Signatures for Short-Range Correlations inO16Observed in the ReactionO

Abstract

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...

Correlations derived from modern nucleon-nucleon potentials

“…Many attempts have been made to measure these correlations in detail. As an example for such measurements we mention the exclusive (e, e ′ N N ) reactions, which have been made possible using modern electron accelerators [1,2]. The hope is that the detailed analysis of such experiments yields information about the correlated wavefunction of the nucleon pair absorbing the virtual photon.…”

Tensor correlations in nuclei and exclusive electron scattering

“…This holds in particular for the short-range correlations that reflect the remnants of the hard-core part of the nucleon-nucleon (N N ) force in the medium. Recently, manifestations for strongly correlated proton pairs and the existence of Jastrow-like correlations emerged from the simultaneous detection of two protons upon absorption of one (virtual) photon by an atomic nucleus [1,2]. Nuclear many-body theories have produced vastly different predictions for the short-range behaviour of nuclei.…”

“…The hadronic current operator J µ (q) in the above expression accounts for the coupling of the electromagnetic field to the nuclear system. In our calculations, the one-body part (J [1] µ ) has been implemented in the standard Impulse Approximation (IA) fashion, whereas the two-body currents (J [2] µ ) include the conventional pion-exchange and intermediate ∆ 33 -resonance terms [11]. The matrix element of Eq.…”

“…The correlation functions g c and f tτ establish the link between the measurements and the nuclear many-body theories and contain the information of the "beyond meanfield" structure of nuclei. In inclusive A(e, e ′ ) and exclusive A(e, e ′ p) reactions, signals from the "correlation" part (4) in the transition matrix elements are frequently obscured by the dominant one-body term J [1] µ in Eq. (3).…”

“…The method was outlined in Ref. [11] and is based on explicitly calculating the contribution from the operator J [1] µ (i; q) + J [1] µ (j; q) −g c (r ij ) + f tτ (r ij ) S ij τ i . τ j to the two-nucleon knockout channels (A(e, e ′ pn) and A(e, e ′ pp)) and integrating over the complete phase space of the undetected nucleon.…”