# I Electron and positron annihilate

1. Jul 4, 2017

### dyn

Hi.
I am just starting to self-study particle physics. Came across the following in some notes -
Electron and positron annihilate to form a photon with zero momentum. I thought all photons had momentum due to the de Broglie equation ?
Also 2 electrons scatter to form a photon with zero energy. Again I can't see how this can be true.
Any help would be appreciated.
Thanks

2. Jul 4, 2017

### Orodruin

Staff Emeritus
Please provide proper references to where you have read this. Without proper references it is impossible to tell what the source has said and what you might have misunderstood from the source. In addition, we need to have more context to figure out what exactly you are looking at.

3. Jul 4, 2017

### dyn

I can't provide references. It came from some handwritten lecture notes. Is it possible for a photon to have zero momentum or energy ?

4. Jul 4, 2017

Staff Emeritus
Explain to me something I'm not going to show you is a hard question to answer.

5. Jul 4, 2017

### willem2

It's probably part of an explanation why the annihilation always produces 2 photons.

6. Jul 4, 2017

### Staff: Mentor

No. But without any context it is impossible to tell what exactly went wrong.

You could write the relevant part of the notes here in the forum.

7. Jul 4, 2017

### dyn

The Feynman diagram shows an electron and positron annihilating to form a photon which then decays into an electron and positron. The notes say " if an electron and a positron annihilate a photon is formed with zero charge , zero momentum and energy 2Ee and hence an apparent mass of 2Ee/c2 "

8. Jul 4, 2017

### Staff: Mentor

This is one of the two leading-order Feynman diagrams for electron-positron scattering (Bhabha scattering).

https://en.wikipedia.org/wiki/Bhabha_scattering

The photon is virtual, not real, so it need not obey the relationship $E^2 - (pc)^2 = (mc^2)^2 = 0$.

9. Jul 5, 2017

### vanhees71

If this was really stated in these lecture notes they are for sure not a "proper reference". So rather get a good textbook.