QCD hard scattering and the sign of the spin asymmetry A(pi)LL.
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.
Jäger
Barbara
B
Institut für Theoretische Physik, Universität Regensburg, D-93040 Regensburg, Germany.
Stratmann
Marco
M
Kretzer
Stefan
S
Vogelsang
Werner
W