I know electrons have helicity but can particles which are not fundamental i.e.protons have helicity?


2 Answers 2


Helicity of a particle is defined to be the projection of its spin vector $\mathbf{s}$ along the direction of its impulse $\mathbf{p}$:

$$H = \frac{\mathbf{s}\cdot\mathbf{p}}{||\mathbf{s}\cdot \mathbf{p}||}$$

  • For a massless particle, the helicity is equivalent to the chirality;
  • If the mass of the particle is not zero, then helicity is not a Lorentz invariant;
  • Neutrino helicity is $-1$

So every particle with spin $\neq 0$ and/or $m \neq 0$ has helicity.


Yes. Helicity is nothing but the projection of the spin of the particle along the direction of the momentum. Protons have spin as well as the other composite particles.


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