Chiral Sigma Model with Pion Mean Field in Finite Nuclei

The properties of infinite matter and finite nuclei are studied by using the chiral sigma model in the framework of the relativistic mean field theory. We reconstruct an extended chiral sigma model in which the omega meson mass is generated dynamically by the sigma condensation in the vacuum in the same way as the nucleon mass. All the parameters of chiral sigma model are essentially fixed from the hadron properties in the free space. In nuclear matter, the saturation property comes out right, but the incompressibility is too large and the scalar and vector potentials are about a half of the phenomenological ones, respectively. This fact is reflected to the properties of finite nuclei. We calculate N = Z even-even mass nuclei between N = 16 and N = 34. The extended chiral sigma model without the pion mean field leads to the result that the magic number appears at N = 18 instead of N = 20 and the magic number does not appear at N = 28 due to the above mentioned nuclear matter properties. The latter problem, however, could be removed by the introduction of the finite pion mean field with the appearance of the magic number at N = 28. We find that the energy differences between the spin-orbit partners are reproduced by the finite pion mean field which is completely a different mechanism from the standard spin-orbit interaction.Comment: 19 pages, 9 figures. Prog. Theor. Phys. to be publishe

Relativistic Hartree approach with exact treatment of vacuum polarization for finite nuclei

We study the relativistic Hartree approach with the exact treatment of the vacuum polarization in the Walecka sigma-omega model. The contribution from the vacuum polarization of nucleon-antinucleon field to the source term of the meson fields is evaluated by performing the energy integrals of the Dirac Green function along the imaginary axis. With the present method of the vacuum polarization in finite system, the total binding energies and charge radii of 16O and 40Ca can be reproduced. On the other hand, the level-splittings in the single-particle level, in particular the spin-orbit splittings, are not described nicely because the inclusion of vacuum effect provides a large effective mass with small meson fields. We also show that the derivative expansion of the effective action which has been used to calculate the vacuum contribution for finite nuclei gives a fairly good approximation.Comment: 15 pages, 8 figure

Variation of hadron masses in nuclear matter in the relativistic Hartree approximation

We study the modification of hadron masses due to the vacuum polarization using the chiral sigma model, which is extended to generate the $\omega$ meson mass by the sigma condensation in the vacuum in the same way as the nucleon mass. The results obtained in the chiral sigma model are compared with those obtained in the Walecka model which includes $\sigma$ and $\omega$ mesons in a non-chiral fashion. It is shown that both the nucleon mass and the $\omega$ meson mass decrease in nuclear medium, while the $\sigma$ meson mass increases at finite density in the chiral sigma model.Comment: 10 pages, 2 figures, accepted for publication in Nucl.Phys.

Loop corrections and the K-matrix formalism

We propose a new model that combines the rigor of full-fledged loop calculations in field theory with the simplicity of the K-matrix formalism for channel-coupling effects. We show, for one example, the equivalence of this new model, the renormalized Im-K-matrix (RIK) model, with a more standard approach in which nonperturbative loop corrections are calculated. In the RIK model an analytic continuation of the K-matrix formalism is combined with a renormalization procedure to arrive at a scattering amplitude that obeys low-energy theorems and is also an analytic function in the complex momentum plane

Dibaryon resonance and two-photon bremsstrahlung in pp scattering

The signature of a possible dibaryon state is studied in two-photon production in proton-proton scattering, that is, in the process pp -> D gamma -> 2 gamma NN. We have investigated the effects on the cross section for cases in which the dibaryon has spin 0(+/-). Special attention is paid to interference of the dibaryon signal with that of the two-photon bremsstrahlung process