Gluon spin in the nucleon

We study the gluon spin contribution (Gamma) to the nucleon's spin as measured in hard QCD processes. Gamma 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 Gamma at the quark model renormalization scale, mu(0)(2). In a generic nonrelativistic quark model Gamma(NQM) = -8/9 (alpha(NQM)/m(q)) [l/r], in the bag model Gamma(bag) = -0.1 alpha(bag). These correspond to Gamma(NQM) approximate to -0.7 and Gamma(bag) approximate to -0.4 at alpha(QCD) approximate to 1.0.