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.