Evidence for Polarization of Gluons in the Proton

We discuss the impact of recent high-statistics Relativistic Heavy Ion Collider data on the determination of the gluon polarization in the proton in the context of a global QCD analysis of polarized parton distributions. We find evidence for a nonvanishing polarization of gluons in the region of momentum fraction and at the scales mostly probed by the data. Although information from low momentum fractions is presently lacking, this finding is suggestive of a significant contribution of gluon spin to the proton spin, thereby limiting the amount of orbital angular momentum required to balance the proton spin budget.

Figure 1

Gluon helicity distribution at $Q^2=10{\mathrm{GeV}}^2$ for the new fit, the original DSSV analysis of [3], and for an updated analysis without using the new 2009 RHIC data sets (DSSV*, see text). The dotted lines present the gluon densities for alternative fits that are within the 90% C.L. limit. The $x$ range primarily probed by the RHIC data is indicated by the two vertical dashed lines.

Figure 2

Latest STAR data [6] for the double-spin asymmetry in jet production for two rapidity ranges compared to the results of our new and original [3] analyses. The inner and outer bands correspond to $\mathrm{\Delta }{\chi }^2=1$ unit and 90% C.L., respectively.

Figure 3

As in Fig. 2, but comparing to the PHENIX data [7, 9] for the double-spin asymmetry in ${\pi }^0$ production at $\sqrt{s}=62.4\mathrm{GeV}$ (upper panel) and $\sqrt{s}=200\mathrm{GeV}$ (lower panel).

Figure 4

Change of the $\mathrm{\Delta }{\chi }^2$ profile of the truncated first moment of $\mathrm{\Delta }g$ in the RHIC $x$ range with $Q^2$. The solid lines at the base of the plot indicate the 90% C.L. interval.

Figure 5

90% C.L. areas in the plane spanned by the truncated moments of $\mathrm{\Delta }g$ computed for $0.05\le x\le 1$ and $0.001\le x\le 0.05$ at $Q^2=10{\mathrm{GeV}}^2$. Results for DSSV, DSSV*, and our new analysis, with the symbols corresponding to the respective values of each central fit, are shown.