15 research outputs found
Muon capture rates: Evaluation within the quasiparticle random phase approximation
The quasiparticle random phase approximation is used in evaluation of the total muon capture rates for final nuclei participating in double-β decay. Several variants of the method are used, depending on the size of the single-particle model space used, or treatment of the initial bound muon wave function. The resulting capture rates are all reasonably close to each other. In particular, the variant that appears to be most realistic results in rates that are in good agreement with the experimental values. There is no necessity for an empirical quenching of the axial current coupling constant g_A. Its standard value g_A = 1.27 seems to be adequate
Reexamining the light neutrino exchange mechanism of the 0νββ decay with left- and right-handed leptonic and hadronic currents
The extension of the Majorana neutrino mass mechanism of the neutrinoless double-beta decay (0νββ) with the inclusion of right-handed leptonic and hadronic currents is revisited. While only the exchange of light neutrinos is assumed, the s_(1/2) and p_(1/2) states of emitted electrons as well as recoil corrections to the nucleon currents are taken into account. Within the standard approximations the decay rate is factorized into a sum of products of kinematical phase-space factors, nuclear matrix elements, and the fundamental parameters that characterize the lepton number violation. Unlike in the previous treatments, the induced pseudoscalar term of hadron current is included, resulting in additional nuclear matrix elements. An improved numerical computation of the phase-space factors is presented, based on the exact Dirac wave functions of the s_(1/2) and p_(1/2) electrons with finite nuclear size and electron screening taken into account. The dependence of values of these phase-space factors on the different approximation schemes used in evaluation of electron wave functions is discussed. The upper limits for effective neutrino mass and the parameters ⟨λ⟩ and ⟨η⟩ characterizing the right-handed current mechanism are deduced from data on the 0νββ decay of ^(76)Ge and ^(136)Xe using nuclear matrix elements calculated within the nuclear shell model and quasiparticle random phase approximation. The differential decay rates, i.e., the angular correlations and the single electron energy distributions for various combinations of the total lepton number violating parameters that can help to disentangle the possible mechanism, are described and discussed
The Nd(He,) and Sm(,He) reactions with applications to decay of Nd
The Nd(He,) reaction at 140 MeV/u and Sm(,He)
reaction at 115 MeV/u were measured, populating excited states in Pm.
The transitions studied populate intermediate states of importance for the
(neutrinoless) decay of Nd to Sm. Monopole and
dipole contributions to the measured excitation-energy spectra were extracted
by using multipole decomposition analyses. The experimental results were
compared with theoretical calculations obtained within the framework of
Quasiparticle Random-Phase Approximation (QRPA), which is one of the main
methods employed for estimating the half-life of the neutrinoless
decay () of Nd. The present results thus provide useful
information on the neutrino responses for evaluating the and
matrix elements. The matrix element
calculated from the Gamow-Teller transitions through the lowest state
in the intermediate nucleus is maximally about half of that deduced from the
half-life measured in direct counting experiments and at least
several transitions through intermediate states in Pm are
required to explain the half-life.
Because Gamow-Teller transitions in the Sm(,He) experiment are
strongly Pauli-blocked, the extraction of Gamow-Teller strengths was
complicated by the excitation of the , ,
isovector spin-flip giant monopole resonance (IVSGMR). However, the near
absence of Gamow-Teller transition strength made it possible to cleanly
identify this resonance, and the strength observed is consistent with the full
exhaustion of the non-energy-weighted sum rule for the IVSGMR.Comment: 18 pages, 13 figures, 2 table
Unique forbidden beta decays and neutrino mass
The measurement of the electron energy spectrum in single β decays close to the endpoint provides a direct determination of the neutrino masses. The most sensitive experiments use β decays with low Q value, e.g. KATRIN (tritium) and MARE (rhenium). We present the theoretical spectral shape of electrons emitted in the first, second, and fourth unique forbidden β decays. Our findings show that the Kurie functions for these unique forbidden β transitions are linear in the limit of massless neutrinos like the Kurie function of the allowed β decay of tritium
The λ Mechanism of the 0νββ-Decay
The λ mechanism (WL − WR exchange) of the neutrinoless double beta decay (0νββ-decay), which has origin in left-right symmetric model with right-handed gauge boson at TeV scale, is investigated. The revisited formalism of the 0νββ-decay, which includes higher order terms of nucleon current, is exploited. The corresponding nuclear matrix elements are calculated within quasiparticle random phase approximation with partial restoration of the isospin symmetry for nuclei of experimental interest. A possibility to distinguish between the conventional light neutrino mass (WL − WL exchange) and λ mechanisms by observation of the 0νββ-decay in several nuclei is discussed. A qualitative comparison of effective lepton number violating couplings associated with these two mechanisms is performed. By making viable assumption about the seesaw type mixing of light and heavy neutrinos with the value of Dirac mass mD within the range 1 MeV < mD < 1 GeV, it is concluded that there is a dominance of the conventional light neutrino mass mechanism in the decay rate
Reactor antineutrino spectra and forbidden beta decays
The exact relativistic shape factors, associated with the nuclear matrix elements governing the first forbidden beta decays, are presented. It is expected that their consideration can allow a more accurate theoretical description of antineutrino fluxes from the power reactor. A qualitative analysis of the uncertainty of reactor antineutrino flux from 235U within the electron spectrum conversion method is performed