Charged pion photoproduction with the Delta(1232) baryon beyond the   resonance region
  We investigate the charged pion photoproduction off the proton target with the Delta(1232) baryon in the final state, i.e. gamma p -> pi^- Delta^++(1232) and gamma p -> pi^+ Delta^0(1232), based on the effective Lagrangian method, beyond the resonance region, E_cm >~ GeV. We employ the pi- and rho-meson Regge trajectories in the t-channel, in addition to the proton- and Delta-pole, in the s- and u-channels respectively, and the contact-interaction contributions. A specific scheme for the form factor which satisfies the Ward-Takahashi identity, crossing symmetry, and on-shell condition, is taken into account. To discuss the validity of the Regge approach within the Effective Lagrangian method, we also consider a smooth interpolation between the Regge- and Feynman-propagators for the t-channel meson exchanges as a function of sqrt(s). We present the numerical results for the energy and angular dependences of the cross sections, and double-polarization observable. It turns out that the present framework shows the significance of the pi-exchange and contact interaction terms to reproduce the experimental data qualitatively well. Especially, the interpolation between the two propagators plays a crucial role to reproduce the high-energy experimental data. The pi^+ decay-angle distribution is also studied using the Delta^++ decay frame, i.e. the Gottfried-Jackson frame. The present results will be a useful guide for future high-energy photon-beam experiments. 
Seung-il Nam, Byung-Geel Yu
hep-ph