Covariant response theory beyond RPA and its application

Abstract

The covariant particle-vibration coupling model within the time-blocking approximation is employed to supplement the relativistic random phase approximation (RRPA) with coupling to collective vibrations. The Bethe—Salpeter equation in the particle—hole channel with an energy-dependent residual particle—hole (ph) interaction is formulated and solved in the shell-model Dirac basis as well as in the momentum space. The same set of coupling constants generates the Dirac—Hartree single-particle spectrum, the static part of the residual ph interaction, and the particle-phonon coupling amplitudes. This approach is applied to a quantitative description of damping phenomenon in even—even spherical nuclei with closed shells 208Pb and 132Sn. Since phonon coupling enriches the RRPA spectrum with a multitude of ph ⊗ phonon states, a noticeable fragmentation of giant monopole and dipole resonances is obtained in the examined nuclei. The results are compared with experimental data and results of the nonrelativistic approach.