Why is the Decay of Neutral Pion to Electron Positron Pair Forbidden?

[ruchika]

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

 

[humanino]

Is this a homework question ? Why not write "Please explain" ?

Measurement of the rare decay pi0 -> e+ e-

 

[blechman]

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]

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 ;-)

 

[Meir Achuz]

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]

it is second order, so it depends on what one means by "forbidden"

 

[humanino]

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).

 

[LongLiveYorke]

it is second order, so it depends on what one means by "forbidden"

Isn't it also helicity suppressed?

 

[malawi_glenn]

Isn't it also helicity suppressed?

yes, see the post above yours =)