= Unpolarized resonance spin factor
{title2=$g=(2J_R+1)/[(2s_a+1)(2s_b+1)]$}
For unpolarized incident particles, the resonant cross section sums over the resonance's magnetic substates and averages over the incident spin states. This gives the displayed statistical weight under the conventional <Breit-Wigner resonance cross section> normalization. A spin-zero resonance formed from two spin-half particles has $g=1/4$, whereas a spin-one resonance has $g=3/4$. This counting is distinct from final-state <color charge> multiplicities.
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