QCD Hard Scattering and the Sign of the Spin AsymmetryALLπ

Zusammenfassung

Recent preliminary PHENIX data are consistent with a negative and sizable longitudinal double-spin asymmetry A(LL)(pi) for pi(0) production at moderate transverse momentum p(perpendicular to)similar or equal to1-4 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 to) produces at best a very small negative asymmetry in this kinematic range.