EDS Blois 2019





  \title{Gluon polarization measurements from longitudinally polarized proton-proton collisions at STAR}


    Zilong Chang, for the STAR Collaboration \\

    Brookhaven National Laboratory, Upton, NY 11973





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 di-jet $A_{LL}$ 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 $A_{LL}$. Di-jet $A_{LL}$ results are shown for four topologies in regions of pseudo-rapidity, effectively scanning the $x$-dependence of the gluon polarization.