Target mass corrections for virtual Compton scattering at twist-2 and generalized, nonforward Wandzura–Wilczek and Callan–Gross relations Original Research Article

The off-cone Compton operator of twist-2 is Fourier transformed using a general procedure which is applicable, in principle, to any QCD tensor operator of definite (geometric) twist. That method allows, after taking the nonforward matrix elements, to separate quite effectively their imaginary part and to reveal some hidden structure in terms of appropriately defined variables, including generalized Nachtmann variables. In this way, without using the equations of motion, generalizations of the Wandzura–Wilczek relation and of the mass-corrected Callan–Gross relation to the nonforward scattering, having the same shape as in the forward case, are obtained. In addition, new relations for those structure functions which vanish in the forward case are derived. All the structure functions are expressed in terms of iterated generalized parton distributions of nth order. In addition, we showed that the absorptive part of twist-2 virtual Compton amplitude is determined by the nonforward extensions of image and image only.