Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
2000-10-25
Phys.Rev. D63 (2001) 114012
Physics
High Energy Physics
High Energy Physics - Phenomenology
34 pages, revtex, 1 eps figure included using epsf. Misprints corrected, one reference added
Scientific paper
10.1103/PhysRevD.63.114012
We study the virtual Compton amplitude in the generalized Bjorken region (q^2 -> Infinity, t small) in QCD by means of a light-cone expansion of the product of e.m. 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 t/q^2. 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 tree level. However, this divergence has zero projection on the polarization vector of the final photon, so that the physical helicity amplitudes are finite.
Radyushkin Anatoly V.
Weiss Carsten
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