Overview

Since the realization that the proton is a composite particle, much work has gone into understanding its structure and how the quarks and gluons that make up this structure lead to the proton's basic properties, such as charge, momentum, spin and magnetic moment. While much has been learned about some of these properties from several decades of experimental studies, knowledge of the spin structure has lagged behind, primarily due to the subtlety required for spin dependent measurements. The first results from polarized deep inelastic scattering (pDIS) experiments in the late 1980s and early 1990s revealed that contrary to the prevailing wisdom, quarks did not carry a significant fraction of the proton's spin. However, those experiments, which effectively probed the quark spins through their interactions via virtual photons, could not probe the charge-neutral gluons, and hence could only weakly constrain the gluon's contribution to the nucleon's spin. Polarized proton-proton collisions at high center of mass energy employing the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) provide a unique and direct way to access the gluon spin (DeltaG) through measurements of the double helicity asymmetry ALL. In the present work, results for the double helicity asymmetry in neutral pion (pi0) production in polarized proton collisions from the 2005 and 2006 RHIC runs (Run5 and Run6, respectively) measured with the PHENIX detector are presented. Pions are abundantly produced in proton-proton collisions and so are a good candidate for a high statistics analysis. The PHENIX Electromagnetic Calorimeter has good energy resolution and high granularity which, when used with a high energy photon trigger, yield a substantial pi 0 sample over a wide range in transverse momentum (p T). The 2005 and 2006 polarized proton runs (at a center of mass energy of 200 GeV) saw large increases in luminosity ( L) and polarization (P) compared to previous runs, resulting in a high figure of merit (P4 L). pi0 ALL from RHIC Run5 and Run6 are precise enough to be sensitive to DeltaG. A measurement of charged pion ALL from RHIC Run5 is also presented, and examined as a complimentary analysis which can help determine the sign of DeltaG. These are the first results from RHIC and BNL addressing the very important question of the role played by the gluons in determining the proton's spin. Finally, the current understanding of the proton spin structure is examined considering this new data. A simple method for estimating the gluon spin structure in the proton is described, and indicates that the current data do offer a significant constraint. Even when using this method to consider theoretical uncertainties, which are large, the significance of the current data remains.

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