  We revisit the derivation of collinear factorization for Deep Inelastic
Scattering at sub-asymptotic values of the four momentum transfer squared,
where the masses of the particles participating in the interaction cannot be
neglected. By using an inclusive jet function to describe the scattered quark
final state, we can restrict the needed parton kinematic approximations just to
the four-momentum conservation of the hard scattering process, and explicitly
expand the rest of the diagram in powers of the unobserved parton transverse
momenta rather than neglecting those. This procedure provides one with more
flexibility in fixing the virtuality of the scattered and recoiling partons
than in the standard derivation, and naturally leads to scaling variables that
more faithfully represent the partonic kinematic at sub-asymptotic energy than
the Bjorken's xB variable.
  We then verify the validity of the obtained factorization formula by
considering a diquark spectator model designed to reproduce the main features
of electron-proton scattering at large $x_B$ in Quantum Chromo-Dynamics. In the
model, the Deep Inelastic Scattering contribution to the cross section can be
explicitly isolated and analytically calculated, then compared to the
factorized approximation. Limiting ourselves to the leading twist contribution,
we then show that use of the new scaling variables maximizes the kinematic
range of validity of collinear factorization, and highlight the intrinsic
limitations of this approach due to the unavoidably approximate treatment of
four momentum conservation in factorized diagrams. Finally, we briefly discuss
how these limitations may be overcome by including higher-twist corrections to
the factorized calculation.
