Nuclear Medium Effects on the Coulomb Sum Values

Nuclear medium effects on the Coulomb sum values are investigated in a relativistic model. The Coulomb sum values of non-relativistic models are strongly quenched by the antinucleon degrees of freedom with effective mass due to the Lorentz scalar potential. The quenching is consistent with the recent analysis of the experimental data. Within the same framework, the quenching is understood as a result of modification of the nucleon size or of the omega-meson mass. Effects of the antinucleon degrees of freedom on giant resonance states etc. are also discussed.