The majorana neutrinoless double-beta decay experiment

The proposed Majorana double-beta decay experiment is based on an array of segmented intrinsic Ge detectors with a total mass of 500 kg of Ge isotopically enriched to 86% in ⁷⁶Ge. A discussion is given of background reduction by material selection, detector segmentation, pulse shape analysis, and electroformation of copper parts and granularity. Predictions of the experimental sensitivity are given. For an experimental running time of 10 years over the construction and operation oft he Majorana setup, a sensitivity of T _1/2 ^0ν ∼4×10²⁷ yr is predicted. This corresponds to 〈mν〉∼0.003−0.004 eV according to recent QRPA and RQRPA matrix element calculations. From Yadernaya Fizika, Vol. 67, No. 11, 2004, pp. 2025–2032. Original English Text Copyright ? 2004 by Aalseth, Anderson, Arthur, Avignone III, Baktash, Ball, Barabash, Brodzinski, Brudanin, Bugg, Champagne, Chan, Cianciolo, Collar, Creswick, Doe, Dunham, Easterday, Efremenko, Egorov, Ejiri, Elliott, Ely, Fallon, Farach, Gaitskell, Gehman, Grzywacz, Hazma, Hime, Hossbach, Jordan, Kazkaz, Kephart, King III, Kochetov, Konovalov, Kouzes, Lesko, Macchiavelli, Miley, Mills, Nomachi, Palms, Pitts, Poon, Radford, Reeves, Robertson, Rohm, Rykaczewski, Saborov, Sandukovsky, Shawley, Stekhanov, Tornow, van de Water, Vetter, Warner, Webb, Wilkerson, Wouters, Young, Yumatov. This article was submitted by the authors in English. The authors represent the Majorana Collaboration     

Non-collapsing QRPA for neutrinoless double beta decay

The standard quasiparticle random phase approximation(QRPA) is widely used to describe the neutrinoless double beta decay process. Although it has been quite successful in many cases of interest, it has some shortcomings. The most important one is that its solutions collapse for physical values of the particle-particle strength. We shall show that modifications can be done which can extend the validity of this standard QRPA beyond the point of collapse. Such modifications are: The introduction of proton-neutron pairing, the inclusion of the Pauli principle and the extension of the Hilbert space. If all these modifications are introduced into the standard QRPA then the collapse does not occur for physical values of the particle-particle strengths. Thus, one might be able to extract more accurate values on the effective neutrino mass by using the best available experimental limits on the half life of neutrinoless double beta decay. Presented by G. Pantis at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’97), Prague, May 27–31, 1997.     

Majorana neutrino masses and the neutrinoless double-beta decay

Neutrinoless double-beta decay is forbidden in the Standard Model of electroweak and strong interaction but allowed in most Grand Unified Theories (GUTs). Only if the neutrino is a Majorana particle (identical with its antiparticle) and if it has a mass is neutrinoless double-beta decay allowed. Apart from 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 one 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 neutrinoless double-beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present work, one discusses the accuracy of the present status of calculating of the nuclear matrix elements and the corresponding limits of GUTs and supersymmetric parameters. The text was submitted by the author in English.     

A possible probe of neutrinoless double-beta decay nuclear matrix elements

Future experiments on the search for the 0??|? decay will be sensitive to the effective Majorana mass in the region of the inverted mass hierarchy. If a positive signal is observed, a possibility to test models of calculation of nuclear matrix elements of the process will appear. We discuss this possibility in some detail.     

无中微子双贝塔衰变实验发展沿革与未来展望

无中微子双贝塔衰变是一种重要的超出标准模型的新物理。在数十年理论和实验的发展与探索的基础上,人们对其可能的物理机制以及实验的技术需求已有较为深刻的理解。国际上有多个实验组通过不同的探测器技术尝试寻找无中微子双贝塔衰变事件,并对半衰期下限给出了10²⁶年量级的限制。目前各实验组正在积极进行下一代实验装置的预研和建设,致力于将半衰期灵敏度提高到10²⁷年以上。中国正依托锦屏地下实验室国际领先的实验环境,开发多个不同路线的探测技术。文章将概述国际上主要的大型无中微子双贝塔衰变实验的现状,并展示在锦屏地下实验室中探索这一前沿物理领域的前景以及基于不同探测器技术的实验方案。     

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.     

A large Hilbert space QRPA and RQRPA calculation of neutrinoless double beta decay

A large Hilbert space is used for the calculation of the nuclear matrix elements governing the light neutrino mass mediated mode of neutrinoless double beta decay (Ovββ-decay) of⁷⁶ Ge,¹⁰⁰ Mo,¹¹⁶ Cd,¹²⁸ Te, and¹³⁶ Xe within the proton-neutron quasiparticle random phase approximation (pn-QRPA) and the renormalized QRPA with proton-neutron pairing (full-RQRPA) methods. We have found that the nuclear matrix elements obtained with the standard pn-QRPA for several nuclear transitions are extremely sensitive to the renormalization of the particle-particle component of the residual interaction of the nuclear hamiltonian. Therefore the standard pn-QRPA does not guarantee the necessary accuracy to allow us to extract a reliable limit on the effective neutrino mass. This behavior already known from the calculation of the two-neutrino double beta decay matrix elements, manifests itself in the neutrinoless double-beta decay but only if a large model space is used. The full-RQRPA, which takes into account proton-neutron pairing and considers the Pauli principle in an approximate way, offers a stable solution in the physically acceptable region of the particle-particle strength. In this way more accurate values on the effective neutrino mass have been deduced from the experimental lower limits of the half-lifes of neutrinoless double beta decay.     

Comparison of various extensions of the QRPA formalism for the double-beta decay

We have used a self-consistent version of the BCS + RQRPA method for a systematic study of the double-beta decay of medium-heavy nuclei with 70 A 100. The results have been compared with the previously used approaches, namely the QRPA and the RQRPA approximations. We have shown that inclusion of the quasiparticle correlations at the BCS level reduces ground state correlations in the particle–particle channel of the proton–neutron interaction, resulting in a systematic reduction of the double-beta-decay matrix elements. We also simplified the RQRPA equations significantly obtaining a low-dimensioned set of linear equations for the quasiparticle densities.     

Mechanisms of neutrinoless double-beta decay: A comparative analysis of several nuclei

The neutrinoless double beta decay of several nuclei that are of interest from the experimental point of view (⁷⁶Ge, ⁸²Se, ¹⁰⁰Mo, ¹³⁰Te, and ¹³⁶Xe) is investigated on the basis of a general Lorentzinvariant effective Lagrangian describing physics effects beyond the Standard Model. The half-lives and angular-correlation coefficients for electrons are calculated for various decay mechanisms associated, in particular, with the exchange of Majorana neutrinos, supersymmetric particles (with R-parity violation), leptoquarks, and right-handed W _R bosons. The effect of theoretical uncertainties in the values of relevant nuclear matrix elements on decay features is considered.     

Angular distribution of electrons in neutrinoless double-beta decay and new physics

For neutrinoless double-beta decay caused by the exchange of light Majorana neutrinos, an expression for the differential width with respect to the angle between the final-electron momenta is obtained on the basis of a Lorentz-invariant effective Lagrangian of the general form. The shape of this angular distribution is analyzed within various extensions of the Standard Model that allow this process—in particular, within theories that involve Majorana super partners and (or) right-handed currents. The angular correlation coefficient for electrons as a function of the mass of the right-handed W boson and the effective Majorana neutrino mass in the decay of the ⁷⁶Ge nucleus is considered within the model involving left—right symmetry.     

The role of the induced currents in the mass mechanism of neutrinoless double-beta decay

The nuclear matrix elements of Majorana neutrino mass mechanism of neutrinoless double-beta decay have so far been calculated using only contributions from vector axial-vector part and weak magnetism of the nuclear current, while other contributions have been neglected. In the present work we are examining the effect of weak magnetism and induced pseudoscalar coupling. We have performed calculations within the proton-neutron renormalized quasiparticle random phase approximation and we have found that these additional contributions of the nucleon current, result in a considerable reduction of the nuclear matrix elements of all nuclei which we have considered. This reduction of the nuclear matrix element makes the extracted limits of the lepton number violating parameters less stringent yielding the best value for less than 0.51 eV for ⁷⁶Ge.     

Chiral two-body currents in nuclei: Gamow-Teller transitions and neutrinoless double-beta decay

We show that chiral effective field theory (EFT) two-body currents provide important contributions to the quenching of low-momentum-transfer Gamow-Teller transitions, and use chiral EFT to predict the momentum-transfer dependence that is probed in neutrinoless double-beta (0νββ) decay. We then calculate for the first time the 0νββ decay operator based on chiral EFT currents and study the nuclear matrix elements at successive orders. The contributions from chiral two-body currents are significant and should be included in all calculations.     

The effect of weak magnetism and induced pseudoscalar coupling in neutrinoless double-beta decay

In calculating the amplitude of the majorana neutrino-mass mechanism of neutrinoless double-beta decay (0νββ-decay), several approximations of the nucleon current have been done. for example, effects from induced current such as weak magnetism and pseudoscalar coupling have been neglected. we shall show in this work that, although such terms do not contribute significantly to the 2νββ-decay amplitude, they are important in the case of 0νββ decay. performing calculations within the renormalized quasiparticle random phase approximation (pn-rqrpa) for all nuclei undergoing double-beta decay in the region a=76 to a=150, we have found that these additional contributions of the nucleon current reduce considerably the matrix elements in all cases for the light neutrino as well as for the heavy neutrino mass. in the light-neutrino mass, we find reductions up to thirty percent, while in the heavy-neutrino mass, up to almost a factor of five. these reductions make the limits on the lepton-number-violating parameters 〈m _ν〉 and η_N less stringent.     

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 Fréjus underground laboratory since February 2003, is devoted to the search for neutrinoless double-beta decay (beta beta 0v). The half-lives of the two neutrino double-beta decay (beta beta 2v) 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 approximately 7 kg of 100Mo and approximately 1 kg of 82Se. The corresponding limits are T1/2(beta beta0v) > 4.6 x 10(23) yr for 100Mo and T1/2(beta beta 0v) > 1.0 x 10(23) yr for 82Se (90% C.L.). Depending on the nuclear matrix element calculation, the limits for the effective Majorana neutrino mass are < 0.7-2.8 e/v for 100Mo and < 1.7-4.9 eV for 82Se.     

Minimal supersymmetric standard model with the gauge mediated supersymmetry breaking and the neutrinoless double-beta decay

Neutrinoless double-beta decay within Minimal Supersymmetric Standard Model with gauge mediated supersymmetry breaking is considered. Limits on R-parity breaking constant coming from non-observability of 0 in ⁷⁶Ge are found. The dependence of on different parameters at the messenger scale M are shown, with special attention paid to nuclear part of calculations. We have found that strongly depends on the effective supersymmetry breaking scale only and deduced limits imposed on this non-standard parameter by the germanium experiment.     

Towards the meV limit of the effective neutrino mass in neutrinoless double-beta decays

We emphasize that it is extremely important for future neutrinoless double-beta(0νββ)decay experiments to reach the sensitivity to the effective neutrino mass|mββ|≈1 meV.With such a sensitivity,it is highly possible to discover the signals of 0νββ decays.If no signal is observed at this sensitivity level,then either neutrinos are Dirac particles or stringent constraints can be placed on their Majorana masses.In this paper,assuming the sensitivity of|mββ|≈1 meV for future 0νββ decay experiments and the precisions on neutrion oscillation parameters after the JUNO experiment,we fully explore the constrained regions of the lightest neutrino mass m1 and two Majorana-type CP-violating phases{ρ,σ}.Several important conclusions in the case of normal neutrino mass ordering can be made.First,the lightest neutrino mass is severely constrained to a narrow range m1∈[0.7,8]meV,which together with the precision measurements of neutrino mass-squared differences from oscillation experiments completely determines the neutrino mass spectrum m2∈[8.6,11.7]meV ing phases is limited to ρ∈[130°,230°],which cannot be obtained from any other realistic experiments.Third,the sum of three neutrino masses is found to beΣ≡m1+m2+m3∈[59.2,72.6]meV,while the effective neutrino mass for beta decays turns out to be mβ≡(|Ue1|2m1^2+|Ue2|2m2^2+|Ue3|2m3^2)1/2∈[8.9,12.6]meV.These observations clearly set up the roadmap for future non-oscillation neutrino experiments aiming to solve the fundamental problems in neutrino physics.     

Testing the single-state dominance in two-neutrino double-beta decay

The single-state-dominance hypothesis (SSDH) states that the decay rates of the two-neutrino double beta decay are governed by a virtual two-step transition connecting the initial and final ground states through the first 1⁺ state, 1 ₁ ⁺ , of the intermediate odd-odd nucleus, for those odd-odd nuclei where the 1 ₁ ⁺ state is the ground state. To investigate the validity the SSDH we have performed a systematical theoretical analysis of all known double-beta-decay transitions where the SSDH conditions are fulfilled. The analysis shows that the SSDH is realized either through a true dominance of the first intermediate 1⁺ state or by cancellations among the contributions of higher lying 1⁺ states of the intermediate nucleus.