$\Delta(1232)$ Resonance Contribution to Two-Photon Exchange in Electron-Proton Scattering Revisited

Understand

We revisit the question of the contributions of two-photon exchange with $\Delta(1232)$ excitation to the electron-proton scattering in a hadronic model.

• Three improvements over the previous calculations are made, namely, correct vertex function for $\gamma N\rightarrow\Delta$, realistic $\gamma N\Delta$ form factors, and coupling constants.
• The discrepancy between the values of $R\equiv \mu_p G_E/G_M$ extracted from Rosenbluth technique and polarization transfer method can be reasonably accounted for if the data of Andivahis {\it et al.} \cite{Andivahis94} are analyzed.
• However, substantial discrepancy remains if the data of Qattan {\it et al.} \cite{Qattan05} are used.