What is "helicity"

n. 1 The quality of being helical. 2 (context physics English) The quantized spin component of a moving particle

In particle physics, helicity is the projection of the angular momentum onto the direction of momentum. The angular momentum is the sum of an orbital momentum and a spin ; its relation to the position operator and the linear momentum is

$$\vec L = \vec r \times \vec p,$$
so 's component in the direction of is zero. Thus, helicity is just the projection of the spin onto the direction of linear momentum. This quantity is conserved.

Because the eigenvalues of spin with respect to an axis have discrete values, the eigenvalues of helicity are also discrete. For a particle of spin , the eigenvalues of helicity are , , ..., −. The measured helicity of a spin particle will range from − to +.

For massless spin- particles, helicity is equivalent to the chirality operator multiplied by /2. 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.

Helicity may refer to:

• Helicity (fluid mechanics), the extent to which corkscrew-like motion occurs
• Helicity (particle physics), the projection of the spin onto the direction of momentum
• Magnetic helicity, the extent to which a magnetic field "wraps around itself"
• Circular dichroism, the differential absorption of left and right circularly polarized light
• A form of axial chirality