Orbital Angular Momentum Parton Distributions in Quark Models

TL;DR

This paper calculates orbital angular momentum distributions in nucleons using quark models and shows their growth with energy scale, highlighting their potential significance in nucleon spin structure at high energies.

Contribution

It provides the first calculation of OAM parton distributions at a low hadronic scale within relativistic and non-relativistic quark models, and tracks their evolution to experimental scales.

Findings

Quark and gluon OAM contributions grow with Q^2.

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

Sign and shape of OAM depend on initial conditions at the hadronic scale.

Abstract

At the low energy, {\sl hadronic}, scale we calculate Orbital Angular Momentum (OAM) twist-two parton distributions for the relativistic MIT bag model and for non-relativistic quark models. We reach the scale of the data by leading order evolution in 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 $Q^2$.