Spin structure of the nucleon: QCD evolution, lattice results and models

TL;DR

This paper reviews the current understanding of the nucleon spin structure, combining lattice QCD results, chiral quark models, and QCD evolution to compare theoretical predictions with experimental data.

Contribution

It introduces a refined model calculation and performs QCD evolution of lattice results up to NNLO, enhancing the comparison between models and lattice data.

Findings

Lattice QCD provides detailed spin and orbital angular momentum contributions.

QCD evolution up to NNLO aligns lattice results with experimental scales.

Comparison with the Myhrer-Thomas scenario offers insights into the proton spin puzzle.

Abstract

The question how the spin of the nucleon is distributed among its quark and gluon constituents is still a subject of intense investigations. Lattice QCD has progressed to provide information about spin fractions and orbital angular momentum contributions for up- and down-quarks in the proton, at a typical scale \mu^2~4 GeV^2. On the other hand, chiral quark models have traditionally been used for orientation at low momentum scales. In the comparison of such model calculations with experiment or lattice QCD, fixing the model scale and the treatment of scale evolution are essential. In this paper, we present a refined model calculation and a QCD evolution of lattice results up to next-to-next-to-leading order. We compare this approach with the Myhrer-Thomas scenario for resolving the proton spin puzzle.