Can the nuclear matrix elements of neutrinoless double beta decay be measured?

A proposal which allows one in principle to measure the neutrinoless double beta decay Fermi matrix element is briefly described. By making use of the isospin conservation by strong interaction, the Fermi 0νββ nuclear matrix element can be transformed to acquire the form of an energy-weighted double Fermi transition matrix element. This allows one to reconstruct the total provided that a small isospin-breaking Fermi matrix element between the isobaric analog state in the intermediate nucleus and the ground state of the daughter nucleus could be measured, for example by charge-exchange reactions. Such a measurement could help to discriminate between the different nuclear structure models in which the calculated may differ by as much as a factor of 5.     

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 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.     

Testing CVC and CKM Unitarity via Superallowed Nuclear Beta Decay

Superallowed nuclear beta decay between 0+ analog states is a sensitive probe of the weak interaction, with the established strength – or Ft value – of each such transition being a direct measure of the vector coupling constant, GV. Each transition’s Ft value depends on the half-life of the parent nucleus as well as on the Q-value and branching ratio for the transition of interest. It also depends on small (1%) transition-dependent theoretical corrections, of which the most sensitive accounts for isospin symmetry breaking. We have recently published a new survey of world superallowed-decay data, which establishes the Ft values of 14 separate superallowed transitions to a precision of order 0.1% or better. The results from this very robust data set yield the value of Vud, the up-down quark mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, and lead to the most demanding test available of CKM unitarity. The survey results and their outcome are described, as is the current direction of experiments that focus on testing the validity of the isospin-symmetry-breaking corrections.     

Status and prospects of searches for neutrinoless double beta decay

The simultaneous beta decay of two neutrons in a nucleus without the emission of neutrinos (called neutrinoless double beta decay) is a lepton number violating process which is not allowed in the Standard Model of particle physics. More than a dozen experiments using different candidate isotopes and a variety of detection techniques are searching for this decay. Some (EXO‐200, Kamland‐Zen, and GERDA) started to take data recently. EXO and Kamland‐Zen have reported first limits of the half life for ¹³⁶Xe. After a decade of little progress in this field, these results start to scrutinize the claim from part of the Heidelberg–Moscow collaboration to have observed this decay. The sensitivities of the different proposals are reviewed.     

The isospin admixture of the ground state and the properties of the isobar analog resonances in medium and heavy mass nuclei

Within the framework of quasiparticle random phase approximation (QRPA), Pyatov-Salamov method [23] for the self-consistent determination of the isovector effective interaction strength parameter, restoring a broken isotopic symmetry for the nuclear part of the Hamiltonian, is used. The isospin admixtures in the ground state of the parent nucleus, and the isospin structure of the isobar analog resonance (IAR) state were investigated with the inclusion of the pairing correlations between nucleons for the medium and heavy mass regions: 80 <A < 90, 102 <A < 124, and 204 <A < 214. It was determined that the influence of the pairing interaction between nucleons on the isospin admixtures in the ground state and the isospin structure of the IAR state is more pronounced for the light isotopes (N ≈ Z) of the investigated nuclei     

Shell Model description of the ββ decay of Xe

We study in this Letter the double beta decay of ¹³⁶Xe with emission of two neutrinos which has been recently measured by the EXO-200 Collaboration. We use the same shell model framework, valence space, and effective interaction that we have already employed in our calculation of the nuclear matrix element (NME) of its neutrinoless double beta decay. Using the quenching factor of the Gamow–Teller operator which is needed to reproduce the very recent high resolution ¹³⁶Xe (³He, t) ¹³⁶Cs data, we obtain a nuclear matrix element M^2ν=0.025 MeV⁻¹${\mathrm{M}}^{2\nu }=0.025{\text{MeV}}^{-1}$ compared with the experimental value M^2ν=0.019(2) MeV⁻¹${\mathrm{M}}^{2\nu }=0.019(2){\text{MeV}}^{-1}$.     

Nuclear 0ν2β decays in B-L symmetric SUSY model and in TeV scale left–right symmetric model

In this paper, we take the B-L supersymmetric standard model (B-LSSM) and TeV scale left–right symmetric model (LRSM) as two representations of the two kinds of new physics models to study the nuclear neutrinoless double beta decays (0ν2β) so as to see the senses onto these two kinds of models when the decays are taken into account additionally. Within the parameter spaces allowed by all the existing experimental data, the decay half-life of the nucleus ⁷⁶Ge and ¹³⁶Xe, ${T}_{1/2}^{0\nu }$(⁷⁶Ge, ¹³⁶Xe), is precisely calculated and the results are presented properly. Based on the numerical results, we conclude that there is greater room for LRSM type models than for B-LSSM type models in foreseeable future experimental observations on the decays.     

Muon-capture rates and their relation with the double-beta decay

We discuss the non-radiative μ ⁻ capture (i.e. ordinary muon capture, OMC) in light nuclei in terms of the nuclear shell model, and in the medium-and heavy-mass nuclei in terms of the quasiparticle random-phase approximation. A new probe of the double-beta-decay matrix elements, namely the use of the OMC to states of the intermediate nucleus of the double beta decay, is also addressed. Presented by J. Suhonen at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.     

An investigation of the influence of the pairing correlations on the properties of the isobar analog resonances inA = 208 nuclei

Within the quasi-particle random phase approximation (QRPA), the method of the self-consistent determination of the isovector effective interaction which restores a broken isotopic symmetry for the nuclear part of the Hamiltonian is given. The effect of the pairing correlations between nucleons on the following quantities were investigated for theA = 208 nuclei: energies of the isobar analog 0⁺ states, the isospin admixtures in the ground state of the even-even nuclei, and the differential cross-section for the²⁰⁸Pb(³He,t)²⁰⁸Bi reaction atE(³He)=450 MeV. Both couplings of the excitation branches withT ^z = T⁰ ± 1, and the analog state with isovector monopole resonance (IVMR) in the quasi-particle representation were taken into account in our calculations. As a result of these calculations, it was seen that the pairing correlations between nucleons have no considerable effect on theT = 23 isospin admixture in the ground state of the²⁰⁸Pb nucleus, and they cause partially an increase in the isospin impurity of the isobar analog resonance (IAR). It was also established that these correlations have changed the isospin structure of the IAR states, and shifted the energies of the IVMR states to the higher values.     

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.     

Uncertainties in the 0νββ-decay nuclear matrix elements

The nuclear matrix elements M ^0ν of the neutrinoless double-beta decay (0νβ β) of most nuclei with known 2νββ-decay rates are systematically evaluated using the Quasiparticle Random Phase Approximation (QRPA) and Renormalized QRPA (RQRPA). The experimental 2νβ β-decay rate is used to adjust the most relevant parameter, the strength of the particle-particle interaction. With such procedure the M ^0ν values become essentially independent of single-particle basis size, the axial vector quenching factor, etc. Theoretical arguments in favor of the adopted way of determining the interaction parameters are presented. It is suggested that most of the spread among the published M ^0ν ’s can be ascribed to the choices of implicit and explicit parameters, inherent to the QRPA method. Presented by V. Rodin at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.     

揭开中微子和反中微子的马约拉纳神秘面纱——无中微子双贝塔衰变低温晶体量热器实验

中微子可能的马约拉纳粒子属性超出了目前标准模型的范畴,是粒子物理与核物理研究领域最重要的科学问题之一。无中微子双贝塔衰变(0vββ)实验是能够确定中微子马约拉纳属性的唯一途径。0vββ的发现可以揭示中微子绝对质量、轻子数破缺、物质—反物质不对称等一系列自然奥秘,是当今粒子物理与核物理研究的前沿课题。在探索无中微子双贝塔衰变的可选择实验方案中,低温晶体量热器具有高能量分辨率、高运行稳定性和低辐射本底的技术优势,成为新一代0vββ实验最具竞争力的探测器技术之一。文章首先介绍无中微子双贝塔衰变的研究历史,之后介绍低温晶体量热器及其最先进的代表——CUORE实验,最后展望关于我国锦屏地下实验室开展低温晶体量热器0vββ实验研究的前景。     

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.     

Double beta decay: A problem of particle, nuclear and atomic physics

A brief review of recent progress in the field of double beta decay is presented. Different aspects from particle, nuclear and atomic physics of both two-neutrino (2νββ) and neutrinoless (0νββ) modes of the double β decay are discussed. It is argued that the R-parity violating supersymmetry (SUSY) contributes to the 0νββ-decay predominantly via charged pion-exchange between decaying nucleons. Further, a problem of reliable determination of the 0νββ-decay nuclear matrix elements (NMEs) is addressed. It is manifested that the uncertainty associated with the calculation of the 0νββ-decay NMEs can be diminished by suitably chosen nuclear probes. A new possibility for the study of lepton number non-conservation is proposed, namely oscillations plus deexcitations of neutral atoms. A phenomenological analysis of this process leads to a resonant enhancement of the neutrinoless double electron capture, that has a Breit-Wigner form.     

Resonance enhancement of neutrinoless double electron capture

The process of neutrinoless double electron (0νECEC$0\nu \text{ECEC}$) capture is revisited for those cases where the two participating atoms are nearly degenerate in mass. The theoretical framework is the formalism of an oscillation of two atoms with different total lepton number (and parity), one of which can be in an excited state so that mass degeneracy is realized. In such a case and assuming light Majorana neutrinos, the two atoms will be in a mixed configuration with respect to the weak interaction. A resonant enhancement of transitions between such pairs of atoms will occur, which could be detected by the subsequent electromagnetic de-excitation of the excited state of the daughter atom and nucleus. Available data of atomic masses, as well as nuclear and atomic excitations are used to select the most likely candidates for the resonant transitions. Assuming an effective mass for the Majorana neutrino of 1 eV, some half-lives are predicted to be as low as 10²² years in the unitary limit. It is argued that, in order to obtain more accurate predictions for the 0νECEC$0\nu \text{ECEC}$ half-lives, precision mass measurements of the atoms involved are necessary, which can readily be accomplished by today?s high precision Penning traps. Further advancements also require a better understanding of high-lying excited states of the final nuclei (i.e. excitation energy, angular momentum and parity) and the calculation of the nuclear matrix elements.     

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.     

β衰变理论的提出

本文试图通过对β衰变理论提出的历史过程的叙述，加深我们对基本物理原理的认识。