title: QCD hard scattering and the sign of the spin asymmetry A(pi)LL.
abstract: Recent preliminary PHENIX data are consistent with a negative and sizable longitudinal double-spin asymmetry A(pi)(LL) for pi(0) production at moderate transverse momentum p( perpendicular ) approximately 1-4 GeV and central rapidity. By means of a systematic investigation of the relevant degrees of freedom, we show that the perturbative QCD framework at leading power in p( perpendicular ) produces at best a very small negative asymmetry in this kinematic range.
authors.affiliation: Institut für Theoretische Physik, Universität Regensburg, D-93040 Regensburg, Germany.
authors.forename: Barbara
authors.initials: B
authors.lastname: Jäger
authors.forename: Marco
authors.initials: M
authors.lastname: Stratmann
authors.forename: Stefan
authors.initials: S
authors.lastname: Kretzer
authors.forename: Werner
authors.initials: W
authors.lastname: Vogelsang