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

TL;DR

This paper reports on measurements of gluon polarization within the proton using polarized proton-proton collisions at RHIC, providing new insights into the proton spin puzzle through jet asymmetry analyses at different energies.

Contribution

It presents new STAR measurements of gluon polarization via inclusive jet and dijet asymmetries at 510 GeV, with advanced analysis techniques for bias and uncertainty estimation.

Findings

Evidence of non-zero gluon polarization at x > 0.05

Exploration of gluon polarization down to x ~ 0.015

Improved analysis methods for jet energy and asymmetry measurements

Abstract

The gluon polarization contribution to the proton spin is an integral part to solve the longstanding proton spin puzzle. At the Relativistic Heavy Ion Collider (RHIC), the STAR experiment has measured jets produced in mid-pseudo-rapidity, $|\eta| < $ 1.0, and full azimuth, $\phi$, from longitudinally polarized $pp$ collisions to study the gluon polarization in the proton. At center of mass energies $\sqrt{s} =$ 200 and 510 GeV, jet production is dominated by hard QCD scattering processes such as gluon-gluon ($gg$) and quark-gluon ($qg$), thus making the longitudinal double-spin asymmetry ($A_{LL}$) sensitive to the gluon polarization. Early STAR inclusive jet $A_{LL}$ results at $\sqrt{s} = $ 200 GeV provided the first evidence of the non-zero gluon polarization at momentum fraction $x > $ 0.05. The higher center of mass energy $\sqrt{s} = 510$ GeV allows to explore the gluon polarization as low as $x \sim$ 0.015. In this talk we will present the recent STAR inclusive jet and dijet $A_{LL}$ results at $\sqrt{s} = 510$ GeV, and discuss the relevant new analysis techniques for the estimation of trigger bias and reconstruction uncertainty, the underlying event correction on the jet energy and its effect on jet $A_{LL}$. Dijet results are shown for different topologies in regions of pseudo-rapidity, effectively scanning the $x$-dependence of the gluon polarization.