150 research outputs found
Self-Energy of Decuplet Baryons in Nuclear Matter
We calculate, in chiral perturbation theory, the change in the self-energy of
decuplet baryons in nuclear matter. These self-energy shifts are relevant in
studies of meson-nucleus scattering and of neutron stars. Our results are
leading order in an expansion in powers of the ratio of characteristic momenta
to the chiral symmetry-breaking scale (or the nucleon mass). Included are
contact diagrams generated by 4-baryon operators, which were neglected in
earlier studies for the isomultiplet but contribute to the self-energy
shifts at this order in chiral perturbation theory.Comment: 11 pages, 2 eps figures, REVTe
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
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
Delta Excitations in Neutrino-Nucleus Scattering
We derive the contribution of -h excitations to quasielastic
charged-current neutrino-nucleus scattering in the framework of relativistic
mean-field theory. We discuss the effect of production on the
determination of the axial mass in neutrino scattering experiments.Comment: 14 pages, revtex, 3 postscript figures (available upon request
Antisymmetrized random phase approximation for quasielastic scattering in nuclear matter: Non-relativistic potentials
Many-body techniques for the calculation of quasielastic nuclear matter
response functions in the fully antisymmetrized random phase approximation on a
Hartree-Fock basis are discussed in detail. The methods presented here allow
for an accurate evaluation of the response functions with little numerical
effort. Formulae are given for a generic non-relativistic potential
parameterized in terms of meson exchanges; on the other hand, relativistic
kinematical effects have been accounted for.Comment: 27 pages, 7 figures; uses revtex and epsfig; minor changes; typos in
some formulae have been corrected and references have been adde
Static properties of nuclear matter within the Boson Loop Expansion
The use of the Boson Loop Expansion is proposed for investigating the static
properties of nuclear matter. We explicitly consider a schematic dynamical
model in which nucleons interact with the scalar-isoscalar sigma meson. The
suggested approximation scheme is examined in detail at the mean field level
and at the one- and two-loop orders. The relevant formulas are provided to
derive the binding energy per nucleon, the pressure and the compressibility of
nuclear matter. Numerical results of the binding energy at the one-loop order
are presented for Walecka's sigma-omega model in order to discuss the degree of
convergence of the Boson Loop Expansion.Comment: 40 pages, 13 figure
Relativistic effects in electromagnetic nuclear responses in the quasi-elastic delta region
A new non-relativistic expansion in terms of the nucleon's momentum inside
nuclear matter of the current for isobar electro-excitation from the nucleon is
performed. Being exact with respect to the transferred energy and momentum,
this yields new current operators which retain important aspects of relativity
not taken into account in the traditional non-relativistic reductions. The
transition current thus obtained differs from the leading order of the
traditional expansion by simple multiplicative factors. These depend on the
momentum and energy transfer and can be easily included together with
relativistic kinematics in non-relativistic, many-body models of isobar
electro-excitation in nuclei. The merits of the new current are tested by
comparing with the unexpanded electromagnetic nuclear responses in the isobar
peak computed in a relativistic Fermi gas framework. The sensitivity of the
relativistic responses to the isobar's magnetic, electric and Coulomb form
factors and the finite width of the isobar is analyzed.Comment: 26 pages plus 6 figure
Exploration of Resonant Continuum and Giant Resonance in the Relativistic Approach
Single-particle resonant-states in the continuum are determined by solving
scattering states of the Dirac equation with proper asymptotic conditions in
the relativistic mean field theory (RMF). The regular and irregular solutions
of the Dirac equation at a large radius where the nuclear potentials vanish are
relativistic Coulomb wave functions, which are calculated numerically.
Energies, widths and wave functions of single-particle resonance states in the
continuum for ^{120}Sn are studied in the RMF with the parameter set of NL3.
The isoscalar giant octupole resonance of ^{120}Sn is investigated in a fully
consistent relativistic random phase approximation. Comparing the results with
including full continuum states and only those single-particle resonances we
find that the contributions from those resonant-states dominate in the nuclear
giant resonant processes.Comment: 16 pages, 2 figure
Collective multipole excitations in a microscopic relativistic approach
A relativistic mean field description of collective excitations of atomic
nuclei is studied in the framework of a fully self-consistent relativistic
random phase approximation (RRPA). In particular, results of RRPA calculations
of multipole giant resonances and of low-lying collective states in spherical
nuclei are analyzed. By using effective Lagrangians which, in the mean-field
approximation, provide an accurate description of ground-state properties, an
excellent agreement with experimental data is also found for the excitation
energies of low-lying collective states and of giant resonances. Two points are
essential for the successful application of the RRPA in the description of
dynamical properties of finite nuclei: (i) the use of effective Lagrangians
with non-linear terms in the meson sector, and (ii) the fully consistent
treatment of the Dirac sea of negative energy states.Comment: 10 figures, submitted to Nucl.Phys.
Loop Corrections and Naturalness in a Chiral Effective Field Theory
The loop expansion is applied to a chiral effective hadronic lagrangian; with
the techniques of Infrared Regularization, it is possible to separate out the
short-range contributions and to write them as local products of fields that
are already present in our lagrangian. (The appropriate field variables must be
re-defined at each order in loops.) The corresponding parameters implicitly
include short-range effects to all orders in the interaction, so these effects
need not be calculated explicitly. The remaining (long-range) contributions
that must be calculated are nonlocal and resemble those in conventional
nuclear-structure calculations. Nonlinear isoscalar scalar and
vector meson interactions are included, which incorporate
many-nucleon forces and nucleon substructure. Calculations are carried out at
the two-loop level to illustrate these techniques at finite nuclear densities
and to verify that the coupling parameters remain natural when fitted to the
empirical properties of equilibrium nuclear matter. Contributions from the
tensor coupling are also discussed.Comment: 22 pages, 6 figure
- …