Initial-state interactions and single-spin asymmetries in Drell–Yan processes

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 H 1 H 2 ↑→ℓ +ℓ − X. The QCD initial-state interactions produce a T-odd spin-correlation S → H 2 · P → H 1 × Q → between the target spin and the virtual photon production plane which is not power-law suppressed in the Drell–Yan scaling limit at large photon virtuality Q 2 at fixed x F . The single-spin asymmetry which arises from the initial-state interactions is not related to the target or projectile transversity distribution δq H ( x, Q). The origin of the single-spin asymmetry in πp ↑→ℓ +ℓ − X is a phase difference between two amplitudes coupling the proton target with J z p=± 1 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 with different orbital angular momentum: ΔL z =1; 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 ↑→ℓ +ℓ − X is similar to the single-spin asymmetries in deep inelastic semi-inclusive leptoproduction ℓ p ↑→ℓ′ πX which arises from the final-state rescattering of the outgoing quark. The Bjorken-scaling single-spin asymmetries predicted for the Drell–Yan and leptoproduction processes highlight the importance of initial- and final-state interactions for QCD observables.