Gluon polarization measurements from longitudinally polarized proton-proton collisions at STAR

TL;DR

This paper reports measurements of gluon polarization in protons using jet asymmetries from polarized proton-proton collisions at STAR, providing insights into gluon contributions at low momentum fractions.

Contribution

It presents new STAR measurements of inclusive jet and dijet asymmetries at 510 GeV, employing novel analysis techniques to probe gluon polarization at lower x values.

Findings

Evidence of non-zero gluon polarization at x > 0.05

Sensitivity to gluon polarization down to x ~ 0.015

Implementation of underlying event correction techniques

Abstract

Jets produced in the pseudo-rapidity range, $-1.0 < \eta < 1.0$, from $pp$ collisions at RHIC kinematics are dominated by quark-gluon and gluon-gluon scattering processes. Therefore the longitudinal double spin asymmetry $A_{LL}$ for jets is an effective channel to explore the longitudinal gluon polarization in the proton. At STAR, jets are reconstructed in full azimuth, from the charged-particle tracks seen by the Time Projection Chamber and electro-magnetic energy deposited in the Barrel and Endcap electro-magnetic calorimeters at both $\sqrt{s} = $ 200 and 510 GeV. Early STAR inclusive jet $A_{LL}$ results at $\sqrt{s} = $ 200 GeV provided the first evidence of the non-zero gluon polarization at $x > $ 0.05. At $\sqrt{s} = $ 510 GeV, the inclusive jet $A_{LL}$ is sensitive to the gluon polarization as low as $x \sim $ 0.015. In this talk, we will discuss recent STAR inclusive jet and dijet $A_{LL}$ results at $\sqrt{s} = $ 510 GeV and highlight the new techniques designed for this analysis, for example the underlying event correction to the jet transverse energy and its effect on the jet $A_{LL}$. Dijet $A_{LL}$ results are shown for four topologies in regions of pseudo-rapidity, effectively scanning the $x$-dependence of the gluon polarization.