Collective nuclear excitations in quantum hadrodynamics

Collective excitations in finite nuclei are studied within the framework of a renormalizable meson-baryon field theory of nuclei (called Quantum Hadrodynamics or QHD). Excited states are investigated in the context of the shell model which arises naturally from a self-consistent, relativistic Hartree description of the ground state. The only inputs are a small number of parameters fixed at experimental values or fitted to static properties of bulk nuclear matter. Attention is focused on low-lying negative-parity excitations in doubly-magic nuclei. Excited energies and wave functions are calculated in the Random Phase Approximation (RPA) using Hartree single-particle energies to form unperturbed levels and Hartree spinors as the particle-hole basis. The particle-hole interaction is determined by the meson masses and coupling constants, and by the Lorentz structure of the meson-nucleon couplings. The full Dirac matrix structure is maintained throughout. Meson retardation effects are shown to be negligible for the low-lying states. Two models of QHD are considered. QHD-I is based on baryons and neutral vector and scalar mesons. The model QHD-II includes, in addition, charged vector and pseudoscalar fields in a renormalizable field theory. Both pseudoscalar and pseudovector pion-nucleon couplings are examined. Expressions for all matrix elements are given along with a discussion of the numerical procedures