Double helicity asymmetry in π^0 production in polarized proton-proton collisions at √ s = 510 GeV with PHENIX mid-rapidity spectrometer

Abstract

PHENIX measurement of longitudinally double helicity asymmetry (A_LL) in inclusive π^0 production at mid-rapidity from p+ p collsions at √ s = 510 GeV from the 2012/2013 RHIC runs is presented. Since the EMC experiment revealed that spin constribution of quarks is surprisingly small, many experimental and theoeretical endeavers have been carried out to understand proton spin structure. The spin contribution of gluon (∆G) might explain the missing part of the proton spin and measuring ∆G is the ultimate goal of the dissertation. To measure ∆G, accessing the helicity gluon distribution (∆g(x,Q^2)) is necessary. The longitudinal polarized p+ p collsions and A_LL measurementss are bes tool for it. A_LL measurements of π^0 (A_LL^π^0) at √ s = 62.4 and 200 GeV and A_LL of jet at √ s = 200 GeV constrain ∆g(x,Q 2 ) significantly. As a result, positive polarization of gluon is discovered within sensed momemtum fraction (x) range, 0.05 ≤ x ≤ 0.2. However large uncertainty remains outside of the x region, especially lower x region. Thus expanding experimental sensitivity to lower x region is a crucial step to understand the ∆g(x,Q 2 ) and the spin structure. To access the lower x region, new measurement of A_LL^π^0 at higher √ s = 510 GeV is carried out and presented in the disseration. The new measurement covers x region, 0.01 ≤ x ≤ 0.1. The measurement is superior to the previous measurements from the point of not only the unique covered x range but also statistical precision. The sophisticated luminosity corrections are also presented in the dissertation to reduce the effects of the multiple collisions in single bunch crossing and the vertex z resolution of detectors. As a result, the world first positive asymmetry in hadron production is measured. The perturbative Quantum Chromodynamics theoretical predition which including the previous measurements is in excellent agreement with the presented A_LL^π^0. With the positive asymmetry and unique x coverage, the presented A_LL^π^0 will contribute to constrain the uncertainty∆g(x,Q 2 ) significantly.