VACUUM POLARIZATION EFFECTS ON THE ELECTROMAGNETIC RESPONSE OF LOW-LYING ISOSCALAR EXCITATIONS

We investigate the effects of vacuum polarization on the electromagnetic response of low-lying isoscalar states. Electromagnetic form factors are calculated in a relativistic random phase approximation to the Walecka model. Vacuum polarization tends to reduce the strong collectivity seen in similar calculations that ignore vacuum effects. We have traced this behavior to the weaker effective residual interaction generated in the presence of vacuum loops. As the effective nucleon mass decreases, nucleon-antinucleon pairs are more easily excited in the medium thus becoming more effective in screening the nucleon charge. While this reduction is consistent with the quenching of the charge response seen in quasielastic electron scattering it, unfortunately, reduces some of the coherence needed to explain the strong collective response. © 1990.