= Polarized muon decay
{title2=$d\Gamma\propto x^2[(3-2x)+(1-2x)\widehat{\mathbf k}\cdot\mathbf s]$}
For a negative <muon> with rest-frame <spin> vector $\mathbf s$, the leading massless-electron distribution is $d\Gamma/(dx\,d\Omega)=G_F^2m_\mu^5x^2[(3-2x)+(1-2x)\widehat{\mathbf k}\cdot\mathbf s]/(384\pi^4)$, where $x=2E_e/m_\mu\in[0,1]$. Its final-spin sum is $64G_F^2(k\cdot q')(q\cdot(p-m_\mu s))$. The <spin> vector is axial whereas the electron direction is polar, so their scalar product reverses under <parity>. Its occurrence demonstrates parity violation by the left-chiral <weak charged current>.
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