Determination of polarized parton distribution functions with recent data on polarization asymmetries

TL;DR

This work presents a NLO QCD global analysis of polarized parton distribution functions incorporating new spin-asymmetry data from JLab, Hermes, Compass, and PHENIX. The approach uses a minimal parameterization tied to unpolarized PDFs, DGLAP evolution, and a Hessian-based uncertainty assessment, yielding significantly reduced uncertainties for Δg(x) thanks to A_LL^{π^0} and improved Δd_v and Δq̄ from A1 measurements. The results show a preference for positive gluon polarization at large x, while small-x behavior remains poorly constrained with multiple viable Δg(x) scenarios, including a potential negative solution. The study highlights the synergy between DIS and hadron-production data and points to future polarized measurements (e.g., at RHIC and eRHIC) to pin down Δg(x) across the full x-range and test scaling violations.

Abstract

Global analysis has been performed within the next-to-leading order in Quantum Chromodynamics (QCD) to determine polarized parton distributions with new experimental data in spin asymmetries. The new data set includes JLab, HERMES, and COMPASS measurements on spin asymmetry A_1 for the neutron and deuteron in lepton scattering. Our new analysis also utilizes the double-spin asymmetry for pi^0 production in polarized pp collisions, A_{LL}^{pi^0}, measured by the PHENIX collaboration. Because of these new data, uncertainties of the polarized PDFs are reduced. In particular, the JLab, HERMES, and COMPASS measurements are valuable for determining Delta d_v(x) at large x and Delta qbar(x) at x~0.1. The PHENIX pi^0 data significantly reduce the uncertainty of Delta g(x). Furthermore, we discuss a possible constraint on Delta g(x) at large x by using the HERMES data on g_1^d in comparison with the COMPASS ones at x~0.05.

Figures (10)

• Figure 1: (Color online) Differences between the data and parametrization results. The dotted curves indicate uncertainty bands of the AAC03 analysis and the shaded areas are the bands of the current analysis.
• Figure 2: (Color online) Comparison with Phenix$\pi^0$-production asymmetry data $A_{LL}^{\pi^0}$. The solid and dashed curves indicates type 1 and 2 analysis results, respectively. The uncertainty range for the type 1 is shown by the shaded band, and the one for the type 2 is by the dotted curves.
• Figure 3: (Color online) Current type-1 analysis results are compared with $g_1^p$ data.
• Figure 4: (Color online) Polarized PDFs obtained by the type-1 $\chi^2$ analysis are shown at $Q^2$=1 GeV$^2$ by the solid curves, and their uncertainties are shown by the shaded bands. In comparison, the AAC03 PDFs are shown by the dashed curves and their uncertainties by the dotted curves.
• Figure 5: (Color online) Comparison with other polarized PDFs at $Q^2$=1 GeV$^2$. The type 1 distributions and their uncertainties are shown by the solid curves and bands. The others are the GRSV, BB, and LSS parametrizations.