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

High Energy Physics - Experiment;High Energy Physics - Phenomenology