Unified picture for single transverse-spin asymmetries in hard-scattering processes.

Using lepton-pair production in hadron-hadron collisions as an example, we explore the relation between two well-known mechanisms for single-transverse-spin asymmetries in hard processes: twist-three quark-gluon correlations when the pair's transverse momentum is large, q perpendicular >> Lambda QCD, and time-reversal-odd and transverse-momentum-dependent parton distributions when q perpendicular is much less than the pair's mass. We find that, although the two mechanisms each have their own domain of validity, they describe the same physics in the kinematic region where they overlap. This unifies the two mechanisms and imposes an important constraint on phenomenological studies.