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