Helicity (particle physics)
In physics, helicity is the projection of the spin onto the direction of momentum. Overview. The angular momentum J is the sum of an orbital angular momentum L and a spin S. The relationship between orbital angular momentum L, the position operator r and the linear momentum (orbit part) 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. The helicity of a particle is positive (" right-handed") if the direction of its spin is the same as the  direction of its motion and negative ("left-handed") if opposite. Helicity is conserved. That is, the helicity commutes with the Hamiltonian, and thus, in the absence of external forces, is time-invariant. It is also rotationally invariant, in that a rotation applied to the system leaves the helicity unchanged. Helicity, however, is not Lorentz invariant; under the action of a Lorentz boost, the helicity may change sign. Consider, for example, a baseball, pitched as a gyroball, so that its spin axis is aligned with the direction of the pitch. It will have one helicity with respect to the point of  view of the players on the field, but would appear to have a flipped helicity in any frame moving faster than the ball ("e.g." a bullet train, as both bullet trains and gyroballs are popular in Japan, while trains are popular in special relativity.) Comparison with chirality. In this sense, helicity can be contrasted to chirality, which is Lorentz invariant, but is "not" a constant of motion for massive particles. For massless particles, the two coincide: the helicity is equal to the chirality, and both are Lorentz invariant, and are constants of motion. In quantum mechanics, angular momentum is quantized , and thus helicity is quantized as well. Because the eigenvalues of spin with respect to an axis have discrete values, the eigenvalues of helicity are also discrete. For a massive particle of spin , the eigenvalues of helicity are , , , ..., −. In massless particles, not all of these correspond to physical degrees of freedom: for example, the photon is a massless spin 1 particle with helicity eigenvalues −1 and +1, and the eigenvalue 0 is not physically present. All known spin  particles have non-zero mass; however, for hypothetical massless spin  particles (the Weyl spinors), helicity is equivalent to the chirality operator  multiplied by . 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. A treatment of the helicity of gravitational waves can be found in Weinberg. In short, they come in only two forms: +2 and -2, while the +1, 0 and -1 helicities are non-dynamical (can be gauged away). Little group. In 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 under a SE(2) rotation by . This is the helicity representation. There is also another unitary representation which transforms non-trivially under the SE(2) translations. This is the continuous spin" representation. In dimensions, the little group is the double cover of SE() (the case where is more complicated because of anyons, etc.). As before, there are unitary representations which don't transform under the SE() "translations" (the "standard" representations) and "continuous spin" representations.