Wandzura-Wilczek approximation from generalized rotational invariance

Abstract

We present the alternative way of derivation of the Wandzura-Wilczek relations between the kinematical twist-3 and twist-2 functions, parameterizing hadronic matrix element in two-photon processes ${\gamma }^{\star }\pi \to \gamma \pi$ and ${\gamma }^{\star }\gamma \to \pi \pi$. The new equations, providing the independence of the physical cross-sections on the choice of the light-cone direction, are suggested and explored. The amplitude of ${\gamma }^{\star }\gamma \to \pi \pi$ up to genuine twist-3 accuracy is found.

Comments: 14 pages, Latex