title: Effect of orbital angular momentum on valence-quark helicity distributions.
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. 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, Deltad/d, will still approach 1 in this limit. By comparing with the experimental data, we find that this ratio should cross zero at x approximately 0.75.
authors.affiliation: Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA.
authors.forename: Harut
authors.initials: H
authors.lastname: Avakian
authors.forename: Stanley J
authors.initials: SJ
authors.lastname: Brodsky
authors.forename: Alexandre
authors.initials: A
authors.lastname: Deur
authors.forename: Feng
authors.initials: F
authors.lastname: Yuan