Gauge invariance and hadron structure

Abstract

We prove that the {\em gauge dependent} gluon spin, gluon and quark orbital angular momenta operators have {\em gauge invariant} expectation values on hadron states with {\em definite} momentum and polarization, therefore the conventional decomposition of nucleon spin into contributions from the spin and orbital angular momentum of quark and gluon is {\em gauge independent}. Similar conclusions apply to the {\em gauge dependent} quark momentum and kinetic energy operators, and accordingly nucleon momentum and mass structures.

This revised version gives a complete, comprehensive proof of our conclusion, and explains why the specific calculations of Hoodbhoy, Ji and Lu (hep-ph/9808305) do not necessarily contradict our theorem