On meson-exchange and Δ-isobar currents in the two-nucleon photoabsorption mechanism

The role of π- and ϱ-meson exchange currents and dynamic Δ-isobar currents with both π- and ϱ-exchange are studied in two-nucleon knockout reactions. The Gottfried approximation is tested for these different absorption mechanisms by making a comparison between a factorized (Gottfried) model and an unfactorized model. For the unfactorized model, analytical expressions are given for the different absorption mechanisms by using harmonic-oscillator wavefunctions. The formalism is applied to the ¹⁶O(γ,pn) reaction in the photon-energy range Eγ = 60–300 MeV. Over the whole energy range, the ϱ-meson degree of freedom is found not to be negligible. It is demonstrated that the unfactorized treatment leads to a sizeable reduction of the cross section and to a change of the shape of the angular cross section.