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Given

https://www.researchgate.net/publication/323384550_ELECTROWEAK_INTERACTION_AND_ITS_TESTS_Bachelor_Thesis

I wonder why are there two photons entering in a) pair production? Isn't it one photon resulting in $e^+e^-$ ?

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It is not possible to satisfy both energy and momentum conservation with only one photon.

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    $\begingroup$ This is easily seen: For the reverse process, pair annihilation, you can go to the rest frame where the momentum is zero. However, a real photon always moves at the speed of light, there is no rest frame. $\endgroup$ Aug 16, 2019 at 12:36
  • $\begingroup$ Right, I remember! :) But another question that arises: When I need a photon that takes a recoil: How/why do the photons interact with each other? Btw, why is a photon resulting in e-e+ in general? :D $\endgroup$
    – Ben
    Aug 16, 2019 at 12:37
  • $\begingroup$ @Ben "why is a photon resulting in e-e+ in general?" That probably deserves a fresh question (if it doesn't exist already). $\endgroup$
    – PM 2Ring
    Aug 16, 2019 at 12:50

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