  The collinear factorization at twist-3 for Drell-Yan processes is studied
with the motivation to solve the discrepancy in literature about the single
spin asymmetry in the lepton angular distribution, and to show how QCD gauge
invariance is realized in the hadronic tensor. The obtained result here agrees
with our early result derived with a totally different approach. In addition to
the asymmetry we can construct another two observables to identify the spin
effect. We show that the gauge invariance of different contributions in the
hadronic tensor is made in different ways by summing the effects of gluon
exchanges. More interestingly is that we can show that the virtual correction
to one structure function of the hadronic tensor, hence to some weighted SSA
observables, is completely determined by the quark form factor. This will
simplify the calculation of higher order corrections. The corresponding result
in semi-inclusive DIS is also given for the comparison with Drell-Yan
processes.
