# Electron-pozitron annihilation, conservation of spin

1. Aug 21, 2009

### Naake

Hello,
I have one question. The most probability annihilation is e+ + e- -> y + y with spins -1/2+1/2=-1+1. But I heard, so, there exist with small probability other combinations of spin. yy is general spin-2 and e+e- is spin-1 and now there is something purportedly with orbitals s, p, d, but I don´t know what, and its my qustion.

2. Aug 21, 2009

### Norman

I think you really need to read up on spin. First of all $$e^+ + e^- \rightarrow 2 \gamma \text{ or } 3 \gamma$$ has either 0 or 1 total spin initially (due to spin statistics). Therefore, if they are in the singlet state initially (S=0) they can go to the $$2\gamma$$ final state. If they are in the triplet state initially (S=1) they can proceed to the $$3 \gamma$$ final state.

3. Aug 22, 2009

### Bob S

Hi Naake-
As pointed out by Norman, positronium can have the two fermions in either a triplet state or singlet state. Their lifetimes are triplet state (142 nanosecond lifetime, 3 photon decay) or singlet state (125 picosecond lifetime, 2 photon decay). The probability ratio of triplet to singlet states is about 3:1.
Bob S