Dynamical coupled-channels effects on pion photoproduction
Physical Review C
The electromagnetic pion production reactions are investigated within the dynamical coupled-channels model developed in Physics Reports, 439, 193 (2007). The meson-baryon channels included in this study are γ N, π N, η N, and the πΔ, ρ N and σ N resonant components of the ππ N channel. With the hadronic parameters of the model determined in a recent study of π N scattering, we show that the pion photoproduction data up to the second resonance region can be described to a very large extent by
... large extent by only adjusting the bare γ N → 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 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 off-shell effects in a dynamical approach is also demonstrated. The meson cloud effects as well as the coupled-channels contributions to the γ N → N^* form factors are found to be mainly in the low Q^2 region. For the magnetic M1 γ N →Δ (1232) form factor, the results are close to that of the Sato-Lee Model. Necessary improvements to the model and future developments are discussed.