Well of course everyone knows...The gluon is a vector boson; like the photon, it has a spin of 1. While massive spin-1 particles have three polarization states, massless gauge bosons like the gluon have only two polarization states because gauge invariance requires the polarization to be transverse.
(
Zumf Ham banjo,
Mon 6 Oct 2008, 13:12,
archived)
