Isobar configurations: $\Delta N$ correlations versus the independent particle model

We present a comparative analysis of two models for the $A( {\gamma,\,\pi N})B$ reaction, which take into account the isobar configurations in the ground state of the nuclei: the $\Delta N$ correlation model and the quasifree pion photoproduction model. The considered models differ in their descriptions of the nucleus states. The $\Delta N$ correlation model takes into account the dynamic correlations of the nucleon and isobar formed in the virtual transition $NN \to \Delta N$, 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$( {\gamma ,\,\pi ^{ +} p} ){}^{15}$C and ${}^{16}$O$( {\gamma ,\,\pi ^{ -} p} ){}^{15}$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 $\Delta N$ correlation model for the $( {\gamma ,\,\pi N} )$ and $( {\gamma ,\,\pi NN})$ reactions with the ${}^{16}$O$( {\gamma ,\, \pi ^{ -} p})$ reaction data measured at BNL. Our results give support to the $\Delta N$ correlation model.