# Overview of Gluon Helicity Measurements at STAR

## TL;DR

This paper reviews recent measurements of gluon helicity contributions to proton spin at STAR, highlighting new data that constrains the polarized gluon distribution at both high and low momentum fractions, advancing understanding of proton spin structure.

## Contribution

It presents new STAR measurements of gluon polarization via $A_{LL}$ in jets, dijets, and $^0$ production, improving constraints on $g(x)$ across a wider $x$ range.

## Key findings

- Positive gluon polarization observed at high $x$
- First insights into low-$x$ gluon polarization from forward $^0$ measurements
- Enhanced precision in $g(x)$ for $x \u2265 0.01$

## Abstract

The contribution to the spin of the proton from the gluon helicity is starting to come into focus: for gluons carrying a large fraction $x$ of the proton momentum, evidence of positive gluon polarization has been observed, via measurements of the longitudinal double-spin asymmetry $A_{LL}$ for inclusive jet and dijet production. $A_{LL}$ is sensitive to the polarized gluon distribution function, $\Delta g(x)$, and while it is positive at high $x$, it is not well constrained for $x<0.05$. Recent measurements at STAR of observables originating dominantly from quark-gluon and gluon-gluon subprocesses aim to improve the precision of $\Delta g(x)$ at high $x$, as well as for the first time provide insight into the low-$x$ contribution. $A_{LL}$ measurements of inclusive jets and dijets at midrapidity $(|\eta|<1)$ and intermediate rapidity $(0.8<\eta<2)$ at STAR at $\sqrt{s}=200$ and $510$ GeV will be shown, along with the statuses of ongoing analyses; these measurements will help improve the $\Delta g(x)$ precision for $x\gtrsim 0.01$. Recent $\pi^0$ $A_{LL}$ measurements in the forward region $(2.65<\eta<3.9)$ at $\sqrt{s}=510$ GeV will also be presented, which probe $\Delta g(x)$ down to $x{\sim}10^{-3}$. Comparisons of these results to recent global analyses and extrapolations will be discussed.