Abstract
We study the virtual Compton amplitude in the generalized Bjorken region t small) in QCD by means of a light-cone expansion of the product of EM currents in string operators in coordinate space. Electromagnetic gauge invariance (transversality) is maintained by including in addition to the twist-2 operators “kinematical” twist-3 operators which appear as total derivatives of twist-2 operators. The non-forward matrix elements of the elementary twist-2 operators are parametrized in terms of two-variable spectral functions (double distributions), from which twist-2 and 3 skewed distributions are obtained through reduction formulas. Our approach is equivalent to a Wandzura-Wilczek–type approximation for the twist-3 skewed distributions. The resulting Compton amplitude is manifestly transverse up to terms of order We find that in this approximation the tensor amplitude for longitudinal polarization of the virtual photon is finite, while the one for transverse polarization contains a divergence already at the tree level. However, this divergence has zero projection on the polarization vector of the final photon, so that the physical helicity amplitudes are finite.
- Received 27 November 2000
DOI:https://doi.org/10.1103/PhysRevD.63.114012
©2001 American Physical Society