on the half-life for the decay of the electron into weakly interacting particles of 1.9*1023 years at 68% confidence level was also derived.

TPC for investigation of double beta decaying nuclei in solid samples

A design of a large volume time projection chamber (TPC) for double beta decay measurements with solid samples is presented. The total surface of measured samples can be up to 2.5 m2. The chamber will operate in a magnetic field up to 0.07 T. The expected spatial resolution is better than 1 mm in electron drift direction and about 2 mm along the sense wires. The momentum resolution of about 10% should be reached. The background of the TPC will be very effectively suppressed by the visualization of each event. The TPC operating in an underground laboratory should reach a limit on the half-life of neutrinoless double beta decay of 1024 years.

Calculation of the neutrinoless ββ decay of 76Ge using a quark model with harmonic confinement

The half-life of the neutrinoless double beta decay of 76Ge into the ground state of 76Se is calculated in a relativistic quark confinement model. The proton-neutron quasi-particle random-phase approximation is used to evaluate the s- and p-wave nuclear matrix elements contained in the decay amplitude. We avoid the closure approximation and calculate the effective vector and axial-vector coupling constants of the hadronic currents using our quark model. In our formulation the recoil matrix element arises from the quark recoil in the decaying neutrons. The recoil and the p-wave effect are discussed and compared with other calculations. From the experimental lower bound for the decay half-life an upper limit for the effective Majorana mass of the neutrino is deduced and the right-handed contribution to the charged weak current estimated.

Description of the ground-state pionic double charge exchange reaction on 128,130Te

The pionic double change exchange reactions 128,130Te( π +, π −) 128,130Xe are investigated within the proton-neutron random-phase approximation. The approach is fpund to give a satisfactory agreement of the calculated differential cross sections and angular distributions with recent data. A strong dependence of the calculated quantities on the strength g pp of the particle-particle force is observed. This behaviour is similar to that reported for the β + decay in neutron-dificient nuclei and the double beta decay. The role of the DCX for the interpretation of the ratio of the double beta decay half-lives of the tellurium isotopes is discussed.

A Bound on the Majoron Coupling to the Neutrino

The nuclear neutrinoless double beta decay with emission of a Majoron is discussed by means of a diagrammatic effective operator approach. In the calculation, the effects of the nuclear short range correlation and nuclear core polarization have been taken into account. The effective Majoron coupling to neutrino is then deduced from the experimental half-life of 48Ca double beta decay with the result ⟨gH0⟩ < (2 ∼ 3) × 10-4.

Low background installation for the 136Xe double beta decay experiment

A pulse ionization chamber filled with xenon at a pressure of 3 MPa is described. A lower limit of 5.5 × 10 19 yr has been obtained for the half-life of the neutrinoless double beta decay of 136Xe.

Double-beta decay and resonances in the nucleus

Double-beta decay half-lives are calculated with the assumption that the emission of electron–neutrino pairs occurs via a Δ(1232) resonance in the nucleus. Numerical results are obtained with a quark model for the hadrons. By assuming that total rates are produced by a combination of neutrinoless and two-nuetrino modes, a lepton-nonconservation parameter η ~ 10−5 is obtained. Although the actual modes of decay and underlying mechanisms are undetermined, the present calculation can be used to obtain an upper limit for the probability admixture of resonances in the nucleus of a few percent.

Double beta decay

The problem of double beta decay is reviewed with emphasis on its relevance to lepton number conservation. Recently, the ratio of the double beta-decay half-lives of $^{128}\mathrm{Te}$ and $^{130}\mathrm{Te}$ has been measured in a geological experiment and a limit for the ratio of the neutrinoless rate to the total rate for $^{82}\mathrm{Se}$ decay has been obtained from a direct-detection experiment. For the first time, these results show conclusively that double beta decay is not primarily a lepton-number-violating neutrinoless process. However, they also do not agree with calculations which assume that only lepton-number-conserving two-neutrino double beta decay occurs. The conclusion that lepton number conservation is violated is suggested by limited experimental information. By considering contributions to the total rate from both the two-neutrino and the neutrinoless channels, we obtain data which are consistent with a lepton nonconservation parameter of order $\ensuremath{\eta}=3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. Roughly the same value of $\ensuremath{\eta}$ is obtained by assuming that the decay occurs either via lepton emission from two nucleons or via emission from a resonance in the nucleus.