Box diagram in the elastic electron-proton scattering

We present an evaluation of box diagram for the elastic $ep$ scattering with proton in the intermediate state. Using analytic properties of the proton form factors we express the amplitude via twofold integral, which involves the form factors in the space-like region only. Therefore experimentally measured form factors can be used in the calculations directly. The numerical calculation is done with the form factors extracted by Rosenbluth, as well as by polarization transfer methods. The dependence of the results on the form factor choice is small for $Q^2 < 6 GeV^2$, but becomes sizable at higher $Q^2$.