Center-of-mass effects in electromagnetic two-proton knockout reactions

The role of center-of mass (CM) effects in the one-body nuclear current in the description of electromagnetically induced two-nucleon knockout reactions is discussed in connection with the problem of the lack of orthogonality between initial bound states and final scattering states obtained by the use of an energy-dependent optical model potential. Results for the cross sections of the exclusive $^{16}$O(e,e$'$pp)$^{14}$C and $^{16}$O($\gamma$,pp)$^{14}$C knockout reactions in different kinematics are presented and discussed. In super-parallel kinematics CM effects produce a strong enhancement of the $^{16}$O(e,e$'$pp)$^{14}$C$_{\rm g.s.}$ cross section which strongly reduces the destructive interference between the one-body and $\Delta$-current and the sensitivity to the treatment of the $\Delta$-current found in previous work.