Effect of Orbital Angular Momentum on Valence-Quark Helicity Distributions

TL;DR

This paper investigates how orbital angular momentum affects valence-quark helicity distributions at large x in perturbative QCD, revealing significant logarithmic contributions and implications for the / ratio behavior.

Contribution

It introduces the role of valence Fock states with nonzero orbital angular momentum in quark helicity distributions, providing new insights into their large-x behavior.

Findings

Orbital angular momentum contributes a large logarithm to negative helicity distributions.

The / ratio approaches 1 as x approaches 1.

The / ratio crosses zero at approximately x=0.75.

Abstract

We study the quark helicity distributions at large x in perturbative QCD, taking into account contributions from the valence Fock states of the nucleon which have nonzero orbital angular momentum. These states are necessary to have a nonzero anomalous magnetic moment. We find that the quark orbital angular momentum contributes a large logarithm to the negative helicity quark distributions in addition to its power behavior, scaling as (1-x)^5\log^2(1-x) in the limit of x\to 1. Our analysis shows that the ratio of the polarized over unpolarized down quark distributions, \Delta d/d, will still approach 1 in this limit. By comparing with the experimental data, we find that this ratio should cross zero at x\approx 0.75.