Resonances in forward π+π− photoproduction on hydrogen.

We show that the cross section of the Δ(1230) production in the γp → pπ+π− 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. [ABSTRACT FROM AUTHOR]