# Neutral pion decay

1. May 15, 2009

### 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?

2. May 15, 2009

### kuon

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

I would say that it is.

3. May 15, 2009

### arivero

EM it is, but anomaly.

4. May 16, 2009

### 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

Last edited: May 16, 2009