Double-spin asymmetries for small-QT Drell-Yan pair production in transversely polarized pp¯ collisions

We discuss the Drell-Yan process at a measured transverse-momentum QT of the produced lepton pair in collisions of transversely polarized protons and antiprotons, to be observed at the proposed spin experiments at GSI. The large logarithmic contributions from multiple soft gluon emission, accompanying the Drell-Yan mechanism at small QT, are resummed to all orders in QCD perturbation theory up to next-to-leading logarithmic (NLL) accuracy. Numerical evaluation shows the impact of the NLL as well as LL effect on the dilepton QT spectra. For the corresponding QT-dependent spin asymmetry ATT(QT), the LL effect gives significant modification while the NLL effect is marginal, leading to QCD prediction that ATT(QT) at GSI is flat at small and moderate QT and almost equals the conventional asymmetry ATT associated with the QT-integrated cross sections. This flat behavior in turn allows us to use analytic saddle-point evaluation of the resummation formula in the limit QT→0, not only to obtain quantitative estimate of ATT(QT), but also to clarify mechanisms behind the relation ATT(QT)≃ATT characteristic of pp¯ collisions at GSI.