We analyze in detail the problem of gauge invariance of the deeply vitual Compton scattering (DVCS) amplitude. Using twist 3 one-gluon exchange diagram contributions and the QCD equations of motion, we derive the general gauge invariant expression of the DVCS amplitude on a (pseudo)scalar particle (pion, He 4 ). Similarly to the case of deep inelastic scattering, the amplitude does not depend on the twist-3 quark-gluon correlations at the Born level. The contribution of the derived amplitude to the single-spin asymmetry with longitudinally polarized lepton is calculated.Deeply Virtual Compton Scattering (DVCS) has recently attracted much attention. One of the main reasons of this interest is the fact that the DVCS process gives information about a new type of parton distributions, called skewed parton distribution (see for example [1][2][3][4] and references therein). The processhas been shown to factorize in the Bjorken region with (q ′ ) 2 = 0, −q 2 large and small transfer t = (p − p ′ ) 2 , as the product of a perturbatively calculable coefficient function and a long distance object, the skewed parton distribution, which generalizes the notion of parton distributions.The fact that there is a problem with the photon gauge invariance of the DVCS amplitude in leading order at Bjorken limit is fairly well-known (see, for instance [4]). The relevant terms are proportional to the transverse component of the momentum transfer and provide the leading contribution to some observables, and in particular, to the Single Spin Asymmetry.As was shown in [5], a fruitful analogy between the transverse spin case of the deep inelastic scattering (DIS) and the DVCS process can be used to derive a general solution of this problem. We elaborate on this approach in the current paper.For simplicity, we concentrate here on the DVCS process off (pseudo)scalar hadrons, which may be pions or helium-4 nuclei, but our calculation may be generalized to any hadrons. We also neglect hadron masses effects, i.e. kinematical power corrections, which may be studied independently.The Lorentz structure of the hard subgraph of the leading order DVCS diagram (Fig.1a) has the form of a transverse projector. All 4-vectors may be presented in the form of the Sudakov decomposition over two light-cone vectors P, n and one component transversal to the given light-cone vectors. Hence, if the virtual photon momentum, which has a transverse component, is convoluted with the hard part of the leading order DVCS amplitude, one obtains a term directly proportional to the transverse component of the virtual photon momentum. In other words, the measure of the photon gauge invariance violation is the non-zero transverse component of the virtual photon momentum. The recent general analysis [6] confirmed that violating terms are indeed kinematically subleading.In this paper we generalize the Ellis-FurmanskiPetronzio (EFP) factorization scheme [7] to the nonforward case and calculate the complete expression for the DVCS amplitude up to twist 3 order. While this schem...

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We derive light-cone sum rules for the electromagnetic nucleon form factors including the next-to-leading-order corrections for the contribution of twist-three and twist-four operators and a consistent treatment of the nucleon mass corrections. The essence of this approach is that soft Feynman contributions are calculated in terms of small transverse distance quantities using dispersion relations and duality. The form factors are thus expressed in terms of nucleon wave functions at small transverse separations, called distribution amplitudes, without any additional parameters. The distribution amplitudes, therefore, can be extracted from the comparison with the experimental data on form factors and compared to the results of lattice QCD simulations. A selfconsistent picture emerges, with the three valence quarks carrying 40% : 30% : 30% of the proton momentum.

We study analytical properties of the hard exclusive processes amplitudes. We found that QCD factorization for deeply virtual Compton scattering and hard exclusive vector meson production results in the subtracted dispersion relation with the subtraction constant determined by the Polyakov-Weiss $D$-term. The relation of this constant to the fixed pole contribution found by Brodsky, Close and Gunion and defined by parton distributions is proved, while its manifestation is spoiled by the small $x$ divergence. The continuation to the real photons limit is considered and the numerical correspondence between lattice simulations of $D$-term and low energy Thomson amplitude is found.Comment: 4 pages, journal versio

We develop a relativistic quark model for pion structure, which incorporates the non-trivial structure of the vacuum of Quantum Chromodynamics as modelled by instantons. Pions are boundstates of quarks and the strong quark-pion vertex is determined from an instanton induced effective lagrangian. The interaction of the constituents of the pion with the external electromagnetic field is introduced in gauge invariant form. The parameters of the model, i.e., effective instanton radius and constituent quark masses, are obtained from the vacuum expectation values of the lowest dimensional quark and gluon operators and the low-energy observables of the pion. We apply the formalism to the calculation of the pion form factor by means of the isovector nonforward parton distributions and find agreement with the experimental data.