Session D7 - Hadron Physics.
ORAL session, Saturday afternoon, May 01
Governor's Square 16, Adam's Mark Hotel
The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) began operation as the first polarized proton collider in 2001. From data collected in the 2001-2002 run, the PHENIX experiment has measured the single-spin transverse asymmetry (A_N) for neutral pion production at x_F\sim0 over a transverse momentum range of~1.0 to 5.0~GeV/c from polarized proton-proton interactions at a center of mass energy (\sqrts) of 200~GeV. Interest in these measurements arises from the observation of large (\sim10-30%) single-spin transverse asymmetries in pp_\uparrow\!\!\rightarrow\!\!\pi X at forward angles by the E704 collaboration at Fermilab (\sqrts = 19.4GeV) and the STAR collaboration at RHIC (\sqrt(s) = 200GeV), as well as single-spin, azimuthal asymmetries in semi-inclusive deep-inelastic scattering by the HERMES collaboration at DESY. Such large asymmetries were initially surprising because, at leading-order, pQCD predicts only small effects. Recently it has been argued that these large asymmetries may be produced by initial-state effects (e.g. the Sivers effect), final-state effects (e.g. transversity with the Collins effect), higher-twist contributions, or a combination of the three. In this talk, we will report on the results ofthis measurement.