Pion photoproduction on nucleus with two-nucleon emission

A model for the pion photoproduction on nuclei in the $A\left({\gamma ,\, \pi NN} \right)B$ reaction is presented. This is an extension of our recent model for the $A\left({\gamma ,\, \pi N} \right)B$ reaction. In this approach we have moved beyond the standard shell-model considering $\Delta$\textit{N} correlations in the nuclear wave functions, which are caused by the virtual transitions $NN \to \Delta N \to NN$ in the ground state of the nucleus. The main ingredients of the model are the two- and three-particle density matrices and the transition operators $\gamma \Delta \to N\pi$ and $\gamma N \to N\pi$. The direct and exchange reaction mechanisms, which follow from the structure of the density matrices, are examined. The model is used to investigate the $^{12}$C$\left({\gamma ,\, \pi ^{+} p} \right)$ and $^{12}$C$\left({\gamma ,\, \pi ^{-} p} \right)$ reactions in the kinematic region of the large momentum transfers to the residual nuclear system, where the pion production occurs with the emission of two nucleons.