A Quark Model Analysis of Orbital Angular Momentum

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.

15 pages; latex; 5 postscript figures included in submission. Final version to appear in Phys. Lett. B. Discussion about the MIT bag model slightly changed

References in corpus (4)

• Nonforward Parton Distributions
• A Study of Off-Forward Parton Distributions
• Evolution Equations for Higher Moments of Angular Momentum Distributions
• A quark model analysis of the transversity distribution

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• Quark Orbital Angular Momentum in the MIT Bag Model
• Orbital Angular Momentum Parton Distributions in Light-Front Dynamics
• On impact parameter dependence of low-x structure functions