Volume 346 - 23rd International Spin Physics Symposium (SPIN2018) - Parallel Session: Nucleon helicity structure (F. Kunne, W. Melnitchouk and B. Page)
Overview of Gluon Helicity Measurements at STAR
C. Dilks
Full text: pdf
Pre-published on: 2019 August 19
Published on: 2019 August 23
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.
DOI: https://doi.org/10.22323/1.346.0130
Open Access