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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...

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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

The cross section for the 3 He͑e, e 0 d͒p reaction has been measured for a range of missing momentum p m at incident electron energies of 370 and 576 MeV and for values of the three-momentum transfer q of 412, 504, and 604 MeV͞c. The longitudinal and transverse structure functions have been separated for q 412 and 504 MeV͞c. The data are compared to exact three-body Faddeev calculations and calculations based on a covariant, gauge-invariant diagrammatic expansion. In general, fair to good agreement is observed, but there are some differences between the data and the calculations, especially for the q dependence and for the transverse structure function W T . [S0031-9007 (98) Many nuclear properties can be described successfully within a mean-field approach. However, phenomena like the depletion of spectroscopic strength and the occurrence of bumps at missing energies characteristic of two-nucleon emission in ͑e, e 0 p͒ reactions indicate that correlations between nucleons, i.e., the motion of two nucleons relative to each other and as a pair inside a nucleus, also play an essential role. The ͑e, e 0 d͒ reaction has proven to be a sensitive tool for the investigation of proton-neutron ͑pn͒ correlations in nuclei. It has been studied on the nuclei Here the cross section for scattering of an electron from a pn pair in the nucleus s e,pn , which depends on the momentum transfer q, reflects the relative protonneutron motion, i.e., the relative pn wave function. The distorted spectral function S pn , which depends on the missing energy E m , the missing momentum p m , and the momentum of the final deuteron p d , contains the information about the center-of-mass (c.m.) motion of the pn pair within the nucleus, modified by the finalstate interaction (FSI). Even though this formula is an approximation, it indicates that the relative and the c.m. behavior of the pn wave function in a nucleus can be studied separately by measuring the q and p m dependence of the ͑e, e 0 d͒ cross section.This approach has proven to be successful for the 6 Li͑e, e 0 d͒ 4 He reaction [4][5][6], and for transitions in the 12 C͑e, e 0 d͒ 10 B reaction [4,7], including one in which the initial pn pair was in a T 1 state. However, the data for the 4 He͑e, e 0 d͒ 2 H reaction [3] could not be explained within the above-mentioned DWIA framework.For the three-nucleon system exact calculations for the ground state and the continuum are now available. By confronting those with detailed accurate experimental data one now can learn about the description of the ͑e, e 0 d͒ process and the pn motion in the nucleus 3 He. A first experiment was performed by Keizer et al. [1], which indicated that in parallel kinematics the q dependence of the cross section for q 350, 380, and 450 MeV͞c follows within error bars that of the elastic electrondeuteron cross section s e,d . This is surprising, since the virtual photon can also interact with a T 1 pn pair in 3 He, which process has a different q dependence from that of the elastic channel. More recently this react...