Vector-Meson Dominance and Pion Production

On the basis of the current-field-identity relation, a unified treatment and quantitatively successful calculation of pion production by $\rho$ mesons and photons is presented. The model, which is independent of free parameters, explains the empirically established facts about strong mass dependences in the $\rho$ density matrix elements (in particular, also in the combination ${\rho }_{11}+{\rho }_{1-1}$) considered as a function of the mass $k^2$ of the $\rho$ meson. These facts have been the basis for a recent criticism of the vector-meson-dominance hypothesis in pion production. But our results indicate that the current-field-identity relation is not in conflict with experiment. We emphasize in our discussion that no model-independent formulation of vector-meson dominance is possible in pion production, according to our present knowledge.