Low-energy vector-dominance-model description ofγγreactions

The threshold enhancement of $\gamma \gamma \to \rho \rho$ is reexamined. We claim that factorization, when explicitly evaluated for low energies, does account for most, if not all, of the reported cross-section data. Our analysis indicates an important low-energy isoscalar-photon contribution yielding a large $\omega \omega$ production cross section. Our approach implies that the vector-dominance model is the major low-energy $\gamma \gamma$ mechanism. This is supported by the preliminary available information on $\gamma \gamma$ total cross sections.