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

## TL;DR

This paper reports measurements of the cross-section and double-helicity asymmetry of inclusive π0 production in polarized proton-proton collisions at 510 GeV, providing new insights into gluon spin contributions at low momentum fractions.

## Contribution

It presents the first measurements of $A_{LL}^{c0}$ at 510 GeV, extending sensitivity to lower x values and constraining gluon polarization models.

## Key findings

- NLO pQCD calculations agree with the cross-section data.
- $A_{LL}^{c0}$ increases with $p_T$ and $c4$ at fixed $x_T$.
- Results support positive gluon polarization contribution.

## 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 $\Delta{}G$.