Gluon spin in the nucleon

We study the gluon spin contribution (Γ) to the nucleonʹs spin as measured in hard QCD 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. These fields give rise to a significant negative contribution to Γ at the quark model renormalization scale, μ02. In a generic non-relativistic quark model ΓNQM = −89(αNQMmq) 〈1r〉, in the bag model Γbag = −0.1 αbag. These correspond to ΓNQM −0.7 and Γbag −0.4 at αQCD 1.0.