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 $γ^{\star}π\to γπ$ and $γ^{\star}γ\toππ$. 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 $γ^{\star}γ\toππ$ up to genuine twist-3 accuracy is found.