 Resonance spin statistical factor

For unpolarized entrance particles, average over their $(2J_a+1)(2J_b+1)$ spin orientations and sum over the isolated resonance's $2J_R+1$ magnetic substates. The resulting weight multiplies the partial-wave cross section. It can exceed one because orbital angular momentum participates; it is not a standalone probability of forming a state. Polarization or several unresolved resonance spins requires more detailed channel counting.