The electromagnetic pion production reactions are investigated within the dynamical coupled-channels model developed in {bf Physics Reports, 439, 193 (2007)}. The meson-baryon channels included in this study are $gamma N$, $pi N$, $eta N$, and the $p
iDelta$, $rho N$ and $sigma N$ resonant components of the $pipi N$ 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 $gamma N to N^*$ 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 $Delta$ (1232) resonance region, and can drastically change the magnitude and shape of the cross sections in the second resonance region. The importance of the off-shell effects in a dynamical approach is also demonstrated. The meson cloud effects as well as the coupled-channels contributions to the $gamma N to N^*$ form factors are found to be mainly in the low $Q^2$ region. For the magnetic M1 $gamma N to Delta$ (1232) form factor, the results are close to that of the Sato-Lee Model. Necessary improvements to the model and future developments are discussed.
As a first step to analyze the electromagnetic meson production reactions in the nucleon resonance region, the parameters of the hadronic interactions of a dynamical coupled-channel model, developed in {it Physics Reports 439, 193 (2007)}, are determ
ined by fitting the $pi N$ scattering data. The channels included in the calculations are $pi N$, $eta N$ and $pipi N$ which has $piDelta$, $rho N$, and $sigma N$ resonant components. The non-resonant meson-baryon interactions of the model are derived from a set of Lagrangians by using a unitary transformation method. One or two bare excited nucleon states in each of $S$, $P$, $D$, and $F$ partial waves are included to generate the resonant amplitudes in the fits. The parameters of the model are first determined by fitting as much as possible the empirical $pi N$ elastic scattering amplitudes of SAID up to 2 GeV. We then refine and confirm the resulting parameters by directly comparing the predicted differential cross section and target polarization asymmetry with the original data of the elastic $pi^{pm} p to pi^{pm} p$ and charge-exchange $pi^- p to pi^0 n$ processes. The predicted total cross sections of $pi N$ reactions and $pi Nto eta N$ reactions are also in good agreement with the data. Applications of the constructed model in analyzing the electromagnetic meson production data as well as the future developments are discussed.
