Effects of non-resonant photoproductions arising from two different πN N couplings are investigated in the γN → ππN reaction. We find that the pseudoscalar (PS) πN N coupling is generally preferable to the pseudovector (PV) πN N coupling and particularly the total cross sections are successfully described by the model with the PS πN N coupling. In order to see the difference between the two couplings, we also show the results of invariant mass spectra and helicity-dependent cross sections in various isospin channels calculated with the PS and PV couplings.

We construct the formula for the photonuclear total absorption cross section using the projection method and the unitarity relation. Our treatment is very effective when interference effects in the absorption processes on a nucleon are strong. The disappearance of the peak around the position of the D 13 resonance in the nuclear photoabsorption can be explained with the cooperative effect of the interference in two-pion production processes, the Fermi motion, the collision broadenings of ⌬ and N*, and the pion distortion in the nuclear medium. The change of the interference effect by the medium plays an important role.

We propose a new model for nonresonant processes that can be used over a wide range of photon energies from the threshold up to 1 GeV. The model is constructed using the pseudoscalar πN N coupling and γN N vertex with the off-shell form factor of the nucleon in a phenomenological way. * )