822 research outputs found
Shell Model Study of the Double Beta Decays of Ge, Se and Xe
The lifetimes for the double beta decays of Ge, Se and
Xe are calculated using very large shell model spaces. The two neutrino
matrix elements obtained are in good agreement with the present experimental
data. For eV we predict the following upper bounds to the
half-lives for the neutrinoless mode: , and . These results are the first from a new generation of Shell
Model calculations reaching O(10) dimensions
A large Hilbert space QRPA and RQRPA calculation of neutrinoless double beta decay
A large Hilbert space is used for the calculation of the nuclear matrix
elements governing the light neutrino mass mediated mode of neutrinoless double
beta decay of Ge76, Mo100, Cd116, Te128 and Xe136 within the proton-neutron
quasiparticle random phase approximation (pn-QRPA) and the renormalized QRPA
with proton-neutron pairing (full-RQRPA) methods. We have found that the
nuclear matrix elements obtained with the standard pn-QRPA for several nuclear
transitions are extremely sensitive to the renormalization of the
particle-particle component of the residual interaction of the nuclear
hamiltonian. Therefore the standard pn-QRPA does not guarantee the necessary
accuracy to allow us to extract a reliable limit on the effective neutrino
mass. This behaviour, already known from the calculation of the two-neutrino
double beta decay matrix elements, manifests itself in the neutrinoless
double-beta decay but only if a large model space is used. The full-RQRPA,
which takes into account proton-neutron pairing and considers the Pauli
principle in an approximate way, offers a stable solution in the physically
acceptable region of the particle-particle strength. In this way more accurate
values on the effective neutrino mass have been deduced from the experimental
lower limits of the half-lifes of neutrinoless double beta decay.Comment: 19 pages, RevTex, 1 Postscript figur
Neutrinoless Double Beta Decay within QRPA with Proton-Neutron Pairing
We have investigated the role of proton-neutron pairing in the context of the
Quasiparticle Random Phase approximation formalism. This way the neutrinoless
double beta decay matrix elements of the experimentally interesting A= 48, 76,
82, 96, 100, 116, 128, 130 and 136 systems have been calculated. We have found
that the inclusion of proton-neutron pairing influences the neutrinoless double
beta decay rates significantly, in all cases allowing for larger values of the
expectation value of light neutrino masses. Using the best presently available
experimental limits on the half life-time of neutrinoless double beta decay we
have extracted the limits on lepton number violating parameters.Comment: 16 RevTex page
A New Class of Majoron-Emitting Double-Beta Decays
Motivated by the excess events that have recently been found near the
endpoints of the double beta decay spectra of several elements, we re-examine
models in which double beta decay can proceed through the neutrinoless emission
of massless Nambu-Goldstone bosons (majorons). Noting that models proposed to
date for this process must fine-tune either a scalar mass or a VEV to be less
than 10 keV, we introduce a new kind of majoron which avoids this difficulty by
carrying lepton number . We analyze in detail the requirements that
models of both the conventional and our new type must satisfy if they are to
account for the observed excess events. We find: (1) the electron sum-energy
spectrum can be used to distinguish the two classes of models from one another;
(2) the decay rate for the new models depends on different nuclear matrix
elements than for ordinary majorons; and (3) all models require a (pseudo)
Dirac neutrino, having a mass of a several hundred MeV, which mixes with
.Comment: 43 pages, 10 figures (included), [figure captions are now included
New results for the two neutrino double beta decay in deformed nuclei with angular momentum projected basis
Four nuclei which are proved to be emitters (Ge,
Se, Nd, U), and four suspected, due to the corresponding
Q-values, to have this property (Nd, Sm, Gd,
Th), were treated within a proton-neutron quasiparticle random phase
approximation (pnQRPA) with a projected spherical single particle basis. The
advantage of the present procedure over the ones using a deformed Woods Saxon
or Nilsson single particle basis is that the actual pnQRPA states have a
definite angular momentum while all the others provide states having only K as
a good quantum number. The model Hamiltonian involves a mean field term
yielding the projected single particle states, a pairing interaction for alike
nucleons and a dipole-dipole proton-neutron interaction in both the
particle-hole (ph) and particle-particle (pp) channels. The effect of nuclear
deformation on the single beta strength distribution as well as on the double
beta Gamow-Teller transition amplitude (M) is analyzed. The
results are compared with the existent data and with the results from a
different approach, in terms of the process half life T. The case of
different deformations for mother and daughter nuclei is also presented.Comment: 45 pages, 13 figure
Constraining Almost Degenerate Three-Flavor Neutrinos
We discuss constraints on a scenario of almost degenerate three-flavor
neutrinos imposed by the solar and the atmospheric neutrino anomalies, hot dark
matter, and neutrinoless double decays. It is found that in the
Majorana version of the model the region with relatively large is
favored and a constraint on the CP violating phases is obtained.Comment: 19 pages (uses revtex), including 6 figures (uses epsf
Multipolar correlations and deformation effect on nuclear transition matrix elements of double- decay
The two neutrino and neutrinoless double beta decay of Zr,
Mo, Ru, Pd, Te and Nd isotopes
for the transition is studied within the PHFB framework along
with an effective two-body interaction consisting of pairing,
quadrupole-quadrupole and hexadecapole-hexadecapole correlations. It is found
that the effect of hexadecapolar correlations can be assimilated substantially
as a renormalization of the quadrupole-quadrupole interaction. The effect of
deformation on nuclear transition matrix elements is investigated by varying
the strength of quadrupolar correlations in the parent and daughter nuclei
independently. The variation of the nuclear transition matrix elements as a
function of the difference in deformation parameters of parent and daughter
nuclei reveals that in general, the former tend to be maximum for equal
deformation and they decrease as the difference in deformation parameters
increases, exhibiting a very similar trend for the and transition matrix elements.Comment: 6 pages, 2 figure
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