How is the parton model justified by QCD?
In short, how does one prove rigorously, starting from the QCD Lagrangian, that in DIS only lightlike parton momentum need to be considered?
Also, how does one prove that all nonperturbative effects can be absorbed into the parton distribution functions? I think such a proof must be nonperturbative in nature. Anything involving Feynman diagrams doesn't count.
