Wandzura-Wilczek approximation from generalized rotational invariance
Résumé
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.