Outlook and Open Questions

The structure of correlated many-body systems, particularly at distance scales small compared to the constituent, in this case nucleon radius, presents a formidable challenge to both experiment and theory. The observed quenching of single-particle spectroscopic factors indicates that mean-field strength is fragmented over a large region of nuclear excitation energies. At large internal momenta, corresponding to distances <0.5 fm, effects from heavier meson exchanges () and the internal structure of the nucleon itself could be important. The possibility exists that one or both nucleons involved in short-range collisions could be excited to the (or to various resonances), producing and components in nuclear wavefunctions. The concept of individual nucleons may even break down for sufficiently small inter-nucleon distances so that quark degrees of freedom become appropriate, with two interacting nucleons being replaced by a six-quark bag. In addition to the intrinsic interest in the two-nucleon correlation function, knowledge of it is important for the understanding of other processes, notably deep inelastic electron scattering and, in particular, the EMC effect. We must understand the short-range correlation structure of nuclei if we are to understand how quark distributions inside nucleons behave in the nuclear environment.

These questions will be addressed by a variety of experimental approaches such as (e,ep) at extreme kinematics, i.e., with both momentum transfers and the internal nucleon momentum in the GeV/c range, a combination only obtainable at CEBAF. This, in turn, has two aspects: the high-momentum structure of single-particle states and that of the continuum. To address the former requires the additional capability of high resolution.

Initial explorations of high-momentum (> 500 MeV/c) components have recently been made with the high duty factor electron beams at NIKHEF [Bo94] where several single-particle states in PB were measured for internal nucleon momenta from 300-500 MeV/c. Mainz [Bl95] reported results on the -hole states in O out to 700 MeV/c where the internal momentum is becoming a significant fraction of the nucleon mass. At such kinematics, one might expect the beginnings of a transition to a relativistic description. The 855 MeV beam at Mainz allowed these high-momentum components to be probed at considerably higher momentum transfers (up to 550 MeV/c) than the NIKHEF measurement which was limited to MeV/c. Both experiments found that the high- momentum components of the hole states were significantly larger than mean field predictions. The NIKHEF results indicate that long-range correlations that couple the hole states to low-lying nuclear excitations play an important role. To go beyond these type of measurements requires considerably higher beam energies that allows increased spatial resolution of the photon, i.e., high . Only at CEBAF can momentum transfers (GeV/c) and internal momenta approaching 1 GeV/c be probed simultaneously.

Virtually all theoretical treatments of two-body correlations predict that much of the high-momentum strength resides in the continuum of the A1 system, with hundreds of times greater strength than in uncorrelated discrete states. This basic feature results essentially from the interaction with a correlated nucleon pair, both of which are ejected. For this reason, at both MIT/Bates and CEBAF energies, detection of multi-particle final states will be important for further elucidation of the role that two- (or more) body correlations play in absorbing the energy and momentum of real and virtual photons.