A note on Wandzura-Wilczek relations

Wandzura-Wilczek (WW) relations between matrix elements of bilocal light-ra y operators have recently regained interest in connection with off-forward scattering processes. Originally derived for matrix elements over leading-t wist 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 scatteri ng. We demonstrate that, for exclusive matrix elements, the inclusion of th ese operators into WW relations is essential for fulfilling constraints imp osed by the conformal symmetry of massless QCD. (C) 2001 Published by Elsev ier Science B.V.