Pion photoproduction on nucleus with two-nucleon emission

A model for the pion photoproduction on nuclei in the A(gamma, pi NN)B reaction is presented. This is an extension of our recent model for the A(gamma, pi N)B reaction. In this approach we have moved beyond the standard shell-model considering AN correlations in the nuclear wave functions, which are caused by the virtual transitions NN -> Delta N -> 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 -> N pi- and gamma n > 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 C-12(gamma, pi+p) and C-12(gamma,pi(-) p) 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. Signatures of the Delta N correlations in the exclusive cross-sections of the (12)c (gamma, pi(+)pn)(10) Be reaction are considered. (C) 2013 Elsevier B.V. All rights reserved.