Unitarity constraints on charged pion photoproduction at large p_perp

Around ${\theta }_{\pi }={90}^{\circ }$, the coupling to the ${\rho }^{\circ }N$ channel leads to a good accounting of the charged pion exclusive photoproduction cross section in the energy range 3 $<E_{\gamma }<$ 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.