Gluon Spin in the Nucleon

Abstract

We study the operator description of the gluon spin contribution ($Γ$) to the nucleon's spin as it is measured in deep inelastic processes. $Γ$ can be related to the forward matrix element of a local gluon operator in $A^+=0$ gauge. In quark models the nucleon contains ambient color electric and magnetic fields. The latter are thought to be responsible for spin splittings among the light baryons. We show that these fields give rise to a significant {\it negative\/} contribution to $Γ$ at the quark model renormalization scale, $μ_0^2$. The non-Abelian character of QCD is responsible for the sign of $Γ$. In a generic non-relativistic quark model $Γ_{NQM}=-{8\over 9}{α_{NQM}\over m_q}\langle{1\over r}\rangle$, in the bag model $Γ_{bag}=-.1α_{bag}$. These correspond to $Γ_{NQM}\approx -0.7$ and $Γ_{bag}\approx -0.4$ at $α_{QCD}\approx 1.0$.