The low-energy pion-nucleus interaction

We discuss the low-energy pion-nucleus interaction within the framework of the optical model. We find that a consistent expansion of the πN amplitude in momentum is required in order that the low-energy pion-nucleus interaction is physically compatible with a simple compositie picture of the nuclear sacttering process. This introduces an energy-dependent component into the usual local part of the Ericson-Ericson optical potential stemming from the πN s-wave effective range. The size of this component is estimated and the corresponding pion-nucleus effective ranges and volume parameters are determined for uniformly dense nuclei. Finally a modified extrapolation to the bound state energies is considered in the light of these large effective ranges.