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The main Feynman diagram I've seen corresponding to this decay is the following:

enter image description here

What I don't understand is why is it not possible for the up-antiup quarks that form the pion to annihilate into a single virtual photon, and then have that virtual photon decay to a positron-electron pair, similar to the annihilation of electron-positron pairs into a muon-antimuon pair in the following fashion?

enter image description here

EDIT: My question was closed and flagged as a duplicate of a question that did not answer what I asked so I repost it. I repeat, it is NOT a duplicate, I am not asking why the loop contribution is suppressed, I am asking why a different Feynman diagram is not acceptable.

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Charge conjugation, C, contrasts even and odd numbers of photons in states and amplitudes.

The $𝜋^0$ has C=+, but one photon has C=—, and hence two photons +.

By contrast, the $\rho^0$, with C=—, can and does couple to one photon, the heart of the Vector Dominance Model.

Note the Z has no well-defined C, as the weak interactions break C. Such a coupling to one Z exists, as the corresponding axial current of PCAC couples to the Z in its inimitable cockeyed way...

All charged fermions couple to an indefinite number of photons, in principle, of course. But the overall amplitude must preserve the C of the incoming and outgoing states, in QED which preserves C (unlike the weak interactions). The daughter $e^+e^-$ of the triangle diagram above are in an even C state, +; unlike the $e^+e^-$ that annihilate to one virtual photon, and then resolve to $\mu^+\mu^-$ in your latter diagram.

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