Double Helicity Asymmetry in $\pi^{0}$ Production at Midrapidity in Polarized $p+p$ Collisions at $\sqrt{s}=510$ GeV

Abstract

PHENIX measurements are presented for the cross-section and double-helicity asymmetry ($A_{LL}$) of inclusive $\pi^{0}$ production ($A_{LL}^{\pi^{0}}$) at midrapidity from $p+p$ collisions at $\sqrt{s}=510$ GeV from data taken in 2012 and 2013 at the Relativistic Heavy Ion Collider (RHIC). The next-to-leading order (NLO) perturbative QCD (pQCD) calculation agrees excellently with the presented cross-section result. The $A_{LL}^{\pi^{0}}$ follows an increasingly positive asymmetry as functions of $p_{T}$ and $\sqrt{s}$ at the fixed $x_{T}$. The latest global analysis results, which support the positive spin contribution of gluon ($\Delta{}G$), agrees excellently with the presented asymmetry result. The asymmetry result extends the experimental sensitivity to the previously unexplored $x$ region down to $x\sim0.01$ and provides additional constraints on $\Delta{}G$.