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A neutral pion decays into two gamma-pho...

A neutral pion decays into two gamma-photons:
`pi^(0)togamma+gamma`
Why cannot a single photon be born? What conservation law is in contradiction with it? What is the energy of the photon?

Text Solution

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The correct Answer is:
It contradicts the law of conservation of momentum ; 67.5 MeV.
Let the pi-meson be at rest in some reference frame. Then its momentum is zero. If a pi-meson decays into two photons, they will fly in opposite directions, so that in this reference frame the combined momentum will be zero although each photon separately has momentum. The transformation of a pion into a single photon is impossible, since it contravenes the law of conservation of momentum.
The energy of each photon is `epsi_(gamma)=135//2=67.5MeV`.
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