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Zero cross section

  1. Mar 19, 2015 #1
    1. The problem statement, all variables and given/known data
    I am making an old exam of a particle physics course, and i know how to calculate the cross section for example for
    bhabha or moller scattering.

    now one of the questions on the old exam is:

    Explain why e-+ e+ -> γ is zero, but i am not sure why this is, can someone explain this or point me in the right direction.

    2.
    I would usually when determining the cross section start with drawing the lowest order feynman diagram, my book put t on the x axis, and the determine the vertex and propagator factors, so i can determine the matrix element.

    than with the formula dσ/dΩ = 1/64*pi*s * M^2
     
    Last edited: Mar 19, 2015
  2. jcsd
  3. Mar 19, 2015 #2

    Orodruin

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    Does it perhaps violate some conservation law? Do you even need to draw a Feynman diagram to come to this conclusion?
     
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