Colour Modification of Effective T-odd Distributions

TL;DR

This paper demonstrates that soft-gluon twist-3 contributions to single-spin asymmetries can be represented as effective T-odd Sivers distributions with process-dependent modifications, extending the applicability of the Sivers mechanism to high transverse momenta.

Contribution

It establishes a connection between twist-3 contributions and T-odd Sivers distributions, including process-dependent color factors, and clarifies their role at large transverse momenta.

Findings

Twist-3 SSA are suppressed by transverse momentum, not virtual-photon momentum transfer.

Transverse-momentum weighted averages of the Sivers function correspond to higher twists.

Full kT-dependent Sivers function represents an infinite tower of higher-twist contributions.

Abstract

We show that soft-gluon twist-3 contributions to single-spin asymmetries (SSA) in hard processes may be expressed in the form of effective T-odd Sivers distributions, whose signs and scales are modified by process-dependent colour factors. We thus prove that the Sivers mechanism may also be applied at large transverse momenta. We stress that twist-3 SSA in semi-inclusive deeply inelastic scattering and Drell-Yan processes are suppressed by transverse momentum rather than a virtual-photon momentum transfer and thus naively correspond to twist two at the hadronic level. More rigorously, the transverse-momentum weighted averages of the Sivers function correspond to increasing twist (3, 5, 7, ...) while the full kT-dependent Sivers function (just as other transverse-momentum dependent distribution and fragmentation functions) corresponds to a resummed infinite tower of higher twists.