Dynamical coupled-channels effects on pion photoproduction

Abstract

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 $\ensuremath{\gamma}N,\ensuremath{\pi}N,\ensuremath{\eta}N$, and the $\ensuremath{\pi}\ensuremath{\Delta},\ensuremath{\rho}N$, and $\ensuremath{\sigma}N$ resonant components of the $\ensuremath{\pi}\ensuremath{\pi}N$ channel. With the hadronic parameters of the model determined in a recent study of $\ensuremath{\pi}N$ 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 $\ensuremath{\gamma}N\ensuremath{\rightarrow}{N}^{*}$ 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 \ensuremath{\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 $\ensuremath{\gamma}N\ensuremath{\rightarrow}{N}^{*}$ form factors are found to be mainly in the low ${Q}^{2}$ region. Necessary improvements to the model and future developments are discussed.