Neutrinoless double-beta decay and double-electron capture

The fundamental importance of searching for neutrinoless double-beta decay is widely recognized. Observation of the decay would tell us that the total lepton number is not conserved and that, consequently, neutrinos are massive Majorana fermions. The same statement could be made in the case of observing neutrinoless double-electron capture. We address the question of the sensitivity of the 0νεε decay to the effective mass of the Majorana neutrino. According to our estimates, in the case of ¹⁵²Gd and ¹⁶⁴Er the sensitivity can be comparable to the favored 0νββ decays of nuclei. The main uncertainty in the prediction of half-lives of the 0νεε decay stems from the lack of sufficient precision in measuring the mass difference between the parent and daughter atoms. More accurate measurements can be accomplished using the modern high-precision ion traps.     

Average and recommended half-life values for two-neutrino double-beta decay: Upgrade ’05

All existing “positive” results on two-neutrino double-beta decay in different nuclei were analyzed. Using the procedure recommended by the Particle Data Group, weighted average values for half-lives of ⁴⁸Ca, ⁷⁶Ge, ⁸²Se, ⁹⁶Zr, ¹⁰⁰Mo, ¹⁰⁰Mo−¹⁰⁰Ru(0 ₁ ⁺ ), ¹¹⁶Cd, ¹⁵⁰Nd, ¹⁵⁰Nd− ¹⁵⁰Sm(0 ₁ ⁺ ) and ²³⁸U were obtained. Existing geochemical data were analyzed and recommended values for half-lives of ¹²⁸Te, ¹³⁰Te, and ¹³⁰Ba are proposed. We recommend the use of these results as presently the most precise and reliable values for half-lives. Presented by A.S. Barabash at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.     

Control of bulk and surface radioactivity in bolometric searches for double-beta decay

The problem of background reduction in bolometers used to search for ββ(0ν) is reviewed with particular emphasis on bulk and surface radioactive contamination of the detectors.     

The neutrinoless double-beta decay: A test for new physics

The neutrinoless double-beta decay is not allowed in the Standard Model (SM) but it is allowed in most Grand Unified Theories (GUTs). The neutrino must be a Majorana particle (identical with its antiparticle) and must have a mass to allow the neutrinoless double-beta decay. Apart of one claim that the neutrinoless double-beta decay in ⁷⁶Ge is measured, one has only upper limits for this transition probability. But even the upper limits allow to give upper limits for the electron Majorana neutrino mass and upper limits for parameters of GUTs and the minimal R-parity violating supersymmetric model. One further can give lower limits for the vector boson mediating mainly the right-handed weak interaction and the heavy mainly right-handed Majorana neutrino in left-right symmetric GUTs. For that, one has to assume that the specific mechanism is the leading one for the neutrinoless double-beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present contribution, one discusses the accuracy of the present status of calculating the nuclear matrix elements and the corresponding limits of GUTs and supersymmetric parameters.     

Search for Cold Dark Matter and Solar Neutrinos with GENIUS and GENIUS-TF

The new project GENIUS will cover a wide range of the parameter space of predictions of SUSY for neutralinos as cold dark matter. Further it has the potential to be a real-time detector for low-energy (pp and ⁷Be) solar neutrinos. A GENIUS Test Facility has been funded and will come into operation by early 2003.     

Neutrinoless ββ decay and the electron neutrino mass

We discuss the nuclear structure elements participant in the calculation of the half-life of the neutrinoless double beta decay, and the consequences upon the adopted limits of the electron-neutrino mass. Presented by O. Civitarese at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.     

The neutron decay retardation spectrometer aSPECT: Electromagnetic design and systematic effects

The apparatus described here, aSPECT, will be used for a measurement of the neutrino-electron angular correlation coefficient a in the decay of free neutrons. The idea of the aSPECT spectrometer is to measure the integrated proton energy spectrum very accurately using an energy filter by electrostatic retardation and magnetic adiabatic collimation. The main ideas of the spectrometer are presented, followed by an explanation of the adiabatic transmission function. Details of the superconducting coil and of the electrode system are given, as well as a discussion of the most important systematic effects: magnetic field and electrostatic potential inhomogeneities, deviation from adiabatic motion, scattering in the residual gas, background, Doppler effect, edge effect, and detector efficiency. Using this spectrometer, the parameter a is planned to be measured with an absolute experimental uncertainty of a 3 ^. 10^-4, from which the axial vector to vector coupling constant ratio can be determined with an accuracy of 0.001.     

The neutron and proton two-particle nucleus 134Sb: New low-spin states observed in the decay of 134Sn and an estimate of the energy of the 7- isomer

Excited states in the ¹³⁴Sb nucleus, populated in the β^--decay of ¹³⁴Sn, have been studied at the mass separator OSIRIS. The ¹³⁴Sn activity was produced via fast neutron-induced fission of ²³⁸U target. A main result was the discovery of a very low-lying first-excited state of ¹³⁴Sb, at 13keV, which has led to a strong revision of the level scheme. The new results are compared with different theoretical calculations and with the known data for the analogous neutron and proton two-particle nucleus in the ²⁰⁸Pb region. On the basis of this comparison, the energy of the ( πg _7/2νf _7/2)_7- isomer is estimated to be about 250keV, some 100keV lower than previously reported. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: anovak@fuw.edu.pl     

Probing the mechanism of neutrinoless double beta decay with SuperNEMO

The SuperNEMO experiment is being designed to search for neutrinoless double beta decay. Its experimental technique of tracking and calorimetry provides the means to discriminate different underlying mechanisms for neutrinoless double beta decay by measuring the angular and energy distributions of electrons. The results of a study by the SuperNEMO Collaboration and F. Deppisch (in preparation) [7] for identifying light Majorana neutrino exchange and right-handed currents are presented.     

Beta decays and muon capture as probes of ββ matrix elements

Detection of the neutrinoless double-beta decay is essential in probing physics beyond the standard model of electroweak and strong interactions. At the moment there is a considerable uncertainty associated with the calculation of the involved nuclear matrix elements. This uncertainty can be diminished by suitably chosen nuclear probes, like beta decays and nuclear muon capture. The related experiments are urgently called for. Presented by J. Suhonen at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.     

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.     

First Evidence for Neutrinoless Double Beta Decay

Double beta decay is indispensable to solve the question of the neutrino mass matrix together with 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.     

A new measurement of 208Tl levels

A new determination for the levels of the nucleus ²⁰⁸Tl was performed by measuring the γ-rays following the β^--decay of ²⁰⁸Hg. Twenty-six γ-rays were assigned and three new levels of ²⁰⁸Tl at 1.728 MeV, 1.652 MeV and 1.362 MeV have been affirmed. A partial ²⁰⁸Hg decaying γ scheme was proposed. The experimental level structure was compared with a shell model calculation. Received: 12 August 2002 / Accepted: 19 December 2002 / Published online: 25 February 2003 RID="a" ID="a"e-mail: Li41zhang@yahoo.com Communicated by W. Henning     

Spin-orbit-like terms in semileptonic weak Hamiltonian

It is shown that new spin-orbit-like terms appear in the effective nonrelativistic weak Hamiltonian for nucleon provided that nuclear potential is taken into account. Arguments for their considerable enhancement, in particular, in relativistic nuclear model of Walecka are advanced. Received: 29 July 1999     

μCR42β: Muon capture rates for double-beta decay

Gamma-rays following nuclear capture of negative muons in enriched ⁴⁸Ti, ⁷⁶Se, ¹⁰⁶Cd, as well as natural Se and Cd targets were measured. Time evolution of several gamma-lines allowed to deduce the total μ-capture rates in Ti, Se and Cd isotopes, which are compared to the Primakoff estimate. The data on the partial μ-capture rates will be used for more precise calculation of the 2β-NMEs of ⁴⁸Ca, ⁷⁶Se and ¹⁰⁶Cd. Presented by V. Egorov at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005. Partly supported by the Russian Foundation for Basic Research (grant 06-02-16587).     

Neutrinoless double beta decay studied with configuration mixing methods

We study neutrinoless double beta decay of several isotopes with state-of-the-art beyond self-consistent mean field methods to compute the nuclear matrix elements (NME). The generating coordinate method with particle number and angular momentum projection (GCM +PNAMP) is used for finding mother and granddaughter states and evaluating transition operators between different nuclei. We analyze explicitly the role of the deformation, pairing and configuration mixing in the evaluation of the NME.     

Gamow-Teller beta decay of 105Sn

The β-decay of ¹⁰⁵Sn was re-investigated at the ISOL facility of GSI Darmstadt by using a total absorption spectrometer. The experimental results include the half-life and β-delayed proton branching ratio of 32.7(5)s and 1.1(4) ^. 10^-4, respectively, and the contribution of electron capture to the ¹⁰⁵Sn decay of 0.420(35). The Gamow-Teller strength distribution peaking at 3.6MeV was measured, yielding a summed Gamow-Teller strength of 3.0(4). The latter data are discussed in comparison with shell-model predictions based on an empirical interaction.     

Double Beta Decay Experiments with Thermal Detectors

Massive low temperature particle detectors and their possible impacts on searches for neutrinoless double beta decay (O-DBD) are presented and discussed. In particular, the experimental work of the Milano group is described. Special relevance is given to the present status of the search for O-DBD of ¹³⁰Te and to the possible expansion of this experiment in the near future. The most recent results obtained by the Milano-Gran Sasso collaboration with a 20 bolometer array are presented. On the basis of these results, the construction of a 42 kg array consisting of 56 TeO₂ bolometers (CUORICINO project), to extend the sensitivity of the present experiment, has been proposed. CUORICINO should represent also a feasibility test for a large array of 1000 bolometers (CUORE project) aiming at the search for neutrinoless Double Beta Decay, Cold Dark Matter and Solar Axions with extremely high sensitivity.     

Interplay of direct and spin-flip contributions in Gamow-Teller decay to odd-odd N = Z nuclei

Strengths of Gamow-Teller decays of T_z = ±1 nuclei to T_z = 0 odd-odd nuclei have been calculated by using spherical shell model and deformed Nilsson wave functions. The role and competition of the microscopic direct and spin-flip mechanisms generating Gamow-Teller transitions are analyzed. Analytical expressions derived for the B(GT) values give useful insight into the regularities of B(GT) values along the N = Z line. The crucial role of configuration mixing is discussed.