Measurements of Cross Sections and Double Longitudinal Asymmetries of pion-antipion production in p + p collisions to constrain the Gluon Spin contribution to the Proton Spin

The spin of the proton is known to be ½ħ. Although its angular momentum sum rule in terms of constituent quark and gluon components has been established, its detailed decomposition is poorly known. What fraction is attributed to the spin (polarization) and orbital angular momentum component is completely unknown, and how much of the spin component is from the quarks and gluons is only partially known. Dedicated experiments in the past few decades have measured the sum of quark and anti-quark spin contribution to account for only ∼25% of the proton spin, whereas separating the sea-quark polarizations or constraining the contribution of gluon polarization is still a subject of active experimental research.;The Relativistic Heavy Ion Collider (RHIC) is a unique facility that provides collisions between polarized protons and thereby excellent tools to study the role of gluons in the proton intrinsic angular momentum. The double longitudinal asymmetry ALL of single inclusive production allows access to the polarized gluon distribution Δ g. It does so when the asymmetry measurements are incorporated into the so-called global analysis where polarized parton distribution functions and fragmentation functions are simultaneously fitted to best describe various measurements from different experiments. While π0 at PHENIX and jets at STAR have mainly been putting constrains on ΔG, the first moment of Δg, other channels that provide complementary information on ΔG are critical.;The high pT charged pion production is expected to be sensitive to the sign of ΔG. The isospin symmetry with other pion species will enables us to visually see the sign via the ordering of ALL of the three pion species even without performing global analysis. The interpretation can also be cross checked with the one drawn from global analysis, where the dominance of q – g scattering in π± production enhances the sensitivity. For this dissertation, high pT charged pion production at mid-rapidity in polarized p+p collisions at s = 200 GeV has been analyzed. In this work, I developed a new analysis including the Hadron Blind Detector, a gas-based Cerenkov detector, to overcome the major challenge, a large fraction of electrons misidentified with π ±, and achieved >98% purity in π± sample. Along with ALL, invariant differential cross section has been measured for different charges separately to validate the current perturbative Quantum Chromo-dynamics framework. Through these first successful measurements, we demonstrated π± is a promising channel to extract crucial information on ΔG in that complete discussions will be available with further constrained charge-separated fragmentation functions and improved statistics.