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Can parallel spin antiproton annihilate?

  1. Oct 1, 2012 #1
    Positronium annihilation opportunities are strongly restricted by the requirement to conserve spin. The spin of a photon is 1; and for some reasons two photons are completely unable to have spins in different directions. They can only have parallel spins (total 2) or antiparallel spins (total 0) - never 1, and this prevents a positron with spin parallel to an electron from annihilating to 2 photons. Only 3 photons are possible (also 5 or larger odd numbers) which is very much slower process.

    Now how about antiproton?
    The problem here is that antiprotons are said to annihilate normally into pions - but pion spin is 0.

    No matter how many pions are produced, whether 3, 4, 5 or more, no combination of pions can possibly handle the combined spin of proton and antiproton with parallel spins.

    So is such annihilation possible?
  2. jcsd
  3. Oct 1, 2012 #2


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    In addition to conservation of angular momentum, the analysis involves conservation of C and P. It gets rather complicated. But quoting the answer,

    "The singlet ground state 1S0 of para-protonium can decay only into three pions. Whereas the ortho-protonium 3S1 can decay either into two or three pions, but never into a system of uncharged pions only. (The 3π decay of ortho-protonium is much less probable than the 2π decay. Not only because the phase space is much smaller, but also because already the simplest 3π state has a high centrifugal barrier.)"
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