Proper identification of the gluon spin

Understand

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 $S_g= \frac 59\Delta g$.
• Furthermore, $\frac 45$ 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 if properly identified, the gluon contribution to the nucleon spin may be drastically smaller than in the conventional wisdom.