 Pole forward-backward asymmetry for charged leptons

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.