Spin, Helicity and Polarisation

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 $\pm\frac{1}{2}$.

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 ${\rm W}^{\pm}$ and ${\rm Z}^{0}$ bosons have spin 1 so may have helicity $\pm$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 $\pm 1$ are said to be transversely polarised and those with zero helicity are longitudinally polarised.