  Single-spin asymmetries were long thought to vanish in high-energy scattering
processes because of their specific time-reversal behavior. Time-reversal-odd
phenomena, however, appear naturally when one includes effects of intrinsic
transverse momenta of partons. The partons, quarks and gluons, enter the
description of high-energy scattering processes in correlators which contain
quark and gluon fields. The correlators, parameterized in terms of distribution
and fragmentation functions, constitute matrix elements of non-local field
configurations. For transverse momentum dependent (TMD) correlators the
non-locality is along a light-front, in contrast to the non-locality along the
light-cone for collinear correlators, integrated over transverse momenta. The
TMD correlators require a careful treatment to assure color gauge invariance,
leading to nontrivial gauge links connecting the parton fields. These give rise
to time-reversal-odd phenomena, showing up as single spin and azimuthal
asymmetries. The gauge links, arising from multi-gluon initial and final state
interactions, depend on the color flow in the process, which has consequences
for issues like universality and factorization.
