Perturbative QCD predictions for two-photon exchange
  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 alpha/alpha_s 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 epsilon 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. 
Dmitry Borisyuk, Alexander Kobushkin