A Quark Model Analysis of Orbital Angular Momentum

TL;DR

This paper investigates the orbital angular momentum distributions of quarks and gluons within the nucleon using quark models and QCD evolution, highlighting their growing significance at higher energy scales and implications for nucleon spin structure.

Contribution

It provides a detailed analysis of OAM distributions at low energy scales using the MIT bag and potential models, and explores their evolution to higher scales relevant for experiments.

Findings

Quark and gluon OAM contributions grow with Q^2.

Quark OAM can dominate nucleon spin at high Q^2.

Gluon OAM is nearly canceled by gluon spin contributions.

Abstract

Orbital Angular Momentum (OAM) twist-two parton distributions are studied. At the low energy, hadronic, scale we calculate them for the relativistic MIT bag model and for non-relativistic potential quark models. We reach the scale of the data by leading order evolution using the OPE and perturbative QCD. We confirm that the contribution of quarks and gluons OAM to the nucleon spin grows with $Q^2$, and it can be relevant at the experimental scale, even if it is negligible at the hadronic scale, irrespective of the model used. The sign and shape of the quark OAM distribution at high $Q^2$ may depend strongly on the relative size of the OAM and spin distributions at the hadronic scale. Sizeable quark OAM distributions at the hadronic scale, as proposed by several authors, can produce the dominant contribution to the nucleon spin at high $Q^2$. As expected by general arguments, we obtain, that the large gluon OAM contribution is almost cancelled by the gluon spin contribution.