Gluon Polarization in Longitudinally Polarized $pp$ Collisions at STAR

TL;DR

The STAR experiment measures the gluon polarization in protons through jet asymmetry data at RHIC, providing evidence of non-zero gluon polarization at certain momentum fractions and exploring lower x regions.

Contribution

This paper presents new experimental measurements of gluon polarization in protons at different energies, including the first evidence at x > 0.05 and ongoing analysis at lower x.

Findings

Evidence of non-zero gluon polarization at x > 0.05

Access to gluon polarization at lower x with 510 GeV data

Ongoing measurements of di-jet asymmetries

Abstract

The STAR Collaboration is performing a wide range of measurements to determine the gluon helicity distribution in the proton. Gluon-gluon and quark-gluon scattering dominate jet production in proton-proton collisions at RHIC, so the longitudinal double-spin asymmetry, $A_{LL}$, for jet production places significant constraints on the gluon polarization in the proton. In recent years STAR has recorded large longitudinally polarized $pp$ data sets at both $\sqrt{s} = 200$ GeV and $510$ GeV. The 2009 STAR inclusive jet $A_{LL}$ measurements at $\sqrt{s} = 200$ GeV show the first experimental evidence of non-zero gluon polarization over the Bjorken-$x$ range, $x > 0.05$. Furthermore, data collected at $\sqrt{s} = 510$ GeV during the 2012 and 2013 RHIC runs allow access to the gluon polarization at lower $x$. In this talk, I will present the final results of the 2009 inclusive jet $A_{LL}$ measurement at $200$ GeV, the analysis status of the 2012 inclusive jet $A_{LL}$ measurement at $510$ GeV, and the status of di-jet and other STAR measurements that are sensitive to gluon polarization.