Transition from hadronic to partonic interactions for a composite   spin-1/2 model of a nucleon
  A simple model of a composite nucleon is developed in which a fermion and a boson, representing quark and diquark constituents of the nucleon, form a bound state owing to a contact interaction. Photon and pion couplings to the quark provide vertex functions for the photon and pion interactions with the composite nucleon. By a suitable choice of cutoff parameters of the model, realistic electromagnetic form factors are obtained. When a pseudoscalar pion-quark coupling is used, the pion-nucleon coupling is predominantly pseudovector. A virtual photopion amplitude is considered in which there are two types of contributions: hadronic contributions where the photon and pion interactions have an intervening propagator of the nucleon or its excited states, and contact-like contributions where the photon and pion interactions occur within a single vertex. At large Q, the contact-like contributions are dominant. The model nucleon exhibits scaling behavior in deep-inelastic scattering and the normalization of the parton distribution provides a rough normalization of the contact-like contributions. Calculations for the virtual photopion amplitude are performed using kinematics appropriate to its occurrence as a meson-exchange current in electron-deuteron scattering. The results show that the contact-like terms can dominate the meson-exchange current for Q > 1 GeV/c. There is a direct connection of the contact-like terms to the off-forward parton distributions of the model nucleon. 
J. A. Tjon (Utrecht) and S. J. Wallace (Jlab and U. Maryland)