Abstract:We develop a consistent technique for the calculation of real photon emission in hard exclusive processes, which is based on the background field formalism and allows a convenient separation of hard electromagnetic and soft hadronic components of the photon. The latter ones are related to matrix-elements of light-cone operators in the electromagnetic background field and can be parametrized in terms of photon distribution amplitudes. We construct a complete set of photon distribution amplitudes up to and including twist-4, for both chirality-conserving and chirality-violating operators. The distribution amplitudes involve several nonperturbative parameters and, most importantly, the magnetic susceptibility of the quark condensate. We review and update previous estimates of the susceptibility and also give new estimates of parameters describing higher-twist amplitudes from QCD sum rules.

We estimate the size of the twist-3 effects on deeply virtual Compton scattering ͑DVCS͒ observables, in the Wandzura-Wilczek approximation. We present results in the valence region for the DVCS cross sections, charge asymmetries, and single spin asymmetries, to twist-3 accuracy.

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We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer Q(2). It is shown that the leading two-photon exchange amplitude behaves as 1/Q(4), and can be expressed in a model independent way in terms of the leading twist nucleon distribution amplitudes. Using several models for the nucleon distribution amplitudes, we provide estimates for existing data and for ongoing experiments.