Gauge invariant gluon spin operator for spinless non-linear wave   solutions
  We consider non-linear wave type solutions with mass scale parameter and vahished canonical spin density operator in a pure SU(2) quantum chtomodynamics (QCD). A new stationary solution which can be treated as a system of static Wu-Yang monopole dressed in off-diagonal gluon field is proposed. A remarkable feature of such a solution is that it possesses a finite energy density everywhere. All considered non-linear wave type solutions have common features: presence of a mass scale parameter, non-vanishing projection of the color magnetic field along the propagation direction and zero spin density. The last property requires revision of the gauge invariant definition of the spin density operator which supposed to be massless vector field in the classical theory. We construct a gauge invariant definition of the classical gluon spin density which is unique and Lorentz frame independent. 
Bum-Hoon Lee, Youngman Kim, D.G. Pak, Takuya Tsukioka and P.M. Zhang
hep-th