Limits of Majorana neutrino mass from combined analysis of data from <Superscript>76</Superscript>Ge and <Superscript>136</Superscript>Xe neutrinoless double beta decay experiments

We present effective Majorana neutrino mass limits <m _ββ> obtained from the joint analysis of the recently published results of ⁷⁶Ge and ¹³⁶Xe neutrinoless double beta decay (0νββ) experiments, which was carried out by using the Bayesian calculations. Nuclear matrix elements (NMEs) used for the analysis are taken from the works, in which NMEs of ⁷⁶Ge and ¹³⁶Xe were simultaneously calculated. This reduced systematic errors connected with NME calculation techniques. The new effective Majorana neutrino mass limits <m _ββ> less than [85.4–197.0] meV are much closer to the inverse neutrino mass hierarchy region.     

Double-beta decay and neutrino mass

Recent achievements in the study of double-beta (ββ) decay are presented. We discuss the potential of this process to search, beyond Standard Model physics, for the QRPA-based methods used for the calculation of the relevant nuclear matrix elements and the derivation of the neutrino mass from both ββ-decay calculations and neutrino oscillation and cosmological data. The key position of the ββ-decay experiments in resolving the neutrino absolute mass is highlighted.     

<Emphasis Type="Italic">O</Emphasis>(1) eV sterile neutrino in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>) gravity

We refer [1] to the role of an additional O(1) eV sterile neutrino in modified gravity models. We find parameter constraints in particular f(R) gravity model using following up-to-dated cosmological data: measurements of the cosmic microwave background (CMB) anisotropy, the CMB lensing potential, the baryon acoustic oscillations (BAO), the cluster mass function and the Hubble constant. It was obtained for the sterile neutrino mass 0.47 eV < m _ν,sterile < 1 eV (2σ) assuming that the sterile neutrinos are thermalized and the active neutrinos are massless, not significantly larger than in the standard cosmology model within the same data set: 0.45 eV < m _ν,sterile < 0.92 eV (2σ). But, if the mass of sterile neutrino is fixed and equals ≈ 1.5 eV according to various anomalies in neutrino oscillation experiments, f(R) gravity is much more consistent with observation data than the CDM model.     

CAMEO project and discovery potential of the future 2<Emphasis Type="Italic">β</Emphasis>-decay experiments

The demands on the future supersensitivity 2β-decay experiments (aiming to observe neutrinoless 2β decay or to advance restrictions on the neutrino mass to m_ν≤0.01 eV) are considered and requirements for their discovery potential are formulated. The most realistic 2β projects are reviewed and the conclusion is obtained that only several of them with high energy resolution would completely satisfy these severe demands and requirements. At the same time, most of the recent projects (CAMEO, CUORE, DCBA, EXO, etc.) could certainly advance the limit on the neutrino mass up to m_ν≤0.05 eV.     

Present and future strategies for neutrinoless double beta decay searches

The renewed interest shown in these days towards neutrinoless double beta decay, a lepton number violating process which can take place only if neutrinos are Majorana particles (ν = \(\bar \nu \)) with a nonvanishing mass, is justified by the fact that the Majorana nature of neutrinos is expected in many theories beyond the Standard Model. We also now know, thanks to the neutrino oscillation experiments, that neutrinos are in fact massive, as expected in these theories and not requested in the Standard Model. Moreover, since neutrino oscillation experiments measure only the absolute value of the difference of the square of the neutrino masses, the discovery of neutrinoless double beta decay would help to disentangle questions that still remain unsolved: what is the absolute mass scale of the neutrinos and which mass hierarchy (normal, inverted or quasi-degenerate) is the correct one?The scope of this paper is not only to review the present results reached in the field by the different groups and technologies worldwide, but also to illustrate and comment on the (near and long-term) future strategies that experimentalists are trying to pursue to reach the needed sensitivity required to explore the inverted hierarchy neutrino mass scale.     

Constraining the lightest neutrino mass and <Emphasis Type="BoldItalic">m</Emphasis> <Subscript> <Emphasis Type="BoldItalic">ee</Emphasis> </Subscript> from general lepton mass matrices

Despite spectacular advances in fixing the neutrino mass and mixing parameters through various neutrino oscillation experiments, we still have little knowledge about the magnitudes of some vital parameters in the neutrino sector such as the absolute neutrino mass scale, effective Majorana mass m_ee measured in neutrinoless double beta decay. In this context, the present work aims to make an attempt to obtain some bounds for m_ee and the lightest neutrino mass using fairly general lepton mass matrices in the Standard Model.     

Neutrinos in the time of Higgs

In this paper, the recent progress in the determination of neutrino oscillation parameters and future prospects have been discussed. The tiny neutrino masses as inferred from oscillation data and cosmology cannot be explained naturally by the Higgs mechanism and warrant some new physics. The latter can be connected to the Majorana nature of the neutrinos which can be probed by neutrinoless double beta decay (0 νββ). The paper also summarizes the latest experimental results in 0 νββ and discusses some implications for the left–right symmetric model which could be a plausible new physics scenario for the generation of neutrino masses.     

First results of the observation of <Superscript>7</Superscript>Be solar neutrinos with the BOREXINO detector

The results of a direct measurement of the counting rate for solar neutrinos from the electron-capture process on ⁷Be, ⁷Be(e ^?, ν _e )⁷Li(E _ν = 0.862 MeV), with the low-background scintillation detector BOREXINO are presented. This is the first ever real-time observation of a signal from solar neutrinos of energy below 1 MeV. The counting rate for monoenergetic beryllium neutrinos in the BOREXINO detector proved to be 47 ± 7 (stat.) ± 12 (syst.) counts/(day × 100 t), which is in agreement with the predictions of the standard solar model and the hypothesis of neutrino oscillations in matter with parameters in the LMA region.     

Dark matter from the SU(4) model

The left-right symmetric Pati-Salam model of the unification of quarks and leptons is based on the SU(4) and SU(2)×SU(2) symmetry groups. These groups are naturally extended to include the classification of families of quarks and leptons. We assume that the family group (the group which unites the families) is also the SU(4) group. The properties of the fourth generation of fermions are the same as those of the ordinary-matter fermions in the first three generations except for the family charge of the SU(4)_F group: F=(1/3, 1/3, 1/3, ?1), where F=1/3 for fermions of ordinary matter and F=?1 for the fourth-generation fermions. The difference in F does not allow mixing between ordinary and fourth-generation fermions. Because of the conservation of the Fcharge, the creation of baryons and leptons in the process of electroweak baryogenesis must be accompanied by the creation of fermions of the fourth generation. As a result, the excess n _B of baryons over antibaryons leads to the excess n_4ν=N?N? of neutrinos over antineutrinos in the fourth generation with n_4ν=n _B . This massive neutrino may form nonbaryonic dark matter. In principle, the mass density of the fourth neutrino n_4νm _N in the Universe can make the main contribution to dark matter, since the lower bound on the neutrino mass m _N from the data on decay of the Z bosons is m _N <m _Z /2. The straightforward prediction of this model leads to the amount of cold dark matter relative to baryons, which is an order of magnitude higher than allowed by observations. This inconsistency may be avoided by nonconservation of the F charge.     

Neutrino properties from high-energy astrophysical neutrinos

It is shown how high-energy neutrino beams from very distant sources can be utilized to learn about some properties of neutrinos such as lifetimes. Furthermore, even mixing elements such as U_e3 and the CP-violating phase in the neutrino mixing matrix can be measured in principle. Pseudo-Dirac mass differences as small as 10^?18 eV² can be probed as well.     

First measurements in search for keV sterile neutrino in tritium beta-decay in the Troitsk nu-mass experiment

We present the first results of precision measurements of tritium -decay spectrum in the electron energy range 16–18.6 keV by the Troitsk nu-mass experiment. The goal is to find distortions that may be caused by the existence of heavy sterile neutrinos. A signature would correspond to a kink in the spectrum with characteristic shape and end point shifted by the value of a heavy neutrino mass. We set new upper limits to the neutrino mixing matrix element U _e4 ² , which improve existing limits by a factor of 2 to 5 in the mass range of 0.1–2 keV.     

Search for a Neutrino with a Mass of 0.01–1.0 MeV in Beta Decays of <Superscript>144</Superscript>Ce–<Superscript>144</Superscript>Pr Nuclei

Spectra of electrons from beta decays of ¹⁴⁴Ce–¹⁴⁴Pr nuclei have been measured and analyzed in order to find a contribution from a heavy neutrino. For the neutrino with the mass m_νH in the interval of 150–350 keV, a new upper bound |U_eH|² ≤ (2?5) × 10^?3 at 90% C.L. has been found for the mixing parameter.     

CAMEO/GEM program and future of double-<Emphasis Type="Italic">β</Emphasis>-decay research

The current results and future prospects of the 2β-decay research are reviewed. The requirements for supersensitivity experiments are formulated and a conclusion is derived that, in the developed CAMEO and GEM projects, the restrictions on the neutrino mass would be pushed down to m_ν≤(0.015–0.05) eV. Moreover, the GEM I setup with natural HPGe detectors could advance the best current limits on the existence of neutralinos—as dark matter candidates—by three order of magnitudes and, at the same time, would be able to identify unambiguously the dark matter signal by detection of its seasonal modulation.     

Effect of Majorana Phases in Neutrino Oscillation

Quantum gravity (Planck scale effects) lead to an effective SU(2) _L ×U(1) invariant dimension-5 Lagrangian involving neutrino and Higgs fields. On symmetry breaking, this operator gives rise to correction to the above masses and mixing. The gravitational interaction M _X =M _pl , we find that for degenerate neutrino mass spectrum, it is shown that the Majorana phase of the neutrino mixing matrix can effects in neutrino oscillation probability.     

Massenspektrometrischer Nachweis von ββ-Zerfallsprodukten

The principles and methods of mass-spectrometric detection ofββ-decay products and the theoretical purposes for such studies are discussed in detail. We report also the results of our new measurements on two tellurides, one native tellurium sample and one selenide. Concerning the decay\(Te^{130} \underrightarrow {\beta - \beta - }Xe^{130} \), we have found that Xe¹³⁰ in a native tellurium sample amounts to 67% of the total xenon with an absence of any other anomaly, which is an important improvement over previous measurements. —For the first time the decay\(Se^{82} \underrightarrow {\beta - \beta - }Kr^{82} \) was studied by massspectrometry. A Kr⁸²-excess was positively found in an copper selenide. A detailed discussion of all processes other than doubleβ-decay which might contribute to the Xe¹³⁰ and Kr⁸²-anomalies is undertaken. It can be shown that these effects are small compared to theββ-decay process. It can be said now positively thatββ-decay occurs in nature. Discussing all available data we obtained the effective half-life of Te¹³⁰ to be 6×10^20±0.3 y and that of Se⁸² equals to 6 × 10^19±0.3 y. The results are in agreement with the theoretical predictions for the 2ν-ββ-decay. Nevertheless, no-neutrinoββ-decay is still possible. However, from the data we calculated that the leptonnonconservation is at most 0.7%.     

Scalar-leptoquark contributions to the neutrino magnetic moment

On the basis of astrophysical data on the neutrino magnetic moment, μ _ν < 3 × 10⁻¹² μ _B , constraints on the scalar-leptoquark masses are found within the minimal model involving four-color symmetry. It is shown that data on the neutrino magnetic moment are compatible with the mixing-parameter range that admits the existence of scalar leptoquarks whose masses are below 1 TeV, reaching experimental limits obtained from direct searches. In the case of mass degeneracy for the scalar leptoquarks S _m of electric charge Q = 2/3, the constraint m _{S m} > 330 GeV is obtained, which is independent of the mixing parameters of the model. The results are compared with the predictions of other leptoquark models. Original Russian Text ? A.V. Povarov, 2007, published in Yadernaya Fizika, 2007, Vol. 70, No. 5, pp. 905–911.     

Status of neutrinoless double beta decay searches

Neutrinoless double beta decay is one of the most sensitive tools in non-accelerator particle physics to probe the regime of physics beyond the standard model. It can provide in fact fundamental informations on the character of neutrinos and their absolute mass scale. The present status of experiments searching for neutrinoless double-beta decay (ββ(0ν)) is reviewed and the most relevant results discussed. Phenomenological aspects of ββ(0ν) are introduced. Given the observation of neutrino oscillations and the present knowledge of neutrino masses and mixing parameters, a possibility to observe ββ(0ν) at a neutrino mass scale m _ν in the range 10–50 meV could actually exist. The achievement of the required experimental sensitivity is a real challenge faced by a number of new proposed projects. A review of the various proposed experiments in the context of their figure-of-merit parameters is given. The most important parameters contributing to the experimental sensitivity are finally outlined. A short discussion on nuclear matrix element calculations is also given.     

On emission from a hydrogen-like atom

A solution of the Dirac equation for an electron in the field of a point nucleus (Ze) has been obtained as an eigenfunction of the Schrödinger Hamiltonian and the spin projection operator Σ₃. With the use of this solution, the probability W^(ν) of the emission of a neutrino per unit time from a hydrogen-like atom, \((Ze)* \to (Ze) + \nu \bar \nu\), has been calculated for the first time in the first order of the parameter Ze ? 1. The probability W^(ν) appears to be rather small, and the corresponding lifetime τ^(ν) = [W^(ν)]^–1 is much larger than the age of the Universe; correspondingly, this process cannot affect the balance of low-energy neutrinos. The smallness of W^(ν) is due not only to the presence of the obvious “weak” factor (Gm_p²)²(m/mp)⁴ in the expression for W^(ν), but also primarily to the “electromagnetic” factor (Zα)¹², which can be revealed only in a particular calculation. It has been argued within quantum electrodynamics with the mentioned wavefunctions that photon emission, (Ze)* → (Ze) + γ, can be absent (analysis of photon emission requires the further development of the method), whereas axion emission, (Ze)* → (Ze) + a, can occur, although the last two effects have not been considered in detail.     

Hierarchical matrices in the see-saw mechanism,large neutrino mixing and leptogenesis

We consider the see-saw mechanism for hierarchical Dirac and Majorana neutrino mass matrices m _D and M _R, including the CP violating phases. Simple arguments about the structure of the neutrino mass matrix and the requirement of successful leptogenesis lead to the situation that one of the right-handed Majorana neutrinos is much heavier than the other two, which in turn display a rather mild hierarchy. It is investigated how for the neutrino mixing one small and two large mixing angles are generated. The mixing matrix element |U _e3|² is larger than 10^-3 and a characteristic ratio between the branching ratios of lepton flavor violating charged lepton decays is found. Successful leptogenesis implies sizable CP violation in oscillation experiments. As in the original minimal see-saw model, the signs of the baryon asymmetry of the universe and of the CP asymmetry in neutrino oscillations are equal and there is no connection between the leptogenesis phase and the effective mass as measurable in neutrinoless double beta decay.Received: 28 May 2003, Revised: 13 September 2003, Published online: 26 November 2003