Polarized Parton Distributions in the Nucleon

TL;DR

This study performs LO and NLO QCD fits to world data on the polarized structure function g1 to extract the nucleon's polarized parton distributions. The authors parametrize Delta f for valence quarks, sea quarks, and gluons at a starting scale Q0^2=4 GeV^2, fix valence moments via the Ellis-Jaffe sum rule, and explore three large-x shapes for the polarized gluon distribution. They find excellent agreement with g1 data, but Delta G(x) remains poorly constrained; valence distributions are better determined and sea flavor decomposition is still uncertain. The work also assesses future probes, including Q^2 evolution of A1, polarized J/ψ photoproduction, and potential polarized g1 measurements at HERA, as routes to pin down gluon polarization and test small-x behavior.

Abstract

The distribution of the spin of the nucleon among its constituents can be parametrized in the form of polarized parton distribution functions for quarks and gluons. Using all available data on the polarized structure function $g_1(x,Q^2)$, we determine these distributions both at leading and next-to-leading order in perturbation theory. We suggest three different, equally possible scenarios for the polarized gluon distribution, which is found to be only loosely constrained by current experimental data. We examine various possibilities of measuring polarized parton distributions at future experiments.