11 research outputs found
Two neutrino double beta decay within the -approximation
We examine the contributions of odd-parity nuclear operators to the
two-neutrino double beta decay amplitude, which come from
the -wave Coulomb corrections to the electron wave functions and the recoil
corrections to the nuclear currents. Although they are formally of higher order
in or of the nucleon than the usual Fermi and Gamow-Teller
matrix elements, explicit calculations performed within the QRPA show that they
are significant when confronted with the experimental data.Comment: 9 pages, latex, no figure
Nuclear moments for the neutrinoless double beta decay II
The recently developed formalism for the evaluation of nuclear form factors
in neutrinoless double beta decay is applied to , ,
, , and nuclei. Explicit analytical
expressions that follows from this theoretical development, in the single mode
model for the decay of , have been worked out. They are useful both
for testing the full numerical calculations, and for analytically checking the
consistency with other formalisms. Large configuration space calculations are
compared with previous studies, where alternative formulations were used. Yet,
besides using the G-matrix as residual interaction, we here use a simple
-force. Attention is paid to the connected effects of the short range
nuclear correlations and the finite nucleon size. Constraints on lepton number
violating terms in the weak Hamiltonian (effective neutrino Majorana mass and
effective right-handed current coupling strengths) are deduced.Comment: 18 pages, latex, minor changes, to appear in Nucl. Phys.
Double Beta Decay in pn-QRPA Model with Isospin and SU(4) Symmetry Constraints
The transition matrix elements for the double beta decays
are calculated for , , , , and
nuclei, using a -interaction. As a guide, to fix the
particle-particle interaction strengths, we exploit the fact that the missing
symmetries of the mean field approximation are restored in the random phase
approximation by the residual interaction. Thus, the T=1, S=0 and T=0, S=1
coupling strengths have been estimated by invoking the partial restoration of
the isospin and Wigner SU(4) symmetries, respectively. When this recipe is
strictly applied, the calculation is consistent with the experimental limit for
the lifetime of and it also correctly reproduces the
lifetime of . In this way, however, the two-neutrino matrix elements
for the remaining nuclei are either underestimated (for and
) or overestimated (for and ) approximately by a
factor of 3. With a comparatively small variation () of the spin-triplet
parameter, near the value suggested by the SU(4) symmetry, it is possible to
reproduce the measured in all the cases. The upper limit for
the effective neutrino mass, as obtained from the theoretical estimates of
matrix elements, is eV. The dependence of the nuclear
matrix elements on the size of the configuration space has been also analyzed.Comment: 25 pages (LaTex) and 3 figures upon request, to be published in Nucl.
Phys.