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Feynman Diagrams, exchange particle?

  1. Apr 2, 2015 #1

    CAH

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    Exchange Particles are to show the transfer of (for example) +/- charge to the other side so the charges balance. But I don't understand...

    Beta plus decay:
    p → n + e+ + νe.

    This is just an example, the Feynman diagram shows a W+ boson transferring the positive charge to the right hand side, but in the equation: +1 →0 + (+1) + 0
    It's all balanced.

    So where does the boson come in? Is it to just reinforce the fact that a positron is produced?
     

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  3. Apr 2, 2015 #2

    mfb

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    The overall reaction is always balanced, and the W does not contribute to it because it is an "internal" (virtual) particle here. There is just no way to write this decay process down without W as the positron and neutrino do not couple to quarks directly. And charge (and also various other numbers) has to be balanced at every vertex.

    It is possible to make an effective theory where this "direct" interaction is possible, and it does reasonably well for beta decays, but that does not match our observations in particle accelerators.
     
  4. Apr 3, 2015 #3

    CAH

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    So which direction does the boson go to?
     
  5. Apr 3, 2015 #4

    mfb

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    It is meaningless to talk about directions for virtual particles.
    You can say "a W+ goes from the quark vertex to the positron/neutrino vertex", but in the same way you can say "a W- goes from the positron/neutrino vertex to the quark vertex".
     
  6. Apr 3, 2015 #5

    CAH

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    Where does the negative/positive charge come from/go to, is it basically showing the electron/positron which is produced to balance charges?
     
  7. Apr 3, 2015 #6

    mfb

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    I don't think that is a useful question.
    Charge is conserved at every vertex. It does not need "directions".
    Also, no particles are produced "to balance charges". Unbalanced reactions simply cannot happen.
     
  8. Apr 3, 2015 #7

    CAH

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    I dont think you understand my question.
    Also you misunderstood my previous reply i was referring to on the diagram. (Where you draw it from, not in real life)
    I know that having a W- boson going to the left has the same effect as a W+ boson going to the right, however marks may be docked in the exam anyway and i still need to know which direction it goes in anyway.
    Im sure the W boson comes from the particle thats acting as i have looked into it further.
     
  9. Apr 3, 2015 #8

    mfb

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    If the exam is asking for the direction of virtual particles, it is a bad exam.
     
  10. Apr 3, 2015 #9

    CAH

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    'If no arrow on W boson then must be clearly slanting in correct direction. E must have - subscript for second mark. If no clear junctions lose second mark. If no arrows on sides -1.'
     

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  11. Apr 3, 2015 #10

    Vanadium 50

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    If they are insisting on arrows on the W, they are wrong. Fermions get arrows. The W is not a fermion.
     
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