= Breit-Wigner resonance cross section
{c}
{title2=$\sigma_{ab\to R\to X}=\frac{\pi g}{p_i^2}\frac{\Gamma_{ab}\Gamma_X}{(E-E_R)^2+\Gamma^2/4}$}
For an isolated <particle resonance> of <spin> $J_R$, the Breit-Wigner approximation relates a production-and-decay <cross section> to its partial <decay widths>. In <natural units>, $p_i$ is either incident particle's momentum magnitude in the <centre-of-momentum frame>, $g=(2J_R+1)/[(2J_a+1)(2J_b+1)]$ for unpolarized incident particles, and $\Gamma$ is the total width. The formula assumes the stated spin counting and neglects nonresonant backgrounds and their interference. Multiplication by $(\hbar c)^2$ converts an inverse-squared-energy cross section to area when momenta are expressed in energy units.
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