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Particle-antiparticle annihilation and spin

  1. May 26, 2005 #1
    I have a question regarding particle-antiparticle annihilation, such as electron-positron, proton-antiproton, etc. Can the annihilation still occur if the two particles are in opposite spin eigenstates, i.e. if the pair has zero net spin?
     
  2. jcsd
  3. May 26, 2005 #2
    The spin of the whole system has to be conserved.

    The particle and the antiparticle will annihilate into two photons. Because of spin conservation (one photon has spin of 1 and particle antiparticle has in your sense net spin of 0). You see? Two photons also with net spin zero.
     
  4. May 27, 2005 #3

    Meir Achuz

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    For positronium (electron-positron bound state), the state with total spin 0 annihilates much faster than the state with spin 1, because the spin 1 state must annihilate into three photons, with 1+1+1=1.
    Incidentally, the two particles are NOT in opposite spin eigenstates.
    If they are in an eigenstate of total spin (1 or 0), the individual particles can not be in spin eigenstates.
     
  5. May 27, 2005 #4
    Sorry, I'm missing something very obvious that I do know, but can't put my finger on right now (note to self - drinking heavily the night before doing Physics doesn't work...). Why can't the spin 0 positronium state decay to two photons with spin +1, -1 respectivaly? That conserves the spin surely?
     
  6. May 27, 2005 #5
    James - who said it doesn't?
     
  7. May 30, 2005 #6
    Does the spin 1 state decay in two stages? Maybe spin 1 positronium -> spin 0 positronium + photon, then spin 0 positronium -> two photons?
     
  8. May 30, 2005 #7

    dextercioby

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    Can u prove it?

    Why?

    Umm,Quantum Mechanics...? :rolleyes:

    Daniel.
     
  9. May 31, 2005 #8
    hehe. No. You learned me that spin has not to be conserved. Only total angular momentum has to be conserved.
     
  10. May 31, 2005 #9
    oh oh, this is a dangerous one


    drop it like it's hot...

    marlon
     
  11. May 31, 2005 #10
    Maybe you're misunderstanding my point, and I'm not sure why you are since it's pretty explicit.

    You're saying that spin 0 positronium CANNOT decay to two photons? That is what I'm referring to.

    If so, I beg to differ:

    http://rockpile.phys.virginia.edu/mmod27.pdf

    Bottom of page 3. (Further up may lie the answer to your inquiry of Meir Achuz's post).
     
    Last edited: May 31, 2005
  12. May 31, 2005 #11

    dextercioby

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    I didn't say that,i missinterpreted your question,since you simply asked it without quoting what in James Jackson's post you were referring to.

    Daniel.
     
  13. Jun 1, 2005 #12

    Meir Achuz

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    So far we have left out charge conjugation invariance, which was tacitly assumed in my original answer. Photons have C=-1. Positronium of spin 0 has C=+1, and so can decay into two photons. Positronium of spin 1 has C=-1, and cannot decay into two photons. Three is the next lowest number. If this leads to more questions, I will try to answer them as asked.
     
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