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Beyond mean-field theory in nuclei at the drip lines

Models based on particle-vibration coupling have been applied for many years to the study of single-particle states and giant resonances (GR) in stable nuclei, and are quite well established. In particular, in the last decade we have developed, within a Milano-Orsay collaboration, a microscopic model aimed to a detailed description of GR excitation and decay. In the present contribution, we discuss an extension of this model to include in a simple way the pairing correlations, which allows the study of GR along an isotopic chain. We apply the model, based on the quasi-particle-phonon coupling and including up to four quasi-particle-type configurations, to the dipole response of neutron-rich oxygen isotopes, recently mesaured at GSI. Low-lying dipole strength is found, which exhausts between 5 and 10% of the Thomas-Reiche-Kuhn (TRK) energyweighted-sum-rule (EWSR) below 15MeV excitation energy, in rather good agreement with the experimental data. The role of the phonon coupling is shown to be crucial in order to obtain this result. We elucidate the possible relations between these calculations and those based on the shell-model.