Calculation of two-photon exchange effects for Δ production in electron-proton collisions

We present an evaluation of a two-photon exchange correction to the cross section for unpolarized Delta isobar production in electron-proton collisions, using a relativistic, crossing symmetric and gauge invariant approach. The calculated box and crossed-box diagrams include nucleon and Delta intermediate states. We find a relation between the angular nonlinearity of the two-photon exchange contribution and the value of the gamma Delta Delta coupling constant. (c) 2006 Elsevier B.V. All rights reserved.