2 00 4 Form factor decomposition of generalized parton distributions at leading twist Ph . Hägler

We extend the counting of generalized form factors presented in PRD63(2000) by Ji and Lebed to the axial vector and the tensor operator at twist-2 level. Following this, a parameterization of all higher moments in x of the tensor (helicity flip) operator is given in terms of generalized form factors. 1 Counting generalized form factors Generalized parton distributions still attract increasing interest among theorists and experimen-talists alike investigating the quark and gluon structure of hadrons. For a complete description of the nucleon structure at (leading) twist 2 level, full knowledge of the corresponding spin independent (vector) not the GPDs themselves, but their Mellin moments in x are needed, see e.g. the recent calculations of moments of GPDs in lattice QCD [4, 5, 6]. General higher Mellin moments of (matrix elements of) bilocal operators lead to towers of local operators, which in turn are parameterized in terms of generalized form factors (GFFs). The correct counting of the number of independent generalized form factors is quite important as a cross check of these parameterizations, as can be seen e.g. from the mistaken application of time reversal in the case of the helicity flip GPDs, see Ref. [3], which lead initially to a wrong number of GPDs but has since then been corrected in 1 since H T (x, ξ → 0, t → 0) = h 1 (x) = δq(x), we will denote H T also as generalized transversity