Precision Measurement of the Longitudinal Double-Spin Asymmetry for Inclusive Jet Production in Polarized Proton Collisions at $\sqrt{s}=200\mathrm{GeV}$

We report a new measurement of the midrapidity inclusive jet longitudinal double-spin asymmetry, $A_{LL}$, in polarized $pp$ collisions at center-of-mass energy $\sqrt{s}=200\mathrm{GeV}$. The STAR data place stringent constraints on polarized parton distribution functions extracted at next-to-leading order from global analyses of inclusive deep-inelastic scattering (DIS), semi-inclusive DIS, and RHIC $pp$ data. The measured asymmetries provide evidence at the $3\sigma$ level for positive gluon polarization in the Bjorken-$x$ region $x>0.05$.

Figure 1

Jet neutral energy fraction (NEF) comparing data (the solid points) with simulations (the histograms), where both are calculated with $p_T$ subtraction. The upper panel shows jets with $8.4<p_T<9.9\mathrm{GeV}/c$, demonstrating the bias in NEF when jet $p_T$ is near the trigger threshold. The lower panel shows jets with $26.8<p_T<31.6\mathrm{GeV}/c$, demonstrating that an apparent bias persists well above threshold when using MIP subtraction (the open circles). The error bars show the simulation statistics. Those for the data are smaller than the points.

Figure 2

Midrapidity ($|\eta |<0.5$, upper panel) and forward rapidity ($0.5<|\eta |<1$, lower panel) inclusive jet $A_{LL}$ vs parton jet $p_T$, compared to predictions from several NLO global analyses. The error bars are statistical. The gray boxes show the size of the systematic uncertainties.