We study two-photon exchange (TPE) in the elastic electron-nucleon scattering at high Q 2 in the framework of pQCD. The obtained TPE amplitude is of order α/αs with respect to Born approximation. Its shape and value are sensitive to the choice of nucleon wavefunction, thus study of TPE effects can provide important information about nucleon structure. With the wavefunctions based on QCD sum rules, TPE correction to the electron-proton cross section has negative sign, is almost linear in ε and grows logarithmically with Q 2 up to 7% at Q 2 = 30 GeV 2 . The results of existing "hadronic" calculations, taking into account just the nucleon intermediate state, can be smoothly connected with pQCD result near Q 2 ∼ 3 GeV 2 . Above this point two methods disagree, which implies that "hadronic" approach becomes inadequate at high Q 2 . Other relevant observables, such as electron/positron cross section ratio, are also discussed.

We calculate two-photon exchange amplitude for the elastic electron-proton scattering in the framework of dispersion relations. The imaginary part of the amplitude is determined by unitarity. Since in the unitarity relation intermediate states are on shell, off-shell form factors are not needed for the calculation. The real part is then evaluated using analytical properties of the amplitude. The expression for the elastic contribution to the amplitude, obtained in our approach, differs from the results of traditional calculations with on-shell form factors. Nevertheless, numerically the difference is minor for Q 2 up to 6 GeV 2 .

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 $Q^2$.Comment: Misprints corrected, two Appendices with technical details adde

We study two-photon exchange for elastic electron-proton scattering at low Q 2 . Compact approximate formulae for the amplitudes are obtained. Numerical calculations are done for Q 2 ≤ 0.1 GeV 2 with several realistic form factor parameterizations, yielding similar results. They indicate that the corrections to magnetic form factor can visibly affect cross-section and proton radii. For low-Q 2 electron-neutron scattering two-photon exchange corrections are shown to be negligibly small.

We consider two-photon exchange (TPE) in the elastic electron-proton scattering and study the contribution arising from the production of ∆(1232) resonance in the intermediate state. We calculate all three TPE amplitudes (generalized form factors), and find that the ∆ contribution mainly influences generalized electric form factor (contrary to the elastic contribution, which affects magnetic form factor), and the effect grows with Q 2 . If the corresponding correction is applied to the recent polarization transfer measurements of proton form factors, their results will change markedly. Thus we suggest that TPE corrections due to inelastic intermediate states are important to polarization experiments at high Q 2 , and should not be neglected.