In particle physics, helicity is the projection of the angular momentum onto the direction of momentum. The angular momentum J is the sum of an orbital momentum L and a spin S ; its relation to the position operator r and the linear momentum p is
so L →'s component in the direction of p→ is zero. Thus, helicity is just the projection of the spin onto the direction of linear momentum. This quantity is conserved.
Because the eigenvalues of spin with respect to an axis have discrete values, the eigenvalues of helicity are also discrete. For a particle of spin S, the eigenvalues of helicity are S, S − 1, ..., −S. The measured helicity of a spin S particle will range from −S to +S.
For massless spin- 1⁄2 particles, helicity is equivalent to the chirality operator multiplied by ħ/2. By contrast, for massive particles, distinct chirality states (e.g., as occur in the weak interaction charges) have both positive and negative helicity components, in ratios proportional to the mass of the particle.
Little group
In 3 + 1 dimensions, the little group for a massless particle is the double cover of SE(2). This has unitary representations which are invariant under the SE(2) "translations" and transform as eihθ under a SE(2) rotation by θ. This is the helicity h representation. There is also another unitary representation which transforms non-trivially under the SE(2) translations. This is the continuous spin representation.
In d + 1 dimensions, the little group is the double cover of SE(d − 1) (the case where d ≤ 2 is more complicated because of anyons, etc.). As before, there are unitary representations which don't transform under the SE(d − 1) "translations" (the "standard" representations) and "continuous spin" representations.