Role of long-range correlations in the quenching of spectroscopic factors.

We consider the proton and neutron quasiparticle orbits around the closed-shell (56)Ni and (48)Ca isotopes. It is found that large model spaces (beyond the capability of shell model applications) are necessary for predicting the quenching of spectroscopic factors. The particle-vibration coupling is identified as the principal mechanism. Additional correlations--due to configuration with several particle-hole excitations--are estimated using shell model calculations and generate an extra reduction which is < or approximately equal to 4% for most quasiparticle states. The theoretical calculations nicely agree with (e, e'p) and inverse kinematics knockout experiments. These results open a new path for a microscopic understanding of the shell model.