$Δ$ resonance contribution to two-photon exchange in electron-proton scattering

We calculate the effects on the elastic electron-proton scattering cross section of the two-photon exchange contribution with an intermediate $Δ$ resonance. The $Δ$ two-photon exchange contribution is found to be smaller in magnitude than the previously evaluated nucleon contribution, with an opposite sign at backward scattering angles. The sum of the nucleon and $Δ$ two-photon exchange corrections has an angular dependence compatible with both the polarisation transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors.

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