  In this work we address several issues associated with orbital angular
momentum relevant for leading twist polarized deep inelastic scattering. We
present a detailed analysis of the light-front helicity operator (generator of
rotations in the transverse plane) in QCD. We explicitly show that, the
operator constructed from the manifestly gauge invariant, symmetric energy
momentum tensor in QCD, in the gauge $A^+=0$, after the elimination of
constraint variables, is equal to the naive canonical form of the light-front
helicity operator plus surface terms. Restricting to topologically trivial
sector, we eliminate the residual gauge degrees of freedom and surface terms.
Having constructed the gauge fixed light-front helicity operator, we introduce
quark and gluon orbital helicity distribution functions relevant for polarized
deep inelastic scattering as Fourier transform of the forward hadron matrix
elements of appropriate bilocal operators. The utility of these definitions is
illustrated with the calculation of anomalous dimensions in perturbation
theory. We explicitly verify the helicity sum rule for dressed quark and gluon
targets in light-front perturbation theory. We also consider internal orbital
helicity of a composite system in an arbitrary reference frame and contrast the
results in the non-relativistic situation versus the light-front (relativistic)
case.
