The helicity of a particle is right-handed if the direction of its spin is the same as the direction of its motion and 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.

Is helicity always conserved?

(a) Helicity is a good quantum number: The helicity is conserved always because it commutes with the Hamiltonian. That is, its value does not change with time within a given reference frame. ... Hence, helicity is conserved for both massive and massless particles.

What is helicity in quantum mechanics?

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.