Helicity

Helicity — characterization of the particle state. Helicity is the projection of the spin onto the direction of motion. Used when describing particles moving at or near the speed of light. Is a conserved Lorentz invariant quantity for massless particles.

Distinguish:

1. Negative or "left" — the spin is directed against the direction of motion of the particle;
2. Positive or "right" — the spin is directed along the motion of the particle.