Neutral Pion Decay: What Forbids EM Force?

[physguy09]

So I know the neutral pion decays as
$$\pi \rightarrow \gamma\gamma$$

my question is though, what forbids this from being an electromagnetic decay? I know it is not decaying via the strong force as there are no quarks resulting from decay. However, I do not see what forbids this from decaying via the EM force, if it is spitting out two gammas, which is the EM force carrier. Parity, G-parity, and Energy are conserved, as well as strangeness and angular momentum, so, what exactly forbids this from being EM?

 

[kuon]

What makes you think it isn't an electromagnetic decay?

I would say that it is.

 

[arivero]

EM it is, but anomaly.

 

[Hans de Vries]

It's not forbidden but it would be under conservation of
the axial current density $j_A^\mu$. (The latter is only conserved
together with the Chern Simons current of the EM field)

Regards, Hans