Why we use the trace calculation?

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The trace calculation is essential in particle physics for determining cross sections as it averages over various possible initial states. This method is particularly useful when the initial state is unknown or when the particle beam consists of a mixture of states. Additionally, using the trace accounts for all possible intermediary (virtual) particles, ensuring comprehensive consideration of these states in calculations. This approach enhances the accuracy and reliability of the results in particle interactions. Understanding the role of trace calculations is crucial for effective analysis in quantum field theory.
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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