Single transverse-spin asymmetry in Drell-Yan lepton angular distribution

TL;DR

This paper provides a gauge-invariant, LO QCD calculation of the single transverse-spin asymmetry in the Drell-Yan lepton angular distribution, expressing the effect in terms of twist-3 quark-gluon correlators $T_F^{(V)}(x_1,x_2)$. The authors perform parallel derivations in both light-cone and covariant gauges to ensure current conservation and color gauge invariance, confirming a result that aligns with the Boer-Mulders–Teryaev formulation rather than the earlier HTS expression that included a derivative term. They show that at tree level the derivative term does not arise and present a numerical estimate using a model for the twist-3 function, finding asymmetries at the percent level that could be probed at RHIC. The work thereby reinforces the consistency of twist-3 factorization for Drell-Yan SSAs and provides a concrete, gauge-invariant prediction for experimental tests.

Abstract

We calculate a single transverse-spin asymmetry for the Drell-Yan lepton-pair's angular distribution in perturbative QCD. At leading order in the strong coupling constant, the asymmetry is expressed in terms of a twist-3 quark-gluon correlation function T_F^{(V)}(x_1,x_2). In our calculation, the same result was obtained in both light-cone and covariant gauge in QCD, while keeping explicit electromagnetic current conservation for the virtual photon that decays into the lepton pair. We also present a numerical estimate of the asymmetry and compare the result to an existing other prediction.

Figures (5)

• Figure 1: Coordinate choice for the virtual photon's rest frame.
• Figure 2: General diagram contributing to the Drell-Yan single transverse-spin asymmetry.
• Figure 3: Lowest order Feynman diagrams contributing to the Drell-Yan single transverse-spin asymmetry.
• Figure 4: Sample Feynman diagram generating a double soft-gluon pole.
• Figure 5: A generic diagram for extracting the special quark propagator.