Effect of Orbital Angular Momentum on Valence-Quark Helicity Distributions

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. 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)⁵log⁡²(1-x) in the limit of x→1. Our analysis shows that the ratio of the polarized over unpolarized down quark distributions, Δd/d, will still approach 1 in this limit. By comparing with the experimental data, we find that this ratio should cross zero at x≈0.75.