We revisit the transverse single spin asymmetry in the angular distribution
of a Drell-Yan dilepton pair. We study this asymmetry by using twist-3
collinear factorization, and we obtain the same result both in covariant gauge
and in the light-cone gauge. Moreover, we have checked the electromagnetic
gauge invariance of our calculation. Our final expression for the asymmetry
differs from all the previous results given in the literature. The overall sign
of this asymmetry is as important as the sign of the Sivers asymmetry in
Drell-Yan.Comment: 9 page
