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Momentum Transfer in Scattering

  1. Aug 9, 2016 #1
    1. The problem statement, all variables and given/known data
    This is just a simple proof of substitution, but after one day struggle, I still can't get where that minus sign appear.

    2. Relevant equations
    Here is the equation:
    $$ (p_1 - p_3)^2=-(\mathbf p_1 - \mathbf p_3)^2$$
    where ##p_1=(E/c,\mathbf p_1)## and ##p_3=(E/c, \mathbf p_3)## is four-momentum of incident and scattered electron respectively, with stationary target.

    3. The attempt at a solution
    Here is my doodling:
    (p_1 - p_3)^2=(E/c+\mathbf p_1-E/c-\mathbf p_3)^2=(\mathbf p_1 - \mathbf p_3)^2.
    Which step that I miss?
  2. jcsd
  3. Aug 10, 2016 #2
    ah, i've got it. solved by myself.
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