Polarized Gluon Distribution Delta g(x) in the Proton
  We study the polarized gluon distribution $\Delta g(x)$ in a longitudinally polarized proton. We argue that the distribution can be calculated approximately from the quark color currents found in simple quark models. The first result from the MIT bag model as well as the non-relativistic quark model shows that $\Delta g(x)$ is positive at all $x$. If this feature holds in QCD, it imposes a strong constraint on phenomenological fits to experimental data. The total gluon helicity $\Delta G$ from the bag model is about $0.3\hbar$ at the scale of 1 GeV, considerably smaller than previous theoretical expectations. 
Panying Chen and Xiangdong Ji