Resolution of discrete final states in the16 O(e,e ′ pp) 14 C reaction may provide an interesting tool to discriminate between contributions from one-and two-body currents in this reaction. This is based on the observation that the 0 + ground state and first 2 + state of 14 C are reached predominantly by the removal of a 1 S0 pair from 16 O in this reaction, whereas other states mostly arise by the removal of a 3 P pair. This theoretical prediction has been supported recently by an analysis of the pair momentum distribution of the experimental data [1]. In this paper we present results of reaction calculations performed in a direct knock-out framework where final-state interaction and one-and two-body currents are included. The two-nucleon overlap integrals are obtained from a calculation of the two-proton spectral function of 16 O and include both long-range and short-range correlations. The kinematics chosen in the calculations is relevant for recent experiments at NIKHEF and Mainz. We find that the knock-out of a 3 P proton pair is largely due to the (two-body) ∆-current. The 1 S0 pair knock-out, on the other hand, is dominated by contributions from the one-body current and therefore sensitive to two-body short-range correlations. This opens up good perspectives for the study of these correlations in the 16 O(e,e ′ pp) reaction involving the lowest few states in 14 C. In particular the longitudinal structure function f00, which might be separated with super-parallel kinematics, turns out to be quite sensitive to the NN potential that is adopted in the calculations.

Spectroscopic factors for one-nucleon knockout from 16 O are calculated for states with low excitation energy in 15 N with the Bonn-C potential. A method is proposed to deal with both short-and long-range correlations consistently. For this purpose a Green's function formalism is used and the self-energy in the Dyson equation is approximated as the sum of an energy-dependent Hartree-Fock ͑HF͒ term and dispersion and correlation terms of higher order in the G-matrix interaction. This G matrix is obtained by solving the Bethe-Goldstone equation with a Pauli operator which excludes just the model space treated in the subsequent calculation of the self-energy. The energy dependence of the HF energies induces an additional reduction of the spectroscopic factors for quasiparticle states close to the Fermi level by about 10%. Experimental data may signal the need of some further improvement in the treatment of intermediate-and long-range correlations. ͓S0556-2813͑96͒04105-2͔

A procedure for the calculation of the two-body spectral function of a finite nucleus is presented. This spectral function is used to calculate the longitudinal part of the 16 O(e,eЈpp) cross section assuming plane waves for the outgoing nucleons. Short-range correlation effects are included in the pair-removal amplitudes by adding corresponding defect wave functions that are obtained from the solution of the Bethe-Goldstone equation in the finite nucleus. The associated G matrix is used as the effective interaction in a large but finite model space to calculate the pair-removal amplitudes in a random-phase approach. The resulting spectral functions exhibit clear differences between different realistic interactions in the momentum range 2-5 fm Ϫ1 for the initial proton momenta. ͓S0556-2813͑96͒01309-X͔

The reaction 16 O͑e, e 0 pp͒ 14 C has been studied at a transferred four-momentum ͑v, jqj͒ ͑210 MeV, 300 MeV͞c͒. The differential cross sections for the transitions to the ground state and the lowest excited states in 14 C were determined as a function of the momentum of the recoiling 14 C nucleus and the angle between the momentum of the proton emitted in the forward direction and the momentum transfer q. A comparison of the data to the results of calculations, performed with a microscopic model, shows clear signatures for short-range correlations in the 16 O ground state. [S0031-9007(98)07083-5] PACS numbers: 21.10. Pc, 21.30.Fe, 25.30.Fj, 27.20. + n In recent years, studies on short-range correlations (SRC) in nuclei have made striking progress. Microscopic many-body calculations in nuclear matter [1][2][3] and nuclei [4][5][6] have shown that SRC can account for a sizable fraction of the depletion in the occupancy of the valence orbits, observed in (e, e 0 p) proton knockout reactions [7]. Furthermore, these calculations predict an enhancement of the high-momentum components in the nucleon wave functions. Signatures of admixtures of highmomentum components in the nuclear ground state are expected to be found in the (e, e 0 p) reaction at high missing energies and in two-nucleon knockout (e, e 0 NN) studies [8,9]. Although experimentally more involved, the latter reactions have distinct advantages as a probe for studying SRC in nuclei.In an exclusive (e, e 0 NN) reaction both ejectiles are identified and the excitation energy of the residual nucleus is determined by energy conservation. This allows the measurement of the cross section for transitions to discrete states, as has recently been shown for the 16 O͑e, e 0 pp͒ 14 C reaction [10,11]. Furthermore, the reaction mechanism for two-nucleon knockout by virtual photons depends on the spin and isospin of the nucleon pair in the initial state. This implies that complementary information on SRC can be extracted from (e, e 0 pp) and (e, e 0 pn) reaction studies.In Ref.[10], we have presented the first results of a triple coincidence 16 O͑e, e 0 pp͒ 14 C experiment. The excitation energy spectrum up to 20 MeV of the residual nucleus 14 C and the corresponding missing-momentum distributions were compared with calculations performed within a simple factorization approximation of the cross section. In this Letter the differential cross sections are presented as a function of the excitation energy, the missing momentum, and the emission angle of the forward proton. The data are compared to the results of calculations performed with the microscopic model, recently described in Ref. [9].The measurements were performed with the high dutyfactor electron beam extracted from the pulse-stretcher AmPS at NIKHEF. The measurements were performed with 584 MeV electrons and the scattered electrons were detected at an angle of 26 ± . The central values of the energy transfer v and three-momentum transfer jqj were 210 MeV and 300 MeV͞c, respectively. Protons, with momenta p 1 a...

The reaction 16 O͑e, e 0 pp͒ has been studied at a transferred four-momentum ͑v, jqj͒ ͑210 MeV, 300 MeV͞c͒. Evidence has been obtained for direct knockout of proton pairs from the 1p shell. The excitation-energy spectrum of the residual nucleus and the missing-momentum densities indicate that knockout of a 1 S 0 pair dominates the reaction, while there is also a noticeable contribution from knockout of 3 P pairs. [S0031-9007(97) The description of short-range correlations (SRC) in complex nuclei is a long-standing problem in many-body physics. These correlations account for the effects of the nucleon-nucleon (NN) interaction at short distance and require a description of the dynamics of nucleons bound in a nuclear system that goes beyond the meanfield approach. Recently, several microscopic calculations of the momentum distribution of nucleons have been performed, both for nuclear matter [1][2][3] and nuclei [4,5], starting from realistic NN interactions. These calculations indicate that, due to the strong repulsive part of the NN force at short range, nucleons can scatter to energies and momenta far above the Fermi energy and momentum.If a nucleon of a strongly correlated pair is knocked out from a nucleus, e.g., after absorption of a virtual photon, the residual A 2 1 nucleus is likely to be left in a state with large excitation energy and momentum. As a consequence, the other nucleon may be emitted as well, which implies that information on SRC in nuclei can be obtained from studies of the semi-exclusive ͑e, e 0 N͒ reaction at large missing energy and momentum [6,7], or from the exclusive ͑e, e 0 NN͒ reaction. The latter reaction is expected to provide the most direct information on the effects of SRC, since in the plane wave impulse approximation (PWIA) its cross section is determined by the correlations in the relative wave function of the nucleon pair. Moreover, the identity of both emitted particles is determined, and the final state is well defined if the residual A 2 2 nucleus is left in its ground state or a low-lying excited state.Beyond PWIA, electromagnetically induced twonucleon knockout may also arise from coupling to mesonexchange currents (MEC) or result from D-excitation with subsequent decay via a DN ! NN reaction. Since SRC, MEC, and D-excitation contribute in a different way to the ͑e, e 0 pn͒ and ͑e, e 0 pp͒ reactions, these reactions are expected to yield complementary information on the different processes that contribute to the cross section.0031-9007͞97͞78(26)͞4893(5)$10.00