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C-violating pion0 decay

  1. Jun 13, 2015 #1

    ChrisVer

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    The neutral pion is dominantly decaying to 2 photons via electromagnetic interaction [itex] \pi^0 \rightarrow 2 \gamma[/itex].
    However if one allows for P-violation in electromagnetic interactions, he will also get C violation (due to CPT theorem). In that case a decay of the form [itex] \pi^0 \rightarrow 3 \gamma[/itex] could be observable.
    I read in "Discrete symmetries and CP Violation: From experiment to theory" by M.S.Sozzi that without the selection rule applied on the ##\pi^0## decay, then the rate of the three-gamma is reduced by a factor of order [itex]\mathcal{O}(\alpha)[/itex] (that means ~130 times less), since we have 3 photon vertices instead of 2. Would that imply that:
    [itex]\frac{Br(\pi^0 \rightarrow 3 \gamma )}{Br(\pi^0 \rightarrow 2 \gamma)} \sim 10^{-2}[/itex]?
    How can this be in agreement with the experimental result of [itex] <3~ 10^{-8}[/itex]? It's not and that's why we say that C-invariance is there...then why looking for the 3-photon decay?
     
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  3. Jun 13, 2015 #2

    mfb

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    To check if the electromagnetic interaction is really C-conserving. Smaller C-violations lead to smaller branching fractions.
     
  4. Jun 13, 2015 #3

    Vanadium 50

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    This.
     
  5. Jun 13, 2015 #4

    Vanadium 50

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    Good reason: to see if the EM interaction is really C-conserving.
    Better reason: to see if there is some new interaction that interferes with electromagnetism, thus making it experimentally accessible.
     
  6. Jun 13, 2015 #5

    ChrisVer

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    Yup but if we find some C-violation it won't be from EM interactions since EM interactions cannot explain the 3photon decay. It would have to be something else.
    So I would go with the "better reason"...which atm I cannot challenge o0)
     
  7. Jun 13, 2015 #6

    mfb

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    It can, if it has a small C-violating term.
    Other C-violating interactions are possible as well, of course (and given the precision experiments done with electromagnetism, I guess that would be a more likely explanation).
     
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