Haruki Kurasawa

Effects of the Dirac sea on the Coulomb sum value for electron scattering

Abstract Polarization effects of the Dirac sea on the Coulomb sum values for electron scattering are investigated within the framework of the random-phase approximation (RPA) in the relativistic σ-ω model. Divergence due to NN excitations in the correlation functions is renormalized using a method of dimensional regularization. Particle-hole correlations reduce the sum values of the relativistic Hartree approximation by 15≈20%, while effects of the Dirac sea reduce them further by about 10≈20%. The quenching phenomena of the Coulomb sum values observed in medium heavy nuclei are rather well explained in the renormalized relativistic RPA.

Giant monopole and quadrupole states in the relativistic σ-ω model

Abstract Giant monopole and quadrupole states in the relativistic σ-ω model are studied with use of a local Lorentz boost and scaling method. In assuming nuclear matter, those excitation energies are expressed in a simple way in terms of the relativistic Landau parameters, and the expressions are formally very similar to nonrelativistic ones. In the σ-ω model, however, the values of the Landau parameters themselves are dominated by relativistic effects. Unlike the restoring forces depending on the Landau parameters, the mass parameters of these states are little affected by relativity.

Effects of the Dirac sea on the giant monopole states

Abstract Effects of the Dirac sea on the excitation energy of the giant monopole states are investigated in an analytic way within the σ − ω model. The excitation energy is determined by the relativistic Landau-Migdal parameters, F 0 and F 1 . Their analytic expressions are derived in the relativistic random phase approximation (RRPA) without the Dirac sea, with the Pauli blocking terms and with the full Dirac sea. It is shown that in the RRPA based on the mean field approximation the Pauli blocking terms should be included in the configuration space, according to the relativistic Landau theory. In the renormalized RRPA, the incompressibility coefficient becomes negative, if NN excitations are neglected.

Role of the Dirac Sea in the Quenching of Gamow-Teller Strength

The issue of the Gamow-Teller sum rule in atomic nuclei is reexamined in the framework of a relativistic description of nuclei. It is shown that the total Gamow-Teller strength in the nucleon sector is reduced by about 12% as compared to the prediction of a nonrelativistic description. This reduction is due to the antinucleon degrees of freedom.

Modification of the nucleon size in nuclei in the σ-ω model☆

Abstract In the σ-ω model, the nucleon size in a nuclear medium is different from that in free space. The modification is due to the change of the vacuum structure. The mean square radius of the proton is swollen by (− 1 2π 2 )( g ω m 0 ) 2 In M ∗ M , where gω, m0 and M ∗ denote the ω-N coupling constant, the bare mass of the ω-meson and the nucleon effective mass, respectively. This increases the RMS radius of the proton in the normal-density nuclear matter by 7.6%. As a result nuclear response functions for electron scattering are reduced by 10–15% at the squared four-momentum transfer 0.2–0.3 GeV2.

Quasielastic electron scattering on 12C and 40Ca in the relativistic Hartree approximation Relativistic corrections to the response functions

Abstract The quenching of the longitudinal response functions observed in high-energy electron scattering is studied within a self-consistent relativistic Hartree approximation in the σ-ω model. The σ-ω model yields a small nucleon effective mass which reduces the response functions around the peak position through the Perey effect and the relativistic nuclear current. This reduction, however, is partially cancelled by effects from the relativistic kinematics. Consequently, the reduction is not enough to explain the quenching phenomena of the longitudinal response functions around the peak. The relativistic Coulomb sum values, on the other hand, are little reduced, compared with those of nonrelativistic models, failing to reproduce experimental data. In contrast to the longitudinal ones, the transverse response functions around the peak are enhanced by relativistic effects on the nuclear current. Nevertheless, the present model underestimates experimental values at high momentum transfer.

The mean energy, strength and width of the double giant dipole states

Abstract Model-independent sum rules for an excitation strength of the “double” giant dipole resonance state are derived. The strength is given in terms of that for the “single” giant dipole resonance state. The width and the mean energy of the double giant resonance state relative to those of the single giant dipole resonance state are estimated rather model-independently without assuming independent phonon excitations. The mean energy of the double giant dipole state is twice that of the single giant dipole resonance state, while the width is larger than 2 times that of the single giant dipole state. The mean energy and the width are consistent with experiments. The relationship between these results and those of the folding model is briefly discussed.

Energy-weighted and non-energy-weighted Gamow-Teller sum rules in the relativistic random phase approximation

The non-energy-weighted Gamow-Teller (GT) sum rule is satisfied in relativistic models, when all nuclear density-dependent terms, including Pauli blocking terms from nucleon-antinucleon excitations, are taken into account in the random phase approximation (RPA) correlation function. The no-sea approximation is equivalent to this approximation for the giant GT resonance state and satisfies the sum rule, but each of the total

The Gamow-Teller States in Relativistic Nuclear Models

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Role of the Landau-Migdal Parameters with the Pseudovector and Tensor Couplings in Relativistic Nuclear Models: — The Quenching of the Gamow-Teller Strength —

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