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I I don't understand this Feynman Diagram

  1. Nov 8, 2016 #1
    Draw the lowest-order Feynman diagrams for the e+e- --> W+W-process

    The answer gives three diagrams. I understand the first two, but the third makes no sense to me. Here it is:


    So this is a t-channel Feynman diagram. As far as I can tell regarding how these types of Feynman diagrams are read, the following processes are taking place:

    e- --> ve + W-
    e+ + ve- --> W+

    But this makes no sense to me. How can an electron decay into a neutrino and a W boson? And how can a positron and a neutrino combine to form a W boson?

    Am I simply reading this wrongly?
  2. jcsd
  3. Nov 8, 2016 #2


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    There are no decays involved. An electron, a (virtual) neutrino and a W boson couple. That coupling is allowed. For a decay the W and the neutrino would have to be real (leave the Feynman diagram).

    Splitting the diagram into those subprocesses doesn't work well because you assign particle properties to things that are not actual particles.
  4. Nov 8, 2016 #3


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    well, the best way to think about it is to not order the time in the intermediate states because a Feynman diagram is really a combination of several time ordered diagrams. If you insist in thinking of it this way, then this Feynman diagram is a combination of what you described (where you put the vertex [itex]e^- W^- [/itex] vertex occurs first) plus the time ordered diagram for which the other vertex occurs first.

    But there is no contradiction here. You ask "how can an electron decay into a neutrino and a W boson?" We are dealing with an off-shell neutrino here (it is a virtual neutrino) so there is no problem with that reaction.
  5. Nov 8, 2016 #4


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    Just to add: Maybe you look upon these Feynmandiagrams too literally as processes. The actual process is given by the full amplitude. Feynmandiagrams are terms in its Taylor expansion.
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