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The Breit-Wigner formula says that the cross section for a resonance is proportional to

[tex]

\frac{1}{(2S_a + 1)(2 S_b + 1)},

[/tex]

where Sa and Sb are the spins of the incident particles. What's the reason for this? I don't understand it because for a general cross section there appears just the inverse of that because that's what enters the density-of-states.

Thanks,

Bruno