An important ingredient of parton or string cascade models for ultrarelativistic heavy-ion reactions is a parton description of the baryon. Whereas previous models needed the concept of a diquark in an essential way, we have developed a new model using Diracs approach of Poincare-covariant many-body dynamics with constraints. In our model, the baryon is described as a dynamical set of three valence quarks and a fourth particle, the junction, which carries the momentum fraction of the sea quarks as well as all of the glue. The models parameters are the quark current masses, and one interaction strength, determined by the proton radius. Thus the model has no adjustable free parameters. Nevertheless, we obtain a remarkably good fit to the valence quark structure functions of the baryon
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