CORE POLARIZATION PHENOMENA IN PION-NUCLEUS CHARGE-EXCHANGE REACTIONS ABOVE THE DELTA-RESONANCE

We study pion-induced single- and double-charge exchange reactions in nuclei at energies above the DELTA(3/2, 3/2) resonance using a microscopic, parameter-free Glauber approach. We introduce corrections in the amplitudes due to the medium polarization from an isospin-flip spin-nonflip source which dominates the reaction in the transitions studied and which has not previously been identified. Using an effective force derived from the study of electromagnetic transitions we obtained sizable reductions of the cross sections in both single- and double-charge exchanges, which bring the results of both reactions into close agreement with experiment. Predictions for angular distributions for both reactions on C-14 O-18, and Ca-42 targets are made which should serve as guidelines for experiments planned in this region and as further tests of the proposed isovector renormalization.