Gauge-independence of gluon spin in the nucleon and its evolution

Understand

In recent papers, we have established the existence of gauge-invariant decomposition of nucleon spin, each term of which can be related to known high-energy deep-inelastic-scattering observables.

• A subtlety remains, however, for the intrinsic spin part of gluons at the quantum level.
• In fact, it was sometimes claimed that the evolution of gluon spin depends on the gauge choice and its physical interpretation makes sense only in the light-cone gauge.
• In the present paper, we will demonstrate explicitly that the gluon spin operator appearing in our decomposition evolves gauge-independently and that it properly reproduces the familiar evolution equation for the 1st moments of polarized quark and gluon distributions obtained with the Altarelli-Parisi method, which cannot directly be checked by the standard operator expansion method.