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Homework Help: Gama gamma interaction

  1. Apr 14, 2008 #1
    Hi guys,

    Could someone show me how can I evaluate/measure the cross section of

    gamma+gamma --> electron + positron

    I'll be thankful for your help
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 15, 2008 #2


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    have you done calculations of cross sections before? Have you done, say, Moller scattering?
    This is a long calculation so it's impossible to show you all the steps if you haven't done any calculations. If you have, I can help you get through steps if you are stuck. First, did you draw the Feynman diagrams?
  4. Apr 15, 2008 #3
    Thanks nrged

    I have done the most of calculation. Where I got four terms of Trace {Amplitude}.

    As you knew in the C.M. we have a specific formula for differential cross section

    Diff. Cross Sect. = {1/{64* Pi^2)*(E1+E2)^2}*{|p3|/|p1|}*|M|^2

    That is for interaction(in general) A+B--> C+D

    My question is about p1 ? Is p1=pA or p1=pB ???

    or do you think A and B have the same momentum in C.M. frame"even if they have different masses"?

    Thanks for you help
  5. Apr 15, 2008 #4


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    By definition in the center of mass frame the magnitudes of pA and pB are equal. That's the definition. we are talking about three-momentum, here, of course.
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