= Massless leptonic Z decay width
{title2=$\Gamma_f=\frac{g^2m_Z}{12\pi c_W^2}(c_V^2+c_A^2)$}
For a vertex $(g/c_W)\gamma^\mu(c_V-c_A\gamma^5)$ into one massless color-singlet particle-antiparticle pair, the <massive-vector spin average> and <two-body Lorentz-invariant phase space> give $\Gamma=g^2m_Z(c_V^2+c_A^2)/(12\pi c_W^2)$. With <Standard Model> <neutral-current vector and axial couplings>, one <neutrino> flavor has $\Gamma_\nu=g^2m_Z/(96\pi c_W^2)$ and one charged-<lepton> flavor has $\Gamma_\ell=\Gamma_\nu(1-4s_W^2+8s_W^4)$. <Chiral projectors> already remove inactive <neutrino> helicities. These are tree-level massless results.
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