Medium effects in photopion reactions

After discussing the status of distorted wave impulse approximation approaches to the photoproduction of pions from nuclear targets, it is shown that medium effects due to the excitation of particle-hole states by pion propagation are quite significant and serve to improve agreement between coordinate space distorted-wave impulse-approximation calculations and recent data on the reaction ${}_{}{}^{14}{}_{}{}^{}(\gamma$,${\pi }^{+}$ ${)}^{14}$C. The need to include such medium effects in distorted-wave impulse-approximation photopion calculations is demonstrated, but evidence is given that one must go beyond the local density approximation for evaluating medium effects. The role of recent nuclear structure information and of pion distorted waves on photopion production calculations are also examined.