Why we use the trace calculation?

Hluf
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Dear all
When we calculating the cross section of particles, we apply the trace calculation. This is not clear for me, why we use the trace calculation?
Thank you all.
 
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The trace averages over many possible initial states. Useful when the initial state is not known or the beam of particles contains a mixture of states.
 
Also, if you're talking about intermediary (virtual) particles than taking the trace ensures that all possible intermediary states are taken into account.
 

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