87 research outputs found
Self Consistent and Renormalized particle-particle RPA in a Schematic Model
The dynamical effects of ground state correlations for excitation energies
and transition strengths near the superfluid phase transition are studied in
the soluble two level pairing model, in the context of the particle-particle
self consistent Random Phase Approximation (SCRPA). Exact results are well
reproduced across the transition region, beyond the collapse of the standard
particle-particle Random Phase Approximation. The effects of two-body
correlation in the SCRPA are displayed explicitly.Comment: 11 pages, revtex, 3ps figures, to appear in Phys. Rev.
Neutrino and antineutrino charge-exchange reactions on 12C
We extend the formalism of weak interaction processes, obtaining new
expressions for the transition rates, which greatly facilitate numerical
calculations, both for neutrino-nucleus reactions and muon capture. Explicit
violation of CVC hypothesis by the Coulomb field, as well as development of a
sum rule approach for the inclusive cross sections have been worked out. We
have done a thorough study of exclusive (ground state) properties of B
and N within the projected quasiparticle random phase approximation
(PQRPA). Good agreement with experimental data achieved in this way put in
evidence the limitations of standard RPA and the QRPA models, which come from
the inability of the RPA in opening the shell, and from the
non-conservation of the number of particles in the QRPA. The inclusive
neutrino/antineutrino () reactions C(N
and C(B are calculated within both the PQRPA, and
the relativistic QRPA (RQRPA). It is found that the magnitudes of the resulting
cross-sections: i) are close to the sum-rule limit at low energy, but
significantly smaller than this limit at high energies both for and
, ii) they steadily increase when the size of the configuration
space is augmented, and particulary for energies MeV,
and iii) converge for sufficiently large configuration space and final state
spin. The quasi-elastic C(N cross section recently
measured in the MiniBooNE experiment is briefly discussed. We study the
decomposition of the inclusive cross-section based on the degree of
forbiddenness of different multipoles. A few words are dedicated to the
-C charge-exchange reactions related with astrophysical
applications.Comment: 21 pages, 13 figures, 1 table, submitted to Physical Review
Neutrino and antineutrino cross sections in C
We extend the formalism of weak interaction processes, obtaining new
expressions for the transition rates, which greatly facilitate numerical
calculations, both for neutrino-nucleus reactions and muon capture. We have
done a thorough study of exclusive (ground state) properties of B and
N within the projected quasiparticle random phase approximation (PQRPA).
Good agreement with experimental data is achieved in this way. The inclusive
neutrino/antineutrino () reactions C(N
and C(B are calculated within both the PQRPA, and
the relativistic QRPA (RQRPA). It is found that the magnitudes of the resulting
cross-sections: i) are close to the sum-rule limit at low energy, but
significantly smaller than this limit at high energies both for and
, ii) they steadily increase when the size of the configuration
space is augmented, and particulary for energies MeV,
and iii) converge for sufficiently large configuration space and final state
spin.Comment: Proceedings of the International Nuclear Physics Conference 2010,
Vancouver, BC - Canada 4-9 Jul 201
Neutrino-Nucleus Reactions and Muon Capture in 12C
The neutrino-nucleus cross section and the muon capture rate are discussed
within a simple formalism which facilitates the nuclear structure calculations.
The corresponding formulae only depend on four types of nuclear matrix
elements, which are currently used in the nuclear beta decay. We have also
considered the non-locality effects arising from the velocity-dependent terms
in the hadronic current. We show that for both observables in 12C the higher
order relativistic corrections are of the order of ~5 only, and therefore do
not play a significant role. As nuclear model framework we use the projected
QRPA (PQRPA) and show that the number projection plays a crucial role in
removing the degeneracy between the proton-neutron two quasiparticle states at
the level of the mean field. Comparison is done with both the experimental data
and the previous shell model calculations. Possible consequences of the present
study on the determination of the neutrino oscillation
probability are briefly addressed.Comment: 29 pages, 6 figures, Revtex4. Several changes were made to the
previous manuscript, the results and final conclusions remain unalterable. It
has been accepted for publication as a Regular Article in Physical Review
The Pauli principle, QRPA and the two-neutrino double beta decay
We examine the violation of the Pauli exclusion principle in the
Quasiparticle Random Phase Approximation (QRPA) calculation of the two-neutrino
double beta decay matrix elements, which has its origin in the quasi-boson
approximation. For that purpose we propose a new renormalized QRPA with
proton-neutron pairing method (full-RQRPA) for nuclear structure studies, which
includes ground state correlation beyond the QRPA. This is achieved by using of
renormalized quasi-boson approximation, in which the Pauli exclusion principle
is taken into account more carefully. The full-RQRPA has been applied to
two-neutrino double beta decay of , , and
. The nuclear matrix elements have been found significantly less
sensitive to the increasing strength of particle-particle interaction in the
physically interesting region in comparison with QRPA results. The strong
differences between the results of both methods indicate that the Pauli
exclusion principle plays an important role in the evaluation of the double
beta decay. The inclusion of the Pauli principle removes the difficulties with
the strong dependence on the particle-particle strength in the QRPA on
the two-neutrino double beta decay.Comment: Accepted for publication in Nucl. Phys. A, 22 pages, including 5
figures, LaTeX (using REVTeX and epsfig-style
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
Non-collapsing renormalized QRPA with proton-neutron pairing for neutrinoless double beta decay
Using the renormalized quasiparticle random phase approximation (RQRPA), we
calculate the light neutrino mass mediated mode of neutrinoless double beta
decay of Ge76, Mo100, Te128 and Te130. Our results indicate that the simple
quasiboson approximation is not good enough to study the neutrinoless double
beta decay, because its solutions collapse for physical values of g_pp. We find
that extension of the Hilbert space and inclusion of the Pauli Principle in the
QRPA with proton-neutron pairing, allows us to extend our calculations beyond
the point of collapse, for physical values of the nuclear force strength. As a
consequence one might be able to extract more accurate values on the effective
neutrino mass by using the best available experimental limits on the half-life
of neutrinoless double beta decay.Comment: 15 pages, RevTex, 2 Postscript figures, to appear in Phys. Lett.
Equivalence between local Fermi gas and shell models in inclusive muon capture from nuclei
Motivated by recent studies of inclusive neutrino nucleus processes and muon
capture within a correlated local Fermi gas model (LFG), we discuss the
relevance of nuclear finite size effects in these reactions at low energy, in
particular for muon capture. To disentangle these effects from others coming
from the reaction dynamics we employ here a simple uncorrelated shell model
that embodies the typical finite size content of the problem. The integrated
decay widths of muon atoms calculated with this shell model are then compared
for several nuclei with those obtained within the uncorrelated LFG, using in
both models exactly the same theoretical ingredients and parameters. We find
that the two predictions are in quite good agreement, within 1--7%, when the
shell model density and the correct energy balance is used as input in the LFG
calculation. The present study indicates that, despite the low excitation
energies involved in the reaction, integrated inclusive observables, like the
total muon capture width, are quite independent of the fine details of the
nuclear wave functions.Comment: 11 pages, 8 figures. Final version to be published in EPJ
Kvazičestična aproksimacija nasumnih faza bez urušavanja za dvojni beta raspad
We show how the longstanding problem of the collapse of the charge–exchange QRPA near the physical value of the force strength can be circumvented. This is done by including the effect of ground state correlations into the QRPA equations of motion. The corresponding formalism, called renormalized QRPA, is briefly outlined and its consequences are discussed in the framework of a schematic model for the two-neutrino double beta decay in the 100Mo → 100Ru system. The question of the conservation of the Ikeda sum rule is also addressed within the new formalism.Pokazuje se kako se može izbjeći tvrdokorni problem urušavanja kvazičestične aproksimacije nasumnih faza (QRPA) s nabojskom izmjenom za realne vrijednosti jakosti sila. To se postiže uključivanjem korelacije u osnovnom stanju u jednadžbe stanja QRPA. Raspravljaju se rezultati za shematski model dvoneutrinskog dvojnog beta raspada 100Mo → 100Ru. U okviru ovog formalizma također se raspravlja pitanje Ikedinog zbrojnog pravila
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