Core-excitation effects in O20(d,p)O21 transfer reactions: Suppression or enhancement?

O20(d,p)O21 transfer reactions are described using momentum-space Faddeev-type equations for transition operators and including the vibrational excitation of the 20O core. The available experimental cross section data at 10.5 MeV/nucleon beam energy for the 21O ground state 52+ and excited state 12+ are quite well reproduced by our calculations including the core excitation. Its effect can be roughly simulated reducing the single-particle cross section by the corresponding spectroscopic factor. Consequently, the extraction of the spectroscopic factors taking the ratio of experimental data and single-particle cross section at this energy is a reasonable procedure. However, at higher energies core-excitation effects are much more complicated and have no simple relation to spectroscopic factors. We found that core-excitation effects are qualitatively very different for reactions with the orbital angular momentum transfer ℓ=0 and ℓ=2, suppressing the cross sections for the former and enhancing for the latter, and changes the shape of the angular distribution in both cases. Furthermore, the core-excitation effect is a result of a complicated interplay between its contributions of the two- and three-body nature.