A note on Wandzura–Wilczek relations

Wandzura–Wilczek (WW) relations between matrix elements of bilocal light-ray operators have recently regained interest in connection with off-forward scattering processes. Originally derived for matrix elements over leading-twist operators, their generalization to off-forward and exclusive processes gets complicated by the presence of higher-twist operators that are total derivatives of leading-twist ones and do not contribute to forward scattering. We demonstrate that, for exclusive matrix elements, the inclusion of these operators into WW relations is essential for fulfilling constraints imposed by the conformal symmetry of massless QCD.