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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 $^{16}O(γ,pp)^{14}C$, 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.