Dominance ofS01Proton-Pair Emission in the

Abstract

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

Momentum distributions and spectroscopic factors of doubly closed shell nuclei in correlated basis function theory

“…The same holds for the electromagnetic form factors of the low-lying states, especially those having large angular momentum [15,16]. The emission of two like nucleons in photon and electron scattering process cannot be described by the IPM [17,18]. Also the charge density distributions extracted by elastic electron scattering data are, in the nuclear interior, smaller than those predicted by the IPM [19,20].…”

Probing short-range correlations in asymmetric nuclei with quasi-free pair knockout reactions

“…While being a fascinating phenomenon in itself, a full understanding of nuclear SRCs and its (N − Z) asymmetry dependence is pivotal for studies of compact objects like neutron stars and of the nuclear equation of state [8,9]. The last couple of decades have marked significant growth in insight into the mass and isospin dependence of nuclear SRCs thanks to an experimental program of quasi-free two-nucleon knockout reactions with hadronic and electroweak probes [10,11,12,13,14,15,16,17,18]. Measuring the multi-fold cross sections for those reactions with at least four particles in the final state, is really challenging which made one to think about alternate ways of addressing SRC physics through nuclear reactions.…”

NN final-state interaction in two-nucleon knockout from 16 O

“…Only with the advent of high-duty-cycle electron beams like NIKHEF, MAMI or TJNAF a systematic investigation of this reaction has become possible. At present, only a few pioneering measurements have been carried out [4,5,6,7], but the prospects are very encouraging [8,9].…”

Nuclear physics with electroweak probes

“…The interest in the experimental investigation of two-nucleon emission processes was triggered, at the end of the 1980s, by the prospect of the upcoming new generation of continuous beam electron accelerators [24]. In the following years a number of measurements of (e, e ′ N N ) cross sections have been carried out at NIKHEF-K [26,27,28] and MAMI [29]. Ideally, the two-nucleon knockout reaction can be regarded as the cleanest source of experimental information on NN correlations, as it may give access to the two-nucleon spectral function [25] P…”

Two-body density matrix for closed s-d shell nuclei

“…Nowadays two-body knock-out reactions such as (γ, N N ) [1,2] and (e, e ′ N N ) [3][4][5][6] are intensively studied in order to extract some information about the nucleon-nucleon short-range correlations (SRC) in nuclei. In particular, the cross section for the two-nucleon emission processes is generally related to the two-body spectral function at least in PWIA [7].…”