A quantum-mechanical variable that specifies the component of spin-angular momentum of a particle along its direction of motion. The helicity of a particle with rest mass depends on the reference frame, because such a particle has velocity less than the velocity of light c in vacuum. Hence one can make a Lorentz transformation along the direction of motion to a frame in which the particle moves in the opposite direction. For a massless particle, reversal in direction of motion by Lorentz transformation is impossible, and helicity is a Lorentz-invariant quantity. For example, a beam of circularly polarized light consists of photons (zero-mass quanta of the electromagnetic field) with helicity h = +1 for right circular polarization, or h = −1 for left circular polarization (see illustration).