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, $q_{\perp }\gg {\Lambda }_{\mathrm{QCD}}$, and time-reversal-odd and transverse-momentum-dependent parton distributions when $q_{\perp }$ 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.

• Received 28 February 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.082002