Proper identification of the gluon spin

Properties of the recently proposed gauge-invariant gluon spin S(g) are studied and compared to the usually defined "gluon polarization" Delta g. By explicit 1-loop calculations in a quark state. it is found that S(g) = 5/9 Delta g. Furthermore, 4/5 of S(g) can actually be identified as a "static-field" contribution and shown to cancel exactly an analogous static term in the gluon orbital angular momentum L(g), leaving S(g) + L(g) unaltered. These observations suggest that by an alternative definition, the gluon contribution to the nucleon spin may be drastically smaller than in the conventional wisdom. (C) 2011 Elsevier B.V. All rights reserved.