A Comprehensive Approach to Structure Functions
  A model is presented based on a dipole picture with a hard and a soft pomeron. It is assumed that large dipoles couple to the soft pomeron and small dipoels couple to the hard pomeron. Most of the parameters of the model are predetermined from proton-proton scattering and the only free parameter is the radius Rc, defining the transition from small to large dipoles. This is fixed by the proton structure function. The model then successfully predicts the proton charm and longitudinal structure functions, J/psi photoproduction, the DVCS cross section, the total gamma-p and gamma-gamma cross sections and the photon structure function. 
A Donnachie