= Peak resonance cross section and branching fractions
{title2=$\sigma_{\rm peak}=4\pi g B_{ab}B_X/p_i^2$}
At $E=E_R$, the <Breit-Wigner resonance cross section> loses its explicit dependence on the total <decay width> after writing $\Gamma_{ab}=B_{ab}\Gamma$ and $\Gamma_X=B_X\Gamma$. For an unpolarized ultrarelativistic <Electron>-<Positron> initial state and a spin-one resonance, $g=3/4$ and $p_i=E_R/2$, giving $\sigma_{\rm peak}=12\pi B_eB_X/E_R^2$ in <natural units>. An elastic resonant channel therefore measures the square $B_e^2$, rather than one power of the electronic <branching fraction>.
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