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 gamma N, pi N, eta N, and the pi Delta, rho N, and sigma N resonant components of the pi pi N channel. With the hadronic parameters of the model determined in a recent study of pi N 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 gamma N -> 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 Delta (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 gamma N -> N* form factors are found to be mainly in the low Q(2) region. Necessary improvements to the model and future developments are discussed.