102 research outputs found
Evolution of the pygmy dipole resonance in nuclei with neutron excess
The electric dipole excitation of various nuclei is calculated with a Random
Phase Approximation phenomenological approach. The evolution of the strength
distribution in various groups of isotopes, oxygen, calcium, zirconium and tin,
is studied. The neutron excess produces strength in the low energy region.
Indexes to measure the collectivity of the excitation are defined. We studied
the behavior of proton and neutron transition densities to determine the
isoscalar or isovector nature of the excitation. We observed that in
medium-heavy nuclei the low-energy excitation has characteristics rather
different that those exhibited by the giant dipole resonance. This new type of
excitation can be identified as pygmy dipole resonance.Comment: 14 pages, 12 figures, 7 table
Evolution of the pygmy dipole resonance in nuclei with neutron excess
The electric dipole excitation of various nuclei is calculated with a Random
Phase Approximation phenomenological approach. The evolution of the strength
distribution in various groups of isotopes, oxygen, calcium, zirconium and tin,
is studied. The neutron excess produces strength in the low energy region.
Indexes to measure the collectivity of the excitation are defined. We studied
the behavior of proton and neutron transition densities to determine the
isoscalar or isovector nature of the excitation. We observed that in
medium-heavy nuclei the low-energy excitation has characteristics rather
different that those exhibited by the giant dipole resonance. This new type of
excitation can be identified as pygmy dipole resonance.Comment: 14 pages, 12 figures, 7 table
Pionic correlations and meson-exchange currents in two-particle emission induced by electron scattering
Two-particle two-hole contributions to electromagnetic response functions are
computed in a fully relativistic Fermi gas model. All one-pion exchange
diagrams that contribute to the scattering amplitude in perturbation theory are
considered, including terms for pionic correlations and meson-exchange currents
(MEC). The pionic correlation terms diverge in an infinite system and thus are
regularized by modification of the nucleon propagator in the medium to take
into account the finite size of the nucleus. The pionic correlation
contributions are found to be of the same order of magnitude as the MEC.Comment: 14 pages, 15 figure
Meson-exchange currents and final-state interactions in quasielastic electron scattering at high momentum transfers
The effects of meson-exchange currents (MEC) are computed for the
one-particle one-hole transverse response function for finite nuclei at high
momentum transfers in the region of the quasielastic peak. A
semi-relativistic shell model is used for the one-particle-emission
reaction. Relativistic effects are included using relativistic kinematics,
performing a semi-relativistic expansion of the current operators and using the
Dirac-equation-based (DEB) form of the relativistic mean field potential for
the final states. It is found that final-state interactions (FSI) produce an
important enhancement of the MEC in the high-energy tail of the response
function for GeV/c. The combined effect of MEC and FSI goes away when
other models of the FSI, not based on the DEB potential, are employed.Comment: 4 pages, 5 figure
Superscaling of non-quasielastic electron-nucleus scattering
The present study is focused on the superscaling behavior of electron-nucleus
cross sections in the region lying above the quasielastic peak, especially the
region dominated by electroexcitation of the Delta. Non-quasielastic cross
sections are obtained from all available high-quality data for Carbon 12 by
subtracting effective quasielastic cross sections based on the superscaling
hypothesis. These residuals are then compared with results obtained within a
scaling-based extension of the relativistic Fermi gas model, including an
investigation of violations of scaling of the first kind in the region above
the quasielastic peak. A way potentially to isolate effects related to
meson-exchange currents by subtracting both impulsive quasielastic and
impulsive inelastic contributions from the experimental cross sections is also
presented.Comment: RevTeX, 34 pages including 11 figure
Superscaling in charged current neutrino quasielastic scattering in the relativistic impulse approximation
Superscaling of the quasielastic cross section in charged current
neutrino-nucleus reactions at energies of a few GeV is investigated within the
framework of the relativistic impulse approximation. Several approaches are
used to describe final state interactions and comparisons are made with the
plane wave approximation. Superscaling is very successful in all cases. The
scaling function obtained using a relativistic mean field for the final states
shows an asymmetric shape with a long tail extending towards positive values of
the scaling variable, in excellent agreement with the behavior presented by the
experimental scaling function.Comment: 5 pages, 4 figures, accepted for publicacion in PRL Text modified and
references adde
Semi-relativistic description of quasielastic neutrino reactions and superscaling in a continuum shell model
The so-called semi-relativistic expansion of the weak charged current in
powers of the initial nucleon momentum is performed to describe
charge-changing, quasielastic neutrino reactions at
intermediate energies. The quality of the expansion is tested by comparing with
the relativistic Fermi gas model using several choices of kinematics of
interest for ongoing neutrino oscillation experiments. The new current is then
implemented in a continuum shell model together with relativistic kinematics to
investigate the scaling properties of and cross
sections.Comment: 33 pages, 10 figures, to appear in PR
Superscaling and Charge-changing Neutrino Cross Sections
The superscaling function extracted from inclusive electron scattering data
is used to predict high energy charge-changing neutrino cross sections in the
quasi-elastic and regions.Comment: 3 pages, 2 figures, to appear in the Proceedings of the 7th
International Workshop on Neutrino Factories and Superbeams, Laboratori
Nazionali di Frascati, Frascati (Rome), June 21 - 26, 200
Low-lying magnetic excitations of doubly-closed-shell nuclei and nucleon-nucleon effective interactions
We have studied the low lying magnetic spectra of 12C, 16O, 40Ca, 48Ca and
208Pb nuclei within the Random Phase Approximation (RPA) theory, finding that
the description of low-lying magnetic states of doubly-closed-shell nuclei
imposes severe constraints on the spin and tensor terms of the nucleon-nucleon
effective interaction. We have first made an investigation by using four
phenomenological effective interactions and we have obtained good agreement
with the experimental magnetic spectra, and, to a lesser extent, with the
electron scattering responses. Then we have made self-consistent RPA
calculations to test the validity of the finite-range D1 Gogny interaction. For
all the nuclei under study we have found that this interaction inverts the
energies of all the magnetic states forming isospin doublets.Comment: 19 pages, 13 figures, 7 tables, accepted for publication in Phys.
Rev.
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