Sigma and omega meson propagation in a dense nuclear medium

The propagation of the scalar ($\sigma$) and vector ($\omega$) mesons in nuclear matter is studied in detail using the Walecka model over a wide range of densities and including the effects of a finite $\sigma$ width through the inclusion of a two-pion loop. We calculate the dispersion relation and spectral functions of the $\sigma$ and (transverse and longitudinal) $\omega$ mesons, including the effect of $\sigma$-$\omega$ mixing in matter. It is shown that the mixing effect is quite important in the propagation of the (longitudinal) $\omega$ and $\sigma$ mesons above normal nuclear matter density. We find that there is a two-peak structure in the spectral function of the $\sigma$ channel, caused by $\sigma$-$\omega$ mixing.Comment: 17 pages including 6 ps files, submitted to Phys. Lett. B. Acknowledgement is revise

High-K Precession modes: Axially symmetric limit of wobbling motion

The rotational band built on the high-K multi-quasiparticle state can be interpreted as a multi-phonon band of the precession mode, which represents the precessional rotation about the axis perpendicular to the direction of the intrinsic angular momentum. By using the axially symmetric limit of the random-phase-approximation (RPA) formalism developed for the nuclear wobbling motion, we study the properties of the precession modes in $^{178}$W; the excitation energies, B(E2) and B(M1) values. We show that the excitations of such a specific type of rotation can be well described by the RPA formalism, which gives a new insight to understand the wobbling motion in the triaxial superdeformed nuclei from a microscopic view point.Comment: 14 pages, 8 figures (Spelling of the authors name was wrong at the first upload, so it is corrected

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 $p\bar p$-pairs by a factor 2.Comment: 19 pages Latex including 12 postscript 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 $^{11}$Be + $^{208}$Pb. The breakup mechanism is found to be different between the channels of $j^{\pi}=\frac{1}{2}^{-}$ and $\frac{3}{2}^{-}$, reflecting the underlying structure of $^{11}$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

Longitudinal and Transverse Spin Responses in Relativistic Many Body Theory

Spin longitudinal and transverse response function are studied by means of the relativistic many-body theory. The spin response functions in the relativistic theory are largely reduced from those in the non-relativistic theory. The local density approximation with the eikonal approximation is applied to the the nuclear absorption in the $(\vec{p},\vec{n})$ reactions on C and Ca. We compare the calculated results with the recent experimental data.Comment: 16 pages including 11 figure

Electromagnetic form factors of the bound nucleon

We calculate electromagnetic form factors of the proton bound in specified orbits for several closed shell nuclei. The quark structure of the nucleon and the shell structure of the finite nuclei are given by the QMC model. We find that orbital electromagnetic form factors of the bound nucleon deviate significantly from those of the free nucleon.Comment: 12 pages including 4 ps figure

Microscopic description of the surface dipole plasmon in large Na_N clusters (950 < N < 12050)

Fully microscopic RPA/LDA calculations of the dipole plasmon for very large neutral and charged sodium clusters, Na_N^Z+, in the size range 950 < N < 12050 are presented for the first time. 60 different sizes are considered altogether, which allows for an in-depth investigation of the asymptotic behavior of both the width and the position of the plasmon.Comment: Latex/Revtex, 4 pages with 4 Postscript figures, accepted for publication in Physical Review

Modified Quark-Meson Coupling Model for Nuclear Matter

The quark-meson coupling model for nuclear matter, which describes nuclear matter as non-overlapping MIT bags bound by the self-consistent exchange of scalar and vector mesons, is modified by introducing medium modification of the bag constant. We model the density dependence of the bag constant in two different ways: one invokes a direct coupling of the bag constant to the scalar meson field, and the other relates the bag constant to the in-medium nucleon mass. Both models feature a decreasing bag constant with increasing density. We find that when the bag constant is significantly reduced in nuclear medium with respect to its free-space value, large canceling isoscalar Lorentz scalar and vector potentials for the nucleon in nuclear matter emerge naturally. Such potentials are comparable to those suggested by relativistic nuclear phenomenology and finite-density QCD sum rules. This suggests that the reduction of bag constant in nuclear medium may play an important role in low- and medium-energy nuclear physics.Comment: Part of the text is reordered, revised version to appear in Phys. Rev. C. 19 pages, ReVTeX, 4 figures embedde

Analysis of exchange terms in a projected ERPA Theory applied to the quasi-elastic (e,e') reaction

A systematic study of the influence of exchange terms in the longitudinal and transverse nuclear response to quasi-elastic (e,e') reactions is presented. The study is performed within the framework of the extended random phase approximation (ERPA), which in conjuction with a projection method permits a separation of various contributions tied to different physical processes. The calculations are performed in nuclear matter up to second order in the residual interaction for which we take a (pi+rho)-model with the addition of the Landau-Migdal g'-parameter. Exchange terms are found to be important only for the RPA-type contributions around the quasielastic peak.Comment: 29 pages, 6 figs (3 in postscript, 3 faxed on request), epsf.st