Vector-Dominance Model and Photoproduction of Pions

Abstract

The relation between the processes $\gamma +N\to \pi +N$ and $\pi +N\to \rho +N$, according to the vector-dominance model (VDM), is reexamined. The VDM is defined in terms of the behavior of the invariants in the matrix elements of the vector current. The consequences of the VDM, and assumptions about asymptotic properties, for measurable quantities are discussed. The inability of the VDM to account for the results of polarized-photoproduction experiments is confirmed. An appendix provides a detailed discussion of the relation between invariants and density matrix elements in various frames.