No abstract

Long-range correlations, which are partially responsible for the observed fragmentation and depletion of low-lying single-particle strength, are studied in the Green's function formalism. The self-energy is expanded up to second order in the residual interaction. We compare two methods of implementing self-consistency in the solution of the Dyson equation beyond Hartree-Fock, for the case of the 16 O nucleus. It is found that the energy-bin method and the BAGEL method lead to globally equivalent results. In both methods the final single-particle strength functions are characterized by exponential tails at energies far from the Fermi level.

Coherent Compton scattering on light nuclei in the ⌬-resonance region is studied in the impulse approximation and is shown to be a sensitive probe of the in-medium properties of the ⌬ resonance. The elementary amplitude on a single nucleon is calculated from the unitary K-matrix approach developed previously. Modifications of the properties of the ⌬ resonance due to the nuclear medium are accounted for through the self-energy operator of the ⌬, calculated from the one-pion loop. The dominant medium effects such as the Pauli blocking, mean-field modification of the nucleon and ⌬ masses, and particle-hole excitations in the pion propagator are consistently included in nuclear matter.The properties of the ⌬ in the nuclear medium are calculated in a relativistic framework for symmetrical nuclear matter, along the lines of Refs. ͓14 -16͔. The medium modifications, which are expressed through the dressing of the ⌬ propagator, are investigated using different levels of approximation. The imaginary part of the ⌬ self-energy ͑or the ⌬ decay width͒ is calculated in different models for the nuclear medium. Dispersion relations are used to determine the real part ͑mass modification͒ of the ⌬ self-energy.