Isoscalar compression modes in relativistic random phase approximation

A fully consistent relativistic RPA calculation is performed for the monopole and dipole compression modes in nuclei. The emphasis is put on the effects of Dirac sea states which are generally neglected in relativistic RPA calculations. It is found that these effects can be quite important for the isoscalar monopole mode. The main contributions from the pairs of Fermi to Dirac sea states are through the exchange of the scalar meson, while the vector mesons play a negligible role. Numerical results of relativistic RPA are checked with the constrained relativistic mean field model in the monopole case. A good agreement beteween monopole energies calculated in RRPA and in time-dependent relativistic mean field approach is achieved. For the monopole compression mode, a comparison of experimental and calculated energies gives a value of 250 $\sim$ 270 MeV for the nuclear matter incompressibility. A large discrepancy remains between theory and experiment in the case of the dipole compression mode.