= Pole forward-backward asymmetry for charged leptons
{title2=$A_{FB}=3\mathcal A_e\mathcal A_\ell/4$}
For a pure resonant <Z boson> with negligible <fermion> masses, $\mathcal A_f=(|g_L^f|^2-|g_R^f|^2)/(|g_L^f|^2+|g_R^f|^2)$ and $A_{FB}=3\mathcal A_e\mathcal A_f/4$. <Lepton universality> makes this $3\mathcal A_\ell^2/4$ for charged leptons. A nonzero value diagnoses <parity violation> within this single-resonance approximation; a general angular forward-backward asymmetry need not be parity odd.
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