  We consider the structure of divergences in Drell-Yan process with small
transverse momentum. The factorization proof is not trivial because various
kinds of divergences are intertwined in the collinear and soft parts at high
orders. We prescribe a method to disentangle the divergences in the framework
of the soft-collinear effective theory. The rapidity divergence is handled by
introducing the $\delta$ regulator in the collinear Wilson lines. The collinear
part, which consists of the transverse-momentum-dependent parton distribution
function (TMDPDF), is free of the rapidity divergence after the soft zero-bin
subtraction. There still remains the problem of mixing between the ultraviolet
and infrared divergences, which forbids the renormalization group description.
We show that the mixing is cancelled by the soft function. This suggests that
the collinear and soft parts should be treated as a whole in constructing a
consistent factorization theorem. The renormalization group behavior of the
combined collinear and soft parts is presented explicitly at one loop. We also
show that the integrated PDF can be obtained by integrating the TMDPDF over the
transverse momentum.
