Initial-State Interactions and Single-Spin Asymmetries in Drell-Yan Processes

TL;DR

The paper investigates the origin of single-spin asymmetries in hard QCD processes, focusing on initial-state interactions in Drell-Yan. It shows that gluon exchange between the annihilating antiquark and the target spectator induces a leading-twist, T-odd phase, requiring Delta L^z = 1 orbital angular momentum overlap of light-front wavefunctions. Through a quark-diquark model calculation and crossing arguments with DIS, the authors derive a DY SSA that has the same magnitude but opposite sign as the DIS SSA, and they discuss the QCD generalization with a color factor. The work implies SSAs probe orbital angular momentum and the spin-flavor content of hadrons, challenging the notion that such effects are power-suppressed in high-energy processes and connecting to Wilson-line descriptions and generalized parton distributions.

Abstract

We show that the initial-state interactions from gluon exchange between the incoming quark and the target spectator system lead to leading-twist single-spin asymmetries in the Drell-Yan process. The QCD initial-state interactions produce a $T-$odd spin-correlation between the target spin and the virtual photon production plane which is not power-law suppressed in the Drell-Yan scaling limit. The origin of the single-spin asymmetry in $πp^\uparrow \to \ell^+ \ell^- X$ is a phase difference between two amplitudes coupling the proton target with $J^z_p = \pm {1\over 2}$ to the same final-state, the same amplitudes which are necessary to produce a nonzero proton anomalous magnetic moment. The calculation requires the overlap of target light-front wavefunctions differing by one unit of orbital angular momentum projection $L_z;$ thus the SSA in the Drell-Yan reaction provides a direct measure of orbital angular momentum in the QCD bound state. The single-spin asymmetry predicted for the Drell-Yan process $πp^\uparrow \to \ell^+ \ell^- X$ is similar to the single-spin asymmetries in deep inelastic semi-inclusive leptoproduction $\ell p^\uparrow \to \ell' πX$ which arises from the final-state rescattering of the outgoing quark.

Figures (2)

• Figure 1: The initial-state interaction in the Drell-Yan process.
• Figure 2: An example of a final-state interaction which can cause a single asymmetry in $\overline{p} p^\uparrow \to \pi X$.