Isobar configurations: $ΔN$ correlations versus the independent particle model
We present a comparative analysis of two models for the $A( {γ,\,πN})B$ reaction, which take into account the isobar configurations in the ground state of the nuclei: the $ΔN$ correlation model and the quasifree pion photoproduction model. The considered models differ in their descriptions of the nucleus states. The $ΔN$ correlation model takes into account the dynamic correlations of the nucleon and isobar formed in the virtual transition $NN \to ΔN$, and in the quasifree pion photoproduction model, isobars and nucleons in the nucleus are considered as independent constituents. The predictions of the models are considered for two reactions ${}^{16}$O$( {γ,\,π^{ +} p} ){}^{15}$C and ${}^{16}$O$( {γ,\,π^{ -} p} ){}^{15}$O. It is shown, that the two models predict the differential cross section significantly differing both in absolute values, and in the shape of the angular dependence. We compare the results of the $ΔN$ correlation model for the $( {γ,\,πN} )$ and $( {γ,\,πNN})$ reactions with the ${}^{16}$O$( {γ,\, π^{ -} p})$ reaction data measured at BNL. Our results give support to the $ΔN$ correlation model.