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

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 H1H2up arrow --> l(+)l(-)X. The QCD initial-state interactions produce a T-odd spin-correlation (S) over right arrowH(2) . (P) over right arrowH(1) x (Q) over right arrow 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 deltaq(H)(x, Q). The origin of the single-spin asymmetry in pipup arrow --> l(+)l(-)X is a phase difference between two amplitudes coupling the proton target with J(p)(z) = +/-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: DeltaL(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 pip(up arrow) --> l(+)l(-)X is similar to the single-spin asymmetries in deep inelastic semi-inclusive leptoproduction lp(up arrow) --> l'piX 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. (C) 2002 Published by Elsevier Science B.V.