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Within left-right symmetric model a generalization of the Majorana neutrino mass for the case of light and and heavy neutrinos is introduced and analyzed. Further, current status of calculation of the neutrinoless double beta decay matrix elements is shortly reviewed. An important connection between them and matrix element of double Gamow-Teller operator is established. A new way of fixing quenching of axial-vector coupling constant gA is presented.

An explicit expression is derived for the mixing matrix of Majorana neutrinos in terms of the mass matrix and its Frobenius covariants. Illustrative scenarios are taken as examples where the suggested formula is used to obtain the mixing matrix from the mass matrix.

A bstract We investigate the possibility to discriminate between different pairs of CP non-conserving mechanisms inducing the neutrinoless double beta ( ββ ) 0 ν -decay by using data on ( ββ ) 0 ν -decay half-lives of nuclei with largely different nuclear matrix elements (NMEs). The mechanisms studied are: light Majorana neutrino exchange, heavy left-handed (LH) and heavy right-handed (RH) Majorana neutrino exchanges, lepton charge non-conserving couplings in SUSY theories with R -parity breaking giving rise to the “dominant gluino exchange” and the “squark-neutrino” mechanisms. The nuclei considered are 76 Ge, 82 Se, 100 Mo, 130 Te and 136 Xe. Four sets of nuclear matrix elements (NMEs) of the decays of these five nuclei, derived within the Self-consistent Renormalized Quasiparticle Random Phase Approximation (SRQRPA), were employed in our analysis. While for each of the five single mechanisms discussed, the NMEs for 76 Ge, 82 Se, 100 Mo and 130 Te differ relatively little, the relative difference between the NMEs of any two nuclei not exceeding 10%, the NMEs for 136 Xe differ significantly from those of 76 Ge, 82 Se , 100 Mo and 130 Te, being by a factor ~ (1.3 − 2.5) smaller. This allows, in principle, to draw conclusions about the pair of non-interfering (interfering) mechanisms possibly inducing the ( ββ ) 0 ν -decay from data on the half-lives of 136 Xe and of at least one (two) more isotope(s) which can be, e.g., any of the four, 76 Ge , 82 Se , 100 Mo and 130 Te . Depending on the sets of mechanisms considered, the conclusion can be independent of, or can depend on, the NMEs used in the analysis. The implications of the EXO lower bound on the half-life of 136 Xe for the problem studied are also exploited.

We show that the presence in the nuclear medium of lepton number violating four-fermion interactions of neutrinos with quarks from a decaying nucleus could account for an apparent incompatibility among the 0 νββ searches in the laboratory, the direct neutrino mass measurement with the nuclear β-decay and cosmological data.

We study a new mode of the neutrinoless and two-neutrino double-beta decays in which a single electron is emitted from the atom. The other electron is assumed to occupy one of the vacant s1/2 or p1/2 subshells of the daughter ion. Such process could manifest itself through an additional background signal in the single-electron spectra, which will be accessible in the next-generation experiment SuperNEMO. We calculate the phase-space factors in terms of relativistic electron wave functions obtained as the solutions to the Dirac equation and evaluated at the nuclear radius, while taking into account the shielding effect of nuclear charge via the multiconfiguration Dirac-Hartree-Fock package Grasp2K. Half-lives are estimated for the most relevant double-beta-decay isotopes and experimental significance is discussed.

The neutrino mass mechanisms of the neutrinoless double beta decay (0νββ-decay) generated within the left-right symmetric models are discussed by taking advantage of an interpolating formula for nuclear matrix element depending on the neutrino mass. Its application is justified by a weak dependence of mean square neutrino momentum, which is a properly scaled ratio of nuclear matrix elements for the light and heavy neutrino mass mechanisms, on decaying isotope. Due to this fact there is a single unified lepton number violating parameter including dependendence on all neutrino masses in the decay rate and the light and heavy neutrino mass mechanisms can be hardly distinguished by observing the 0νββ-decay on different isotopes. Further, a see-saw type neutrino mixing, which relates neutrino mixing in light and heavy neutrino sectors, is proposed. By considering the current limit on the 0νββ-decay half-life of 136Xe the regions of dominance of exchange of light and heavy neutrinos are identified.

A review of the Baikal-GVD neutrino telescope status after the winter 2024 deployment campaign that results in 4 104 opticalmodules installed on 114 vertical strings is presented. The results of analysis of Baikal-GVD data collected in 2018-2023 show the presence of cosmic neutrino flux in high-energy cascade channel consistent with observations by the IceCube neutrino telescope. Track-like events analysis results in identification of first high-energy muon neutrino candidates.

Baikal-GVD is a cubic-kilometer scale neutrino telescope, which is currently under construction in Lake Baikal. Baikal-GVD is an array of optical modules arranged in clusters. The first cluster of the array has been deployed and commissioned in April 2015. To date, Baikal-GVD consists of 3 clusters with 864 optical modules. One of the vital conditions for optimal energy, position and direction reconstruction of the detected particles is the time calibration of the detector. In this article, we describe calibration equipment and methods used in Baikal-GVD and demonstrate the accuracy of the calibration procedures.

The 2 ν 2 β decay of 150 Nd to the first excited 740.5 keV 0 1 + level of 150 Sm was measured over 5.845 years with the help of a four-crystal low-background HPGe γ spectrometry system in the underground low-background laboratory STELLA of LNGS-INFN. A 2.381 kg highly purified Nd-containing sample was employed as the decay source. The expected de-excitation gamma-quanta of the 0 1 + level with energies 334.0 keV and 406.5 keV were observed both in one-dimensional spectrum and in coincidence data resulting in the half-life T 1 / 2 = [ 0 . 83 - 0.13 + 0.18 ( stat ) - 0.19 + 0.16 ( syst ) ] × 10 20 year. Interpreting an excess of the 334.0-keV peak area as an indication of the 2 β decay of 150 Nd to the 334.0 keV 2 1 + excited level of 150 Sm with a half-life of T 1 / 2 = [ 1 . 5 - 0.6 + 2.3 ( stat ) ± 0.4 ( syst ) ] × 10 20 year, the 2 ν 2 β half-life of 150 Nd for the transition to the 0 1 + level is T 1 / 2 = [ 1 . 03 - 0.22 + 0.35 ( stat ) - 0.19 + 0.16 ( syst ) ] × 10 20 year, in agreement with the previous experiments. Both half-life values reasonably agree with the theoretical calculations in the framework of proton-neutron QRPA with isospin restoration combined with like nucleon QRPA for description of excited states in the final nuclei. For 2 ν 2 β and 0 ν 2 β transitions of 150 Nd and 148 Nd to several excited levels of 150 Sm and 148 Sm , limits were set at level of T 1 / 2 > 10 20 - 10 21 year.

Baikal-GVD is a cubic kilometer-scale neutrino telescope currently under construction in Lake Baikal. The detector’s components are mobile and may drift from their initial coordinates or change their spatial orientation. This introduces a reconstruction error, particularly a timing error for PMT hits. This problem is mitigated by a combination of a hydroacoustic positioning system and per-component acceleration and orientation sensors. Under regular conditions, the average positioning accuracy for a GVD component is estimated to be less than 13 cm.