Counting form factors of twist-two operators - art. no. 076005

We present a simple method to count the number of hadronic form factors bas ed on the partial wave formalism and crossing symmetry. In particular, we s how that the number of independent nucleon form factors of spin-n, twist-2 operators (the vector current and energy-momentum tensor being special exam ples) is n + 1. These generalized form factors define the generalized (off- forward) parton distributions that have been studied extensively in the rec ent literature. In proving this result, we also show how the J(PC) rules fo r quarkonium states arise in the helicity formalism.