Overview of Gluon Helicity Measurements at STAR

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.