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)(5)log(2)(1-x) in the limit of x -> 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 approximate to 0.75.