Unitarity constraints on charged pion photoproduction at large pperpendicular

Around θπ=90 deg., the coupling to the ρ0N channel leads to a good accounting of the charged pion exclusive photoproduction cross section in the energy range 3<Eγ<10 GeV, where experimental data exist. Starting from a Regge pole approach that successfully describes vector meson production, the singular part of the corresponding box diagrams (where the intermediate vector meson-baryon pair propagates on-shell) is evaluated without any further assumptions (unitarity). Such a treatment provides an explanation of the s-7 scaling of the cross section. Elastic rescattering of the charged pion improves the basic Regge pole model at forward and backward angles.