Dynamical Coupled-channels Effects on Pion Photoproduction

The electromagnetic pion production reactions are investigated within the dynamical coupled-channels model developed in Physics Reports, 439, 193 (2007). The meson-baryon channels included in this study are γN, πN, ηN, and the πΔ, ρN and σN resonant components of the ππN channel. With the hadronic parameters of the model determined in a recent study of πN scattering, we show that the pion photoproduction data up to the second resonance region can be described to a very large extent by only adjusting the bare γN → N* helicity amplitudes, while the non-resonant electromagnetic couplings are taken from previous works. It is found that the coupled-channels effects can contribute about 10-20 % of the production cross sections in the Δ (1232) resonance region, and can drastically change the magnitude and shape of the cross sections in the second resonance region. The importance of the off-shell effects in a dynamical approach is also demonstrated. The meson cloud effects as well as the coupled-channels contributions to the γN → N* form factors are found to be mainly in the low Q2 region. For the magnetic M1 γN → Δ (1232) form factor, the results are close to that of the Sato-Lee Model. Necessary improvements to the model and future developments are discussed