We show that respecting the underlying Lorentz structure in the parton model
has very strong consequences. Failure to insist on the correct Lorentz
covariance is responsible for the existence of contradictory results in the
literature for the polarized structure function g_2(x), whereas with the
correct imposition we are able to derive the Wandzura-Wilczek relation for
g_2(x) and the target-mass corrections for polarized deep inelastic scattering
without recourse to the operator product expansion. We comment briefly on the
problem of threshold behaviour in the presence of target-mass corrections.
Careful attention to the Lorentz structure has also profound implications for
the structure of the transverse momentum dependent parton densities often used
in parton model treatments of hadron production, allowing the k_T dependence to
be derived explicitly. It also leads to stronger positivity and Soffer-type
bounds than usually utilized for the collinear densities.Comment: RevTeX4, 17 pages, 7 eps figure
