303 research outputs found
Sum Rules of the Multiple Giant Dipole States
Various sum rules for multiple giant dipole resonance states are derived. For
the triple giant dipole resonance states, the energy-weighted sum of the
transition strengths requires a model to be related to those of the single and
double giant dipole resonance states. It is also shown that the non-diagonal
matrix elements of the double commutator between the dipole operator and the
nuclear Hamiltonian give useful identities for the excitation energy and
transition strength of each excited state. Using those identities, the
relationship between width of the single dipole state and those of the multiple
ones is qualitatively discussed.Comment: 8 pages, 1 figure, using PTPTeX styl
The Gamow-Teller States in Relativistic Nuclear Models
The Gamow-Teller(GT) states are investigated in relativistic models. The
Landau-Migdal(LM) parameter is introduced in the Lagrangian as a contact term
with the pseudo-vector coupling. In the relativistic model the total GT
strength in the nucleon space is quenched by about 12% in nuclear matter and by
about 6% in finite nuclei, compared with the one of the Ikeda-Fujii-Fujita sum
rule. The quenched amount is taken by nucleon-antinucleon excitations in the
time-like region. Because of the quenching, the relativistic model requires a
larger value of the LM parameter than non-relativistic models in describing the
excitation energy of the GT state. The Pauli blocking terms are not important
for the description of the GT states.Comment: REVTeX4, no figure
Causality in relativistic many body theory
The stability of the nuclear matter system with respect to density
fluctuations is examined exploring in detail the pole structure of the
electro-nuclear response functions. Making extensive use of the method of
dispersion integrals we calculate the full polarization propagator not only for
real energies in the spacelike and timelike regime but also in the whole
complex energy plane. The latter proved to be necessary in order to identify
unphysical causality violating poles which are the consequence of a neglection
of vacuum polarization. On the contrary it is shown that Dirac sea effects
stabilize the nuclear matter system shifting the unphysical pole from the upper
energy plane back to the real axis. The exchange of strength between these real
timelike collective excitations and the spacelike energy regime is shown to
lead to a reduction of the quasielastic peak as it is seen in electron
scattering experiments. Neglecting vacuum polarization one also obtains a
reduction of the quasielastic peak but in this case the strength is partly
shifted to the causality violating pole mentioned above which consequently
cannot be considered as a physical reliable result. Our investigation of the
response function in the energy region above the threshold of nucleon
anti-nucleon production leads to another remarkable result. Treating the
nucleons as point-like Dirac particles we show that for any isospin independent
NN-interaction RPA-correlations provide a reduction of the production amplitude
for -pairs by a factor 2.Comment: 19 pages Latex including 12 postscript figure
The mean energy, strength and width of triple giant dipole resonances
We investigate the mean energy, strength and width of the triple giant dipole
resonance using sum rules.Comment: 12 page
Medium modifications of nucleon electromagnetic form factors
We use the Nambu-Jona-Lasinio model as an effective quark theory to
investigate the medium modifications of the nucleon electromagnetic form
factors. By using the equation of state of nuclear matter derived in this
model, we discuss the results based on the naive quark-scalar diquark picture,
the effects of finite diquark size, and the meson cloud around the constituent
quarks. We apply this description to the longitudinal response function for
quasielastic electron scattering. RPA correlations, based on the
nucleon-nucleon interaction derived in the same model, are also taken into
account in the calculation of the response function.Comment: 46 pages, 16 figure
Coulomb Breakup Mechanism of Neutron-Halo Nuclei in a Time-Dependent Method
The mechanism of the Coulomb breakup reactions of the nuclei with
neutron-halo structure is investigated in detail. A time-dependent
Schr\"odinger equation for the halo neutron is numerically solved by treating
the Coulomb field of a target as an external field. The momentum distribution
and the post-acceleration effect of the final fragments are discussed in a
fully quantum mechanical way to clarify the limitation of the intuitive picture
based on the classical mechanics. The theory is applied to the Coulomb breakup
reaction of Be + Pb. The breakup mechanism is found to be
different between the channels of and
, reflecting the underlying structure of Be. The
calculated result reproduces the energy spectrum of the breakup fragments
reasonably well, but explains only about a half of the observed longitudinal
momentum difference.Comment: 15 pages,revtex, 9 figures (available upon request
Complementary cooperation of an ambulance helicopter and car with medical doctors: meaning of simultaneous dispatch
Effects of the Neutron Spin-Orbit Density on Nuclear Charge Density in Relativistic Models
The neutron spin-orbit density contributes to the nuclear charge density as a
relativistic effect. The contribution is enhanced by the effective mass
stemming from the Lorentz-scalar potential in relativistic models. This
enhancement explains well the difference between the cross sections of elastic
electron scattering off Ca and Ca which was not reproduced in
non-relativistic models. The spin-orbit density will be examined in more detail
in electron scattering off unstable nuclei which would be available in the
future.Comment: 4 pages with 3 eps figures, revte
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