Treatment of the Δ current in electromagnetic two-nucleon knockout reactions

The contribution of the Δ(1232) isobar to the electromagnetic current of the two-nucleon system and its role in (γ,NN) processes is investigated. The difference between the genuine Δ-excitation current and that part of the current connected to the deexcitation of a preformed Δ in the target nucleus is stressed. The latter cannot lead to a resonant behaviour of matrix elements for energies in the Δ region. The reaction 16O(γ,pp)14C, where the Δ contribution is dominant at intermediate energies, is considered. The large variations found in the cross sections for different treatments stress the need for a proper treatment of the Δ current for a clear understanding of the reaction mechanism of two-nucleon emission processes. (orig.)