Model-independent extraction of two-photon effects in elastic electron-proton scattering

We address the discrepancy between the Rosenbluth and polarization transfer data for the electromagnetic form factors of the nucleon. Assuming that the effect of two-photon corrections on the polarization transfer data is negligible, we obtain a model-independent estimate of the two-photon correction Delta^(2\gamma). We analyze the polarization transfer data and the cross section data separately using dispersion relations. A central value as well as an error estimate for Delta^(2\gamma) is then obtained from a comparison of the two analyses. The resulting values for Delta^(2\gamma) are in good agreement with direct calculations available in the literature.