Inclusive cross section and double-helicity asymmetry for $\pi^{0}$ production at midrapidity in $p$$+$$p$ collisions at $\sqrt{s}=510$ GeV

TL;DR

This paper reports measurements of the inclusive $$ production cross section and double-helicity asymmetry in proton-proton collisions at 510 GeV, providing new constraints on gluon polarization and confirming nonzero gluon contribution to proton spin.

Contribution

It presents the first measurements of $$ asymmetry at this energy and extends the kinematic range to lower gluon momentum fractions, refining the understanding of gluon polarization.

Findings

Cross section agrees with NLO pQCD calculations using DSS14 fragmentation functions.

Double-helicity asymmetry $A_{LL}$ shows a positive trend with $p_T$ and $$ energy.

Results support a positive gluon polarization contribution to proton spin for $x>0.01$.

Abstract

PHENIX measurements are presented for the cross section and double-helicity asymmetry ($A_{LL}$) in inclusive $\pi^0$ production at midrapidity from $p$$+$$p$ collisions at $\sqrt{s}=510$~GeV from data taken in 2012 and 2013 at the Relativistic Heavy Ion Collider. The next-to-leading-order perturbative-quantum-chromodynamics theory calculation is in excellent agreement with the presented cross section results. The calculation utilized parton-to-pion fragmentation functions from the recent DSS14 global analysis, which prefer a smaller gluon-to-pion fragmentation function. The $\pi^{0}A_{LL}$ results follow an increasingly positive asymmetry trend with $p_T$ and $\sqrt{s}$ with respect to the predictions and are in excellent agreement with the latest global analysis results. This analysis incorporated earlier results on $\pi^0$ and jet $A_{LL}$, and suggested a positive contribution of gluon polarization to the spin of the proton $\Delta G$ for the gluon momentum fraction range $x>0.05$. The data presented here extend to a currently unexplored region, down to $x\sim0.01$, and thus provide additional constraints on the value of $\Delta G$. The results confirm the evidence for nonzero $\Delta G$ using a different production channel in a complementary kinematic region.