Spin has been compared to the way the Earth rotates about its axis as it travels around the Sun. While quantum spin is indeed a fundamental property of matter, when it is applied to particles such as protons this does not imply that these particles do rotate about their central axis. The proton is made of quarks and gluons. Experimental data may not have revealed quarks and gluons directly, yet it is understood that the behavior of these internal constituents dictates the gross properties of a proton. In the case of a spin-1/2 particle such as a proton, it can exist in one of two quantum spin states (± ħ/2). The spin 1/2 nature of the proton itself can help explain the structure of matter. Double-helicity asymmetry (A_LL) measurements may yield valuable information on the role that gluons play in the total spin of the proton: 1/2 = 1/2ΔΣ + Δ G + ΔL, where ΔΣ is the contribution due to all quarks within the proton and L is the orbital angular momentum. At large transverse momenta and at midrapidity, quark-gluon scattering dominates pion production at RHIC energies. π^± then provide a special opportunity for studying processes sensitive to ΔG: the gluon contribution to the proton spin. Preferentially, up quarks fragment into π⁺ and down quarks to π^- . This preference leads to the dominance of up-quark gluon, and down-quark gluon contributions in the sum over flavors in a factorized pQCD calculation of pion production.

The work presented here details a study which measures A _LL of final state π^± proceeding from polarized proton proton collisions at [special characters omitted] = 200GeV at RHIC. Measurement of charged separated differential cross sections proceeding from such collisions is also presented. As will be demonstrated, the measurements are compatible with the underlying theoretical framework (pQCD), and have directly ruled out maximum ΔG scenarios. The measurements presented form part of an international effort to understand how gluons influence and participate in the spin of the proton.