Calculation of the (e,e'2p) Cross Section

The combined effects of the short-range NN interaction and of long-range correlations associated with the finite size of the nucleus have been investigated publ.70 for the two-proton removal amplitudes from 16O to discrete final states of the final nucleus 14C. As a first step, the sp spectral function has to be obtained by including the depletion of shell-model orbits by short-range correlations. This is accomplished by treating the energy dependence of the sp energies obtained from the G-matrix effective interaction. With the same interaction, the Tamm-Dancoff coupling of the sp motion to two-particle-one-hole and two-hole-one-particle states was included to describe the effect of long-range correlations publ.69. With this description of the sp propagator, the two-proton removal amplitudes were calculated using the dressed RPA method (DRPA). The effect of short-range correlations on this two-proton removal spectral function is then taken into account by the replacement of the relative uncorrelated 1S0 and 3PJ wave functions by correlated ones, using defect wave functions from the solution of the Bethe-Goldstone equation in 16O. For low-lying discrete final states, the resulting spectral functions display a considerable sensitivity to the chosen realistic NN interaction.

Calculations of the triple-coincidence cross section have been completed using the reaction code of the Pavia group ref.1, employing as input the two-nucleon removal amplitudes described above. This code includes the effects of two-body currents due to the excitation of the D resonance as well as a treatment of FSI by means of spin-dependent optical potentials. The results of this study publ.73 suggest that the (e,e'2p) reaction may be highly selective since the calculations indicate the exclusive population of the lowest 0+ and 2+ states by the removal of a 1S0 proton pair, whereas the 1+ state is dominated by 3PJ removal. The calculations reported in publ.73 were performed for kinematical settings relevant to recent experiments at NIKHEF and Mainz. Under these conditions, the knockout of a 3PJ proton pair is largely due to the (two-body) D current. The 1S0 pair knockout, on the other hand, is dominated by the one-body current and therefore sensitive to two-body short-range correlations. Accordingly, good prospects for the study of these long-sought correlations exist in the 16O(e,e'2p) reaction involving the lowest states of 14C. Our group has collaborated on a publication of the analysis of the NIKHEF data publ.75 in which actual triple-coincidence cross sections are reported for the 16O(e,e'2p)14C reaction. The comparison of the data with our calculations is also published in this letter and shows clear signatures of short-range correlations in the 16O ground state. Further data in different kinematical conditions confirm this conclusion and were published in publ.79.


    1. C. Giusti and F. D. Pacati, Nucl. Phys. A615, 373 (1997).