The12C(γ,NN)reaction studied over a wide kinematic range

Abstract

The ${}^{12}\mathrm{C}(\ensuremath{\gamma},np)$ and ${}^{12}\mathrm{C}(\ensuremath{\gamma},pp)$ reactions have been studied using the Glasgow photon tagging spectrometer at the Mainz MAMI electron microtron for ${E}_{\ensuremath{\gamma}}=150--700$ MeV over a kinematic range which extends well beyond the approximately back-to-back detector arrangements of previous work. For ${}^{12}\mathrm{C}(\ensuremath{\gamma},np)$ the general trends of the missing energy distributions are reproduced over a wide range of kinematics and photon energies by the theory developed by the Valencia group. The corresponding ${}^{12}\mathrm{C}(\ensuremath{\gamma},pp)$ channel is overestimated by a factor of \ensuremath{\sim}3. Detailed comparisons of the experimental data with a Monte Carlo simulation of the direct $2N$ knockout process provide the first evidence above the $\ensuremath{\Delta}$ resonance for direct $2N$ knockout and show that this process dominates the ${}^{12}\mathrm{C}(\ensuremath{\gamma},np)$ reaction at low missing energies up to ${E}_{\ensuremath{\gamma}}\ensuremath{\sim}700$ MeV. The ${}^{12}\mathrm{C}(\ensuremath{\gamma},pp)$ reaction is somewhat less well described by the Monte Carlo simulation. A possible explanation of the observed discrepancies within a direct $2N$ framework is presented. At high recoil momenta both $2N$ reaction channels exhibit an excess yield compared to the Monte Carlo prediction of direct $2N$ knockout. The excess yield in this region is compared with the predicted effects of short-range correlations and with the predicted contributions due to other reaction mechanisms.