Models for the Polarized Parton Distributions of the Nucleon

TL;DR

The paper assesses polarized parton distributions using LO and NLO QCD within two contrasting frameworks: a flavor-symmetric, unbroken light sea and a fully flavor-broken, flavor-asymmetric light sea modeled by a Pauli-blocking ansatz at low input scales. It finds that both scenarios can describe current inclusive polarized DIS data, though flavor breaking in the light sea is partially supported by external models and SIDIS hints. The analysis shows a weak constraint on the polarized gluon density, with a vanishing $\Delta g$ still compatible, and a tendency in the broken-ssea scenario toward a positive $\Delta\bar u$ and negative $\Delta\bar d$, consistent with chiral quark-soliton and statistical parton model predictions. The work provides LO/NLO polarized parton densities, tests sum rules, discusses orbital angular momentum at low scales, and offers a public code for the distributions, contributing to a clearer spin decomposition of the nucleon and guiding future SIDIS and polarized collider measurements.

Abstract

Polarized deep inelastic scattering (DIS) data are analyzed in leading and next-to-leading order of QCD within the common `standard' scenario of polarized parton distributions with a flavor-symmetric light sea (antiquark) distribution $δ\bar{q}$, and a completely SU(3)$_f$ broken `valence' scenario with totally flavor-asymmetric light sea densities $(δ\bar{u}\neqδ\bar{d}\neqδ\bar{s})$. The latter flavor-broken light sea distributions are modelled with the help of a Pauli-blocking ansatz at the low radiative/dynamical input scales of $μ_{\rm LO(NLO)}^2=0.26$ (0.40) GeV$^2$ which complies with predictions of the chiral quark-soliton model and expectations based on the statistical parton model as well as with the corresponding, well established, flavor-broken unpolarized sea ($\bar{d}>\bar{u}$). Present semi-inclusive DIS data cannot yet uniquely discriminate between those two flavor-symmetric and flavor-broken polarized light sea scenarios.