The intrinsic angular momentum of a particle is known as its spin. The fermions all have spin half, whereas the bosons have spin one. The component of the spin in the direction of motion of a particle is called helicity. This means that the fermions can have helicity .
Massless particles may exist in just one helicity state. Neutrinos only exist in negative helicity states, known as left handed states, and anti-neutrinos in positive helicity, right handed states.
The and bosons have spin 1 so may have helicity 1 or zero. As the photon and gluon are massless, they cannot exist in a helicity zero state. However, virtual photons and gluons do have mass, so they may have the zero helicity states
Particles with helicity are said to be transversely polarised and those with zero helicity are longitudinally polarised.