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Why is hadronic vacum polarization to be a matrix element of product of currents?

  1. Feb 15, 2012 #1
    Please teach me this:
    In chapter 18.4 Peskin&Schoeder(QFT) they consider the annihilation of electron and positron to hadron.Ignoring the mass of the electron,we have:


    We have:


    I do not understand why they can write:


    Where J[itex]^{\mu}[/itex] is the electromagnetic current of quarks.
    Thank you very much for your kind helping.
  2. jcsd
  3. Mar 2, 2012 #2
    Now,I think that it is because the fluctuation of vacum(that is excited by virtue photon) making a loop that is product of two currents(not more complex including many hadronics).Is that correct?
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