The helicity of a particle is defined as the projection of a spin vector in the direction of its momentum vector,

Therefore, if a particle's spin vector points in the same direction as the momentum vector, the helicity is positive, and if they point in opposite directions, the helicity is negative. However, the helicity of a massless particle is always equal to its chirality. The helicity of a particle is a Lorentz invariant.

Chirality, Neutrino, Spin