Abstract

Nucleon-nucleon correlations, especially those of short-range character, can be well studied with electron-induced two-nucleon knockout reactions at intermediate electron energies. However, these reactions are not only driven by one-body currents, i.e.,
coupling of the virtual photon to one of the nucleons of a correlated pair, a process that
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directly probes NN-correlations. Also
two-body currents, resulting from intermediate Delta-excitation and coupling to exchanged mesons, as well as final state
interactions, influence the experimental cross section. Exclusive measurements of the three-body breakup of 3He offer the
opportunity to compare data to microscopic calculations. The relative importance of competing two-proton knockout
mechanisms can be investigated by varying the energy and momentum of the virtual photon. The experiment was performed
with the electron beam extracted from the Amsterdam Pulse Stretcher (AmPS) at NIKHEF; the incident electron energy was 564
MeV. A cryogenic, high-pressure 3He gas target was used with a thickness of 270 mg/cm^2. Scattered electrons were detected
in the QDQ magnetic spectrometer and both emitted protons in the HADRON plastic scintillator arrays. Cross sections were
determined for three values of the three-momentum transfer of the virtual photon (q=305, 375, and 445 MeV/c) at an energy
transfer value omega of 220 MeV. At q=375 MeV/c, measurements were performed over a continuous range in energy transfer
from 170 to 290 MeV. The data are compared to results of continuum-Faddeev calculations performed by Golak et al., that
account for rescattering among the emitted nucleons. Various potential models were used in the calculations: Bonn-B,
CD-Bonn, Nijmegen-93 and Argonne v18 . Presentation of the data as a function of the missing or neutron momentum, pm,
shows that the cross section decreases exponentially as a function of pm. Calculations performed with only a one-body
hadronic current operator show fair agreement with data obtained at pm < 100 MeV/c at omega = 220 MeV for all q-values. It can
therefore be concluded that at omega = 220 MeV and pm < 100 MeV/c the cross section is dominated by direct knockout of two
protons via a one-body hadronic current. At higher neutron momentum values, data and theoretical predictions differ up to a
factor of five for all values of omega. Within the range of energy transfer values probed in this experiment, the high pm domain
is expected to be strongly influenced by intermediate excitation in the proton-neutron pair. Within specific regions of phase
space, where two nucleons are emitted with comparable momentum vectors, rescattering processes strongly influence the cross
section. For a such a region, measured at q=445 MeV/c, good agreement was found between data and the continuum- Faddeev
calculations as a function of the pn momentum difference in the final state. Information on the wave function of 3He may be
obtained in the domain omega = 220 MeV and pm < 100 MeV/c by representing the cross section as a function of pdiff1, which
can be related to the relative momentum of the constituents of the two-proton pair in the initial state. The observed decrease of
the cross section reflects the behaviour of the wave function and is well reproduced by calculations. At present, the data do
not permit to express preference for any one of the potential models considered.
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