QCD Hard Scattering and the Sign of the Spin Asymmetry A_LL^pi

Abstract

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_perp \simeq 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_perp produces at best a very small negative asymmetry in this kinematic range.

4 pages, 3 figures, final version published in PRL (only minor changes; note: title changed in published version)