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Why 2 photons are formed from positron-electron annihilation at rest

  1. May 26, 2013 #1
    This isn't a homework question, though it kind of relates to a practical I did recently. Sorry if it's posted in the wrong section!

    So, why are only 2 photons formed from positron-electron annihilation at rest? I understand why you can't have just one, as then you won't get conservation of momentum. But why do you only get 2 photons instead of say, 10 or 1000? I've looked up various particle physics textbooks and one on radiation detection, but they only explain that you can't get a single photon.
     
    Last edited: May 26, 2013
  2. jcsd
  3. May 26, 2013 #2

    mfb

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    As far as I know, you can get any number of photons except 1, but the reaction to two photons is really dominating, as it just needs [strike]one[/strike] two vertices ("interaction points") (edit: fixed) in the Feynman graph. In addition, the photons always have the same energy, so it is easy to detect.

    3-photon annihilation, suppressed by a factor of 370.

    Positronium is interesting, as it has one short-living state (which decays to two photons) and one long-living state (which cannot do this, and has to decay to three photons)
     
    Last edited: May 26, 2013
  4. May 26, 2013 #3
    Thanks for the reply. I don't quite understand, though. I thought that positron-electron annihilation always occurs when a positron captures an electron to form positronium, which then decays. If positronium can only decay into 2 and 3 photons, doesn't this rule out the decay into other numbers of photons?
     
  5. May 26, 2013 #4

    mfb

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    Those numbers for positronium are just the most likely numbers (as all lower numbers are forbidden), and a positron/electron pair does not have to form positronium to annihilate.
     
  6. May 26, 2013 #5
    there are two vertex in annihilation.There is a electron or/and positron internal line between the two photons.
     
  7. May 26, 2013 #6

    mfb

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    Oops, thanks. I imagined two and wrote one.
     
  8. May 26, 2013 #7
    Ah, I see. I guess the other mechanism is a plan old collision? The wording of Das and Ferbel's intro to nuclear and particle physics seemed to imply that positronium always formed before an annihilation, so I just assumed that it formed in a collision.
     
  9. May 26, 2013 #8
    This:
    http://arxiv.org/pdf/hep-ph/0310099v1.pdf
    gives the theoretic ratios of 4 photon annihilation to 2, and 5 photon annihilation to 3, in the region of 1/1 000 000. Experiments are similar, though they have measurement errors.
     
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