# Homework Help: Particle Interaction Question

1. Nov 29, 2011

### GrantB

1. The problem statement, all variables and given/known data

Which of the following reactions are forbidden and why?

$\pi^{0}\rightarrow\gamma$

2. Relevant equations

None?

3. The attempt at a solution

I'm pretty sure that the decay doesn't occur (it's forbidden) but I don't know why. I know that it decays most of the time into two gammas.

I think the problem is that I don't understand what characteristics $\gamma$ (photon) has. As in, when I do other problems like this I look at the quark content and the flavors, compare Strangeness and Lepton numbers etc etc.

Thanks!

2. Nov 30, 2011

### fzero

You need to consider conservation of the spin quantum number.

3. Nov 30, 2011

### GrantB

So if the pion has s=0 but gamma has s=1, why does the pion decay to two gammas but not to one?

4. Nov 30, 2011

### fzero

What are the allowed values of total spin for a two photon system?

5. Nov 30, 2011

### GrantB

Okay I looked it up, and found that they combine so that the spin projection cancels and are spin-0.

Sorry I had to ask some menial stuff from you, my book doesn't spend any time going into detail about these things. There was no mention of the photons :[

Thanks for the help.