Pion photoproduction in a dynamical coupled-channels model

The charged and neutral pion photoproduction reactions are investigated in a dynamical coupled-channels approach based on the formulation of Haberzettl, Huang, and Nakayama [ Phys. Rev. C 83, 065502 (2011)]. The hadronic final-state interaction is provided by the Jülich $\pi N$ model, which includes the channels $\pi N$ and $\eta N$ comprising stable hadrons as well as the effective $\pi \pi N$ channels $\pi \Delta$, $\sigma N$, and $\rho N$. This hadronic model has been quite successful in describing $\pi N\to \pi N$ scattering for center-of-mass energies up to $1.9$ GeV. By construction, the full pion photoproduction current satisfies the generalized Ward-Takahashi identity and thus is gauge invariant as a matter of course. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions $\gamma p\to {\pi }^{+}n$, $\gamma p\to {\pi }^{0}p$, $\gamma n\to {\pi }^{-}p$, and $\gamma n\to {\pi }^{0}n$ are in good agreement with the experimental data.