Coupling effects in the extraction of spectroscopic factors

Often one-nucleon spectroscopic factors for loosely-bound nuclei are extracted from peripheral reaction data. This is done under the assumption that, aside from the normalization, the one- nucleon overlap function for the reaction can be approximated in terms of a pure single-particle orbital. The latter can be expected to be valid when the spectroscopic approaches 1, but not necessarily otherwise. To test the validity of the single- particle assumption, we study the core+n system with coupling to excitation of the core. Interactions are fitted to reproduce a realistic nucleus (11Be). We also study an extreme-coupling toy- model, with only two states coupled through a surface delta force. Our results generally render support for the single- particle approximation in the extraction of the spectroscopic factor, even for small values of the factor. When the small values of the spectroscopic factor are combined, though, with a strong localization of the coupling, as in our toy model, deviations from the single particle form can become important.