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.     

Limitation on scalar neutrino masses from Tau decay

The apparent absence of the decay $\text{τ →}\text{ν}{}_{\mathrm{\tau }}\text{ν}{}_{\mathrm{?}}{}^1\text{?,}\text{where}\text{? =}\text{e or}\text{μ}\text{and}\text{ν}{}_{\mathrm{i}}$ is the spin-zero supersymmetric partner of ν_i, implies that the $\text{ν}{}_{\mathrm{i}}$ masses are sufficiently large that the decay is suppressed (assuming the supersymmetric partners exist). We give the resulting limits, which depend on the mass of the W? (the supersymmetric partner of the W) which mediates the decay, and on whether one or both $\text{ν}$ is massive. These are the only experimental limits on $\text{ν}$ masses.     

The link between neutrino masses and proton decay in supersymmetric unification

Following recent joint works with K. Babu and F. Wilczek, I stress here that supersymmetric unification, based on symmetries like SO(10) or a string-derived G(224)=SU(2) _L ×SU(2) _R ×SU(4) ^C possesses some crucial features that are intimately linked to each other. They are (a) gauge-coupling unification; (b) the masses and mixings of all fermions, including especially the neutrinos; and last but not least (c) proton decay. In this context, it is noted that the value of $m_{v_L^\tau } \sim 1/20{\text{ }}eV$ , suggested by the Super-Kamiokande result, goes extremely well with the unification-hypothesis, based on the ideas of (i) SU(4)-color, (ii) left-right symmetry, and (iii) supersymmetry. A concrete proposal is presented within an economical SO(10)-framework that makes five successful predictions for the masses and mixings of the quarks and the charged leptons. The same framework explains why the ν_π?ν_τ oscillation angle is so large $(\sin ^2 2\theta _{\nu _\mu \nu _\tau }^{osc} \approx 0.82 - 0.96)$ and yet V _bc is so small (≈0.04), both in accord with observation. The influence of the masses of the neutrinos and of the charged fermions on proton decay is discussed concretely, within this framework. The $\bar \nu K^ + $ mode is expected to be dominant for SUSY SO(10) as well as SU(5). A distinctive feature of the SO(10) model, however, is the likely prominence of the μ⁺ K ⁰ mode, which, for SU(5), is highly suppressed. Our study shows that while current limits on the rate of proton decaying into $\bar \nu K^ + $ are compatible with theoretical expectations, improvements in these limits by a factor of 5–10 should either turn up events, or else the SO(10)-framework described here, which is otherwise so successful, will be in jeopardy. Prominence of theμ⁺ K ⁰ mode, if observed, will be most significant in that it will reveal the intriguing link that exists between neutrino masses and proton decay in the context of supersymmetric unification.     

Probing grand unification with fermion masses,neutrino oscillations and proton decay

It is noted that a set of facts points to the relevance in four dimensions of conventional supersymmetric unification based on minimally a string-unifiedG(224) symmetry, or maximallySO(10). These include: (i) the observed family structure, (ii) quantization of electric charge, (iii) meeting of the three gauge couplings, (iv) neutrino oscillations (in particular the value of δm ²(νμ−δ _τ), suggested by SuperK), (v) the intricate pattern of the masses and mixings of the fermions, including the smallness ofV _cb and the largeness ofθ _μνμτ ^osc , and (vi) the need for B-L as a generator to implement baryogenesis (via leptogenesis). A concrete proposal is presented within a predictiveSO(10)/G(224) framework that successfully describes the masses and mixings of all fermions, including the neutrinos — with eight predictions, all in agreement with observation. Within this framework, a systematic study of proton decay is carried out, which (a) pays special attention to its dependence on the fermion masses, (b) limits the threshold corrections so as to preserve natural coupling unification, and (c) uses recently improved values of the matrix element and renormalization effects. Allowing for both minimal supersymmetric standard model (MSSM) and its proposed variant, the so-called extended supersymmetric standard model (ESSM), as effective low-energy the ories, the study shows that a conservative upper limit on the proton lifetime is about (l–2)× 10³⁴ years, with ·⁻K⁺ being the dominant decay mode, and quite possibly μ⁺ K ⁰ ande ⁺π⁰ being prominent. This in turn strongly suggests that an improvement in the current sensitivity by a factor of five to ten ought to reveal proton decay. For comparison, some alternatives to the conventional approach to unification pursued here are mentioned at the end. Invited paper presented at the International Summer School held at ICTP, Trieste (June, 2001) and at WHEPP-7 Conference, Allahabad, India (January, 2002). This is an updated version of the paper presented at the Erice School (September, 2000), hep-ph/0106082.     

Neutron-proton pairing and double-beta decay in algebraic models

Nuclear models based on Lie algebras provide insight into collective phenomena. Here neutron-proton pairing, which plays an important role in double beta decay, is discussed in two such models, one based on SO(5) and the second on SO(8). The latter model is used to test the accuracy of the Quasiparticle Random Phase Approximation and improvements to it. Presented at Workshop on calculation of double-beta-decay matrix elements (MEDEX’97), Prague, May 27–31, 1997. This work was supported in part by the U.S. Department of Energy under Grant DE-FG02-97ER41019.     

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.     

Neutrinoless double beta decay with small and hierarchical neutrino mass

We construct a model where neutrino Majorana masses are small and hierarchical but where neutrinoless double beta decay occurs at an observable rate potentially detectable by present day experiments.     

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。     

Unconventional model of neutrino masses

Gelmini and Roncadelli have proposed a model of neutrino masses in which B?L symmetry is spontaneously broken by a small vacuum expectation value of a Higgs triplet. We give an exegesis of this model. We show that the massive neutrinos in this model cannot be cosmologically relevant today and that conflicting analyses of double beta decay experiments can be reconciled.     

Investigation of double-beta decay with the NEMO-3 detector

The double-beta-decay experiment NEMO-3 has been taking data since February 2003. The aim of this experiment is to search for neutrinoless (0 νββ) decay and investigate two neutrino doublebeta decay in seven different isotopically enriched samples (¹⁰⁰Mo, ⁸²Se, ⁴⁸Ca, ⁹⁶Zr, ¹¹⁶Cd, ¹³⁰Te, and ¹⁵⁰Nd). After analysis of the data corresponding to 3.75 yr, no evidence for 0 νββ decay in the 100Mo and 82Se samples was found. The half-life limits at the 90% C.L. are 1.1 × 10²⁴ and 3.6 × 10²³ yr, respectively. Additionally for 0 νββ decay the following limits at the 90% C.L. were obtained, >1.3 × 10²² yr for ⁴⁸Ca, >9.2 × 10²¹ yr for ⁹⁶Zr, and >1.8 × 10²² yr for 150Nd. The 2 νββ decay half-life values were precisely measured for all investigated isotopes.     

Status and perspectives of the COBRA double-beta decay experiment

The COBRA experiment is going to use a large amount of CdZnTe semiconductor detectors to perform a search for various double-beta-decay modes. The current status of the experiment is presented, as well as first results. Improved half-life limits for the ground-state transitions of ⁶⁴Zn and ¹²⁰Te for 0νβ ⁺/EC and 0νECEC have been obtained and the rate of the 4-fold forbidden ¹¹³Cd decays has been measured. A short outlook on future activities is also given.     

Radiative neutrino decay in superstring models

The rate for the radiative decay of neutrinos is calculated in E₆ superstring models with light colour triplet isosinglets h and h. In contrast to the standard model, the rate is not suppressed either by the GIM mechanism or by helicity matching. Comparison with astrophysical data allows one to set an upper limit on the Yukawa couplings QL and λd^cv^ch. Taking it is found that , compatible with m_vc, m_vμ in the range 10–100 eV. From the cosmological requirement of not photo-ionizing light elements after nucleosynthesis, it follows that the v_τ must lie in the same mass range and consequently .