Dynamical coupled-channels effects on pion photoproduction

The electromagnetic pion production reactions are investigated within the dynamical coupled-channels model developed by Matsuyama, Sato, and Lee [Phys. Rep. 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 nonresonant electromagnetic couplings are taken from previous works. It is found that the coupled-channels effects can contribute about 30–40 % 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 loop-integrations 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. Necessary improvements to the model and future developments are discussed.