J. Suhonen

Weak-interaction and nuclear-structure aspects of nuclear double beta decay

Abstract Weak-interaction and nuclear-structure aspects of double beta decay are reviewed. Starting from effective electroweak lagrangians, decay rates for the two-neutrino and neutrinoless modes of the nuclear double beta decay transitions are defined and second-order perturbative expressions for the nuclear decay amplitudes are given. Nuclear matrix elements of the relevant operators are presented, as extracted from data and from shell-model and QRPA calculations as well as from other theoretical approximations. The analysis is performed both for the two-neutrino and neutrinoless modes of the decay. The expressions for ground-state-to-ground-state and ground-state-to-excited-state transitions are presented. Updated experimental and theoretical information on β−β− decays in 48Ca, 76Ge, 82Se, 96Zr, 100Mo, 116Cd, 124Sn, 128Te, 130Te, 136Xe, 150Nd, and on β+β+, β+ EC and double EC decays in 78Kr, 92Mo, 96Ru, 106Cd, 124Xe, 130Ba, 136Ce is analyzed and compared with theoretical results. The relevance of single-beta-decay transitions feeding some of the nuclei where double-beta-decay transitions occur is pointed out. The systematics of various phase-space factors and extracted matrix elements is presented.

First results of the search for neutrinoless double-beta decay with the NEMO 3 detector.

The NEMO 3 detector, which has been operating in the Frejus underground laboratory since February 2003, is devoted to the search for neutrinoless double beta decay (bb0nu). Half-lives of the two neutrino double beta decays (bb2nu) have been measured for 100Mo and 82Se. After 389 effective days of data collection from February 2003 until September 2004 (Phase I), no evidence for neutrinoless double beta decay was found from ~7kg of 100Mo and ~1 kg of 82Se. The corresponding lower limits for the half-lives are 4.6 x 10^23 years for 100Mo and 1.0 x10^23 years for 82Se (90% C.L.). Depending on the nuclear matrix elements calculation, limits for the effective Majorana neutrino mass are<0.7-2.8 eV for 100Mo and<1.7-4.9 eV for 82Se

Systematic study of beta and double beta decay to excited final states

Abstract A systematic study of the two-neutrino double beta (2νββ) decay to the final ground state and excited states is performed within an extension of the quasiparticle random-phase approximation (QRPA) model. In this extension, the multiple commutator model (MCM), a simultaneous treatment of the double-odd and double-even nuclei is possible by assuming their states to have the structure of one or two QRPA phonons. The MCM calculation is done for nine 2νβ−β− decays and six 2νECEC decays, and systematics of the relevant matrix elements and half-lives have been created. The study of the 2νββ-decay rates is complemented with the MCM study of the single-beta-decay properties of the relevant nuclei within the double-beta isobaric chains. The Woods-Saxon single-particle energies have been corrected near the Fermi surface by comparing the BCS quasiparticle energies with spectroscopic data of odd-mass nuclei. Pairing gaps, energy systematics of the Gamow-Teller states and the available beta-decay data have been used to obtain effective, model-space adapted, two-body matrix elements starting from the G-matrix elements of the Bonn one-boson-exchange potential. This enables a parameter-free calculation of the double Gamow-Teller matrix elements and theoretical prediction of double-beta half-lives.

Measurement of the Double Beta Decay Half-life of Nd-150 and Search for Neutrinoless Decay Modes with the NEMO-3 Detector

The half-life for double-{beta} decay of {sup 150}Nd 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 {sup 150}Nd, we measured the half-life for 2{nu}{beta}{beta} decay to be T{sub 1/2}{sup 2{nu}}=(9.11{sub -0.22}{sup +0.25}(stat.){+-}0.63(syst.))x10{sup 18} yr. The observed limit on the half-life for neutrinoless double-{beta} decay is found to be T{sub 1/2}{sup 0{nu}}>1.8x10{sup 22} yr at 90% confidence level. This translates into a limit on the effective Majorana neutrino mass of <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.

Nuclear matrix elements of 0νββ decay with improved short-range correlations

Nuclear matrix elements of the neutrinoless double beta decays of 96Zr, 100Mo, 116Cd, 128Te, 130Te and 136Xe are calculated for the light-neutrino exchange mechanism by using the proton-neutron quasiparticle random-phase approximation (pnQRPA) with a realistic nucleon-nucleon force. The g_pp parameter of the pnQRPA is fixed by the data on the two-neutrino double beta decays and single beta decays. The finite size of a nucleon, the higher-order terms of nucleonic weak currents, and the nucleon-nucleon short-range correlations (s.r.c) are taken into account. The s.r.c. are computed by the traditional Jastrow method and by the more advanced unitary correlation operator method (UCOM). Comparison of the results obtained by the two methods is carried out. The UCOM computed matrix elements turn out to be considerably larger than the Jastrow computed ones. This result is important for the assessment of the neutrino-mass sensitivity of the present and future double beta experiments.

Calculation of allowed and first-forbidden beta-decay transitions of odd-odd nuclei

Abstract A method for calculating the β − and β + EC transitions from an odd-odd nucleus to the excited states of the adjacent even-even nucleus is presented. The beta transitions are treated combining the charge-conserving and the charge-changing mode of the quasi-particle random- phase approximation assuming a common vacuum. The beta-decay transitions are treated in their allowed and first-forbidden approximations, including also the ground-state transition. As a test case of the present formalism, detailed beta-decay properties of the following odd-odd-to-even-even transitions have been calculated and compared with experiment: the β − decay transitions 136 I (2 − )→ 136 Xe (J π ) and 136 Cs (5 + ) → 136 Ba (J π ) , as well as the β + EC transitions 136 La (1 + → 136 Ba (J π ) . The calculated results agree nicely with data.

Short-range correlations and neutrinoless double beta decay

Abstract In this work we report on the effects of short-range correlations upon the matrix elements of neutrinoless double beta decay ( 0 ν β β ). We focus on the calculation of the matrix elements of the neutrino-mass mode of 0 ν β β decays of 48 Ca and 76 Ge. The nuclear-structure components of the calculation, that is the participant nuclear wave functions, have been calculated in the shell-model scheme for 48 Ca and in the proton–neutron quasiparticle random-phase approximation (pnQRPA) scheme for 76 Ge. We compare the traditional approach of using the Jastrow correlation function with the more complete scheme of the unitary correlation operator method (UCOM). Our results indicate that the Jastrow method vastly exaggerates the effects of short-range correlations on the 0 ν β β nuclear matrix elements.

Review of the properties of the 0νβ−β− nuclear matrix elements

In this work we investigate general properties of the nuclear matrix elements (NMEs) related to the neutrinoless double ?? decays (0????? decays) of several nuclei of interest for double-beta-decay experiments. A summary of the values of the NMEs calculated along the years by the Jyv?skyl??La Plata collaboration is given. These NMEs are compared with those of the other available calculations and general conclusions are drawn based on these comparisons.

Double beta decay of 96Zr

Abstract After 10357 h of running the NEMO-2 tracking detector with an isotopically enriched zirconium source (0.084 mol yr of 96Zr), a ββ2ν decay half-life of T1/2=(2.1+0.8(stat)−0.4(stat)±0.2(syst))·1019 y was measured. Limits with a 90% C.L. on the 96Zr half-lives of 1.0·1021 y for ββ0ν decay to the ground state, 3.9·1020 y to the 2+ excited state and 3.5·1020 y for ββ0νχ0 decay with a Majoron (χ0) were obtained. The data also provide direct limits at the 90% C.L. for the 94Zr half-lives. These limits are 1.1·1017 y for ββ2ν decay to the ground state, 1.9·1019 y for ββ0ν decay to the ground state and 2.3·1018 y for ββ0νχ0 decay to ground state.

Q values of the 76Ge and 100Mo double-beta decays

Penning trap measurements using mixed beams of 100 Mo - 100 Ru and 76 Ge - 76 Se have been utilized to determine the double-beta decay Q-values of 100 Mo and 76 Ge with uncertainties less than 200 eV. The value for 76 Ge, 2039.04(16) keV is in agreement with the published SMILETRAP value. The new value for 100 Mo, 3034.40(17) keV is 30 times more precise than the previous literature value, suffcient for the ongoing neutrinoless double-beta decay searches in 100 Mo. Moreover, the precise Q-value is used to calculate the phase-space integrals and the experimental nuclear matrix element of double-beta decay.