Neutrino helicity ($h_{\nu }=2{\mathbf{S}}_{\nu }\cdot {\hat{\mathbf{p}}}_{\nu }$)

For a relativistic neutrino, the normalized helicity is approximately $-1$ in ordinary weak production. For a massive particle, helicity and fermion chirality differ; suppressed opposite-helicity amplitudes and frame dependence prevent promoting the massless selection rule to an exact statement about arbitrary boosts.