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, ∆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 ∆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 (|η| < 1) and intermediate rapidity (0.8 < η < 2) at STAR at √ s = 200 and 510 GeV will be shown, along with the statuses of ongoing analyses; these measurements will help improve the ∆g(x) precision for x 0.01. Recent π 0 A LL measurements in the forward region (2.65 < η < 3.9) at √ s = 510 GeV will also be presented, which probe ∆g(x) down to x∼10 −3 . Comparisons of these results to recent global analyses and extrapolations will be discussed.