A Unified Picture for Single Transverse-Spin Asymmetries in Hard Processes

TL;DR

This work explores 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 couple's mass.

Abstract

Using Drell-Yan pair production 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, $q_\perp \gg Λ_{\rm QCD}$, 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 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 of single spin asymmetries.

Figures (3)

• Figure 1: A generic Feynman diagram contributing to the single transverse-spin asymmetry for the Drell-Yan process through the quark-antiquark scattering channel, containing both soft- and hard-pole contributions.
• Figure 2: Decomposition of the generic diagram in Fig. 1 into different regions in the transverse-momentum-dependent factorization approach. The large transverse momentum of the Drell-Yan pair may come from (a) the Sivers function, (b) the antiquark TMD distribution, and (c) the soft factor.
• Figure 3: A typical Feynman diagram contributing to the Sivers functions at a large transverse momentum, $k_\perp \gg \Lambda_{\rm QCD}$, arising from the twist-three quark-gluon correlation. The bars indicate the soft and hard poles.