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We calculate the quark orbital angular momentum of the nucleon from the quark energy-momentum tensor form factors on the lattice with the quenched approximation. The disconnected insertion is estimated stochastically which employs the Z₂ noise with an unbiased subtraction. This reduced the error by a factor of 3--4 with negligible overhead. The total quark contribution to the proton spin is found to be 0. 300. 07. From this and the quark spin content we deduce the quark orbital angular momentum to be 0. 170. 06 which is 34% of the proton spin. We further predict that the gluon angular momentum is 0. 200. 07; i. e. , 40% of the proton spin is due to the glue.
Mathur et al. (Wed,) studied this question.