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Why we use the trace calculation?

  1. Mar 11, 2014 #1
    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.
  2. jcsd
  3. Mar 11, 2014 #2
    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.
  4. Mar 11, 2014 #3
    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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