On the role of quark orbital angular momentum in the proton spin

TL;DR

This paper investigates how quark orbital angular momentum influences the proton spin within a covariant quark-parton model, aligning theoretical predictions with experimental data on spin structure functions.

Contribution

It introduces a Lorentz invariant 3D approach to analyze the interplay of quark spins and orbital angular momentum in proton spin structure, improving upon the collinear parton model.

Findings

Spin structure functions satisfy experimental constraints.

Proton spin content $\

The approach aligns with experimental data on proton spin.

Abstract

We study the covariant version of the quark-parton model, in which the general rules of the angular momentum composition are accurately taken into account. We demonstrate how these rules affect the relativistic interplay between the quark spins and orbital angular momenta, which collectively contribute to the proton spin. The spin structure functions $g_{1}$ and $g_{2}$ corresponding to the many-quark state $J=1/2$ are studied and it is shown they satisfy constraints and relations well compatible with the available experimental data including proton spin content $\Delta\Sigma\lesssim1/3$. The suggested Lorentz invariant 3D approach for calculation of the structure functions is compared with the approach based on the conventional collinear parton model.