Distorted-wave approximation and nuclear correlations in pion reactions

The final- (or initial-) state interactions of pions participating in reactions with a complex nucleus are usually treated in distorted-wave impulse approximation with optical model wave functions. We improve this approximation by using an "effective pion field" which takes into account the hard-core nuclear correlations. This is illustrated here by the example of charged pion photoproduction from a ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ target.