Quasielastic Electron Scattering and Vacuum Polarization

The longitudinal response function for quasielastic electron scattering from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ at a momentum transfer $q=550\mathrm{}$ MeV is calculated in a relativistic random-phase approximation (RPA). The particle-hole response is calculated exactly in the finite system, while vacuum-polarization effects are included in a local-density approximation. The RPA result with vacuum polarization is about 25% below the Hartree response and comparable to Saclay data.