  Distribution functions in hard processes can be described by quark-quark
correlators, nonlocal matrix elements of quark fields. Color gauge invariance
requires inclusion of appropriate gauge links in these correlators. For
transverse momentum dependent distribution functions, in particular important
for describing T-odd effects in hard processes, we find that new link
structures containing loops can appear in abelian and non-abelian theories. In
transverse moments, e.g. measured in azimuthal asymmetries, these loops may
enhance the contribution of gluonic poles. Some explicit results for the link
structure are given in high-energy leptoproduction and hadron-hadron
scattering.
