Decay of pion

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Why the decay of neutral pion to electron positon pair is forbidden?
 
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blechman

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you know, way back when i wrote down on a test that this decay was forbidden. my professor has a good laugh: he told me that i just "disproved" his PhD thesis!
 

malawi_glenn

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it is not forbidden, I did my Diploma thesis on that decay! ;-P

i.e I am in the same situation as blechman's teacher was ;-)
 

clem

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The decay into electron-positron has to proceed through two virtual photons, one leading to the e-p pair and the other being absorbed by one of the pair. This makes the pi-->e p
rate much slower than the two gamma decay.
If the term "forbidden" is used in the way it is used in beta decay, then the e-p is
"forbidden", as confirmed by the experimental rate.
 

malawi_glenn

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it is second order, so it depends on what one means by "forbidden"
 
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it is second order, so it depends on what one means by "forbidden"
Here I think it means that it violates helicity conservation (which is expected for the leading order in the Bjorken limit).
 

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