Helicity¶

source

Every matter particle (electrons, quarks, etc.) is spinning, i.e. each matter particle carries some intrinsic angular momentum. This spin is an inherently quantum mechanical property of fundamental particles. (red: indicates spin and gray: indicates the direction). The particle shows right-handedness. This is called helicity of the particle. (specifically right helicity)

Right Helicity

Left Helicity

Mass is a something that tells us whether helicity is an “intrinsic” property of the particle. If a particle is massless, then its helicity has a fixed value in all reference frames. On the other hand, if a particle has any mass, then helicity is not an intrinsic property since different observers (in valid reference frames) can measure different values for the helicity (left- or right-helicity). So even though helicity is something which is easy to visualize, it is not a “fundamental” property of most particles.

Common convention: Right-handed Helicity \(\rightarrow\) spin and momentum parallel Left-handed Helicity \(\rightarrow\) spin and momentum antiparallel

Also Related to Chirality.