High-momentum Protons and the (e,e'p) Cross Section

The presence of high-momentum nucleons in the nuclear ground state remains an issue of great interest. The possible experimental demonstration of such high-momentum components has been explored in a theoretical calculation of the cross section for the (e,e'p) reaction. The spectral function of 16O was obtained earlier in publ.53 and publ.59 by solving the Dyson equation in the finite nucleus using a realistic nucleon-nucleon (NN) interaction. A meaningful theoretical determination of the coincidence cross section for the (e,e'p) reaction involves both the nucleon spectral function and a realistic description of the wave function of the ultimately detected proton inside the nucleus. In order to proceed to a more complete theoretical treatment of the coincidence cross section and to establish the possible detectability of removal processes involving protons with high initial momentum, our group, in collaboration with the Pavia group, has used the theoretical spectral functions obtained in publ.53 and publ.59 to determine the coincidence cross section for the (e,e'p) reaction on 16O. The results of this collaboration have been published in publ.71 and publ.72. At low missing energy the removal of protons from p1/2 and p3/2 quasihole states has been studied. The comparison of the missing momentum dependence of these transitions with experimental data from NIKHEF ref.1 and Mainz ref.2 is very favorable. The slight difference between theory and experiment at high missing momenta can at most account for a very tiny fraction of the single-particle (sp) strength. This strength is predicted to be present at these momenta publ.53, publ.59 based on information of the momentum distribution. A comparison for the p3/2 quasihole state with the results of ref.3 using the Argonne v14 NN interaction ref.4 suggests that the Green's function result yields a slightly better description of the momentum dependence of the cross section. While the shape of the cross section is nicely described by these new results, the associated spectroscopic factors are overestimated substantially. A substantial fraction of this discrepancy can be ascribed to the influence of long-range correlations publ.69. A nonnegligible discrepancy remains, however, since a correct treatment of the center-of-mass motion cannot explain the remaining difference ref.5. Our results publ.71, publ.72 clearly support the notion that the presence of high-momentum components in the nuclear ground state can only be explored by considering high missing energies in the (e,e'p) reaction. Although other processes may contribute at these energies, our results demonstrate that detectable cross sections can be expected, indicating the presence of high-momentum nucleons in the nucleus. On the other hand, we obtain negligible cross sections for high-momentum nucleons at low missing energy, further supporting the results obtained in publ.53, publ.59. Experimental work at JLab has recently confirmed the presence of high-momentum components at high missing energy ref.6.


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