Lattice simulation of the anomalous gluon contribution to the proton spin

The magnitude of the gluon contribution to the proton spin induced by the triangle anomaly diagram in the quark axial current is estimated by means of a lattice QCD simulation (in the temporal axial gauge) of the polarized-proton matrix element of the anomalous current 〈ps‖${\mathit{K}}_{\mathrm{\mu }}$‖ps〉. The result of the simulation is that the fractional contribution of the gluon anomaly to the proton spin, -(3α/2π)Δg, is less than 0.05 in magnitude. This is much too small to affect the basic interpretation of the E.M.C. experiment, namely that the quark spins are responsible for very little, if any, of the proton spin.