Nondiffractive feature of γN→ρ±N\gamma N\to \rho^\pm N with ρ\rho-meson electromagnetic multipole moments

We investigate photoproduction of charged $\rho$ off the nucleon using $\rho(770)+\pi(140)$ Regge pole exchanges by considering the $\rho$-meson electromagnetic multipole moments. The significance of the Ward identity at the $\gamma\rho\rho$ vertex is emphasized for current conservation in the process. Given $\pi$ exchange with the well-known coupling constants for $\gamma\pi\rho$ and $\pi NN$, we analyze the role of the $\rho$ exchange in the $\gamma p\to \rho^+n$ and $\gamma n\to\rho^-p$ processes without model-dependences except for the magnetic moment $\mu_{\rho^\pm}=\pm2.01$ and electric quadrupole moment ${\cal Q}_{\rho^\pm}=\pm0.027$ fm$^2$ taken from theoretical estimates. The nondiffractive feature of both cross sections is reproduced with a rapid decrease beyond the resonance region by the dominance of $\pi$ exchange over the $\rho$\,. % Cross sections for differential and density matrix elements are presented to compare with existing data. The parity and photon polarization asymmetries are predicted to demonstrate the apparent roles of the $\rho$-meson electromagnetic multipole moments.Comment: 5 pages, 8 figure

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.Comment: 14 pages, 24 figure