ar X iv : h ep - p h / 01 09 14 3 v 1 1 7 Se p 20 01 Two - Quark Correlations in the Hard Electromagnetic Nucleon Form Factors

The, so called, “hard-scattering approach” represents a suitable framework for the perturbative treatment of exclusive hadronic processes at large energies and (transverse) momentum transfers. In this context, diquarks can serve as a useful phenomenological concept to model nonperturbative effects which are still observable in the kinematic range accessible by present-day experiments. We outline how a description of baryons as quark-diquark systems can be understood as an effective theory in the sense that the pure quark hard-scattering approach is recovered in the limit of asymptotically large momentum transfers. Our arguments are based on a reformulation of the hard-scattering formalism in terms of quark-diquark degrees of freedom. This reformulation provides the exact form of photonand gluon-diquark vertices and corresponding vertex functions (diquark form factors) in the limit of asymptotically large momentum transfers – and thus also asymptotic constraints which should be fulfilled by phenomenological quark-diquark models for hard scattering. As an application of this reformulation we present an analysis of the hard electromagnetic nucleon form factors with respect to their quark-diquark content. It is generally accepted that the, so called, “hard scattering approach” (HSA) gives the correct description of exclusive hadronic processes in the limit of asymptotically large (transverse) momentum transfers Q2 (for an overview on the HSA see, e.g., Ref. [1]). The HSA is based on the factorization of hadronic amplitudes in process dependent, perturbatively calculable hard-scattering amplitudes and process independent distribution amplitudes (DAs) which contain the bound-state dynamics of the hadronic constituents. The corresponding analytical expression for one of the simplest exclusive quantities, the nucleon magnetic form factor, is given by