# Parity conservation in electromagnetic decays

When a $\pi^0$ decays into photons, the only possible number of photons to which it can decay is 2$\times n$, with $n$ a natural number. This is because in electromagnetic decays charge conjugation is conserved, let's assume that it decays into two photons

$$\pi^0 -> \gamma+\gamma$$

and:

C|$\pi^0$> = 1

C|$\gamma$> = -1

in this decay, parity also needs to be conserved, and

P|$\pi^0$> = -1

P|$\gamma$> = -1

the parity of the system of two photons is given by $(-1)^l$P|$\gamma$>P|$\gamma$> where $l$ is the orbital angular momentum of the system. Does this mean that if parity is conserved, the orbital angular momentum of the resulting system must be $l=1$ (or 3, 5...) or am I deducing something wrong?

• You are doing fine. In real life, you may confirm the l=1 directly by inspection of the explicit amplitude you find in QFT texts, calculated by J Steinberger in the late 40s. – Cosmas Zachos Sep 3 '18 at 5:09

## 1 Answer

You are deducing it right. Both C and P should be conserved in the decay, so to obtain a negative parity, since you have two identical paricles in the final state, you need the proper value of angular momentum l.