Measurement of the Transverse Single-Spin Asymmetry for Mid-rapidity Production of Neutral Pions in Polarized p+p Collisions at 200 GeV Center-of-Mass Energy

摘要

The spin structure of the proton has revealed itself to be extremely complexand is an area of ongoing research. The Relativistic Heavy Ion Collider (RHIC)at Brookhaven National Laboratory (BNL) inaugurated its operation as the firstpolarized-proton collider during the 2001-2002 run, marking the beginning of anew era in the study of proton spin structure. From the data collected in this run, the PHENIX experiment measured thetransverse single-spin asymmetry (A_N) for neutral pion production at x_F~0.0over a transverse momentum range of 1 to 5 GeV/c from polarized proton-protoninteractions at a center-of-mass energy of 200 GeV and found it to be zerowithin a few percent. Interest in these measurements arises from theobservation of large (~30%) transverse single-spin asymmetries in pionproduction at forward angles by the E704 collaboration at Fermilab (sqrt{s} =19.4 GeV), found by the STAR and BRAHMS experiments to persist at RHICenergies, as well as single-spin, azimuthal asymmetries observed recently insemi-inclusive deep-inelastic scattering experiments. Such large asymmetrieswere initially surprising because at leading order, pQCD predicted only smalleffects. Several possible origins of these large asymmetries have been proposed.Despite great theoretical progress in recent years, no single, clear formalismhas emerged in which to interpret the available data. Further theoretical workand a variety of additional experimental measurements will be necessary tounderstand current results and elucidate the transverse spin structure of theproton.