Resonances in forward $\pi^+\pi^-$ photoproduction on hydrogen

Abstract

We show that the cross section of the $\Delta(1230)$ production in the $\gamma p \to p \pi^+\pi^-$ reaction at forward angles and with unpolarized photons can be described in terms of gauge invariant one pion t-channel exchange amplitudes. Proper description of the polarized cross sections requires, however, the inclusion of contributions from the baryon exchanges.