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Interaction of photon+electron with parallel spins

  1. Apr 20, 2015 #1
    As I understand it, the fundamental unit of interaction in QED is a term with a pair of (spinor) electron factors and a (vector) photon factor, represented in a Feynman diagram as two (anti-)electron lines and one photon line meeting at a vertex.

    I get the case where the photon and electron have opposite spins: e.g. incoming electron, spin=+1/2, incoming photon, "spin"(helicity)=-1, outgoing electron, spin=-1/2.

    But how do you handle incoming photon "spin"=+1, incoming electron spin=+1/2, outgoing electron spin=???

    I can see how it would work with *four* lines -- i.e. topologically, the same as if there was no interaction at all (or an "X", rather than the three-line "Y" diagram)...but IIRC that's just not allowed (what would the term look like, with two vector and two spinor indices?)

    At first, I thought that the interactions of photons/electrons with parallel spins just had to be a second-order effect (e.g. two Y diagrams back-to-back), but it seems like even in that case you would need to have an intermediate particle with spin=+3/2...but that's not available in QED.

    What am I getting horribly wrong? I get the usual caveats, that Feynman diagrams only represent perturbative expansions etc etc...but even with those constraints, I'm sure that there's a way for this to work.
  2. jcsd
  3. Apr 25, 2015 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
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