Single Transverse-Spin Asymmetries in Drell-Yan Processes

The asymmetry in the angular distribution of Drell--Yan dilepton pairs generated in collisions where just one nucleon is transversely polarised has been examined in the literature with a variety of results, mainly differing by factors of two. We re-evaluate the asymmetry via twist-3 contributions in collinear factorisation. In order to allow complete and in-depth comparison with existing calculations, we supply all calculational details.