Proton Spin Content From Lattice QCD

We calculate the form factor of the quark energy momentum tensor and thereby extract the quark orbital angular momentum of the nucleon. The calculation is done on a quenched 163 x 24 lattice at β = 6.0 and with Wilson fermions at κ = 0.148, 0.152, 0.154 and 0.155. We calculate the disconnected insertion stochastically which employs the Z2 noise with an unbiased subtraction. This proves to be an efficient method of reduce the error from the noise. We find that the total quark contribution to the proton spin is 0.29 ± 0.07. From this we deduce that the quark orbital angular momentum is 0.17 ± 0.08 and predict the gluon spin to be 0.21 ± 0.07, i.e. about 40% of the proton spin is due to the glue