Dispersive effects in low energy pion-nucleus scattering

Dispersive effects on intermediate nucleons and deltas in the pion-nucleon scattering amplitude are shown to be very important for low energy (Tπ≤80 MeV) pion-nucleus reactions. The inclusion of this interaction in comparison with familiar three-body approaches is shown to alter the predicted differential cross sections of a first-order optical potential by more than an order of magnitude and total cross sections by more than a factor of four. In the presence of a second-order potential which incorporates the effect of pion true absorption, differential cross sections differ by a factor of more than three and total cross sections by a factor of two. The large effect is caused by the rapid energy dependence of the imaginary part of the two-body amplitude near the elastic threshold. (orig.)