# Pair Production

1. Apr 6, 2009

### Zman

In Pair Production where a high energy photon collides with a nucleus a positron and an electron may result.
But I have also come across references that say that two high energy photons can collide with each other to produce an electron and a positron.
Is this correct?

2. Apr 6, 2009

I think it is correct but the probability of a photon/photon encounter is very small.

3. Apr 6, 2009

Staff Emeritus
Two separate processes.

4. Apr 6, 2009

### ZapperZ

Staff Emeritus
Just in case it isn't clear, the gamma-gamma collision so far is only a prediction of QED and has not been shown yet experimentally, whereas the pair production from a single photon near a nucleus is a common occurrence.

Zz.

5. Apr 6, 2009

Staff Emeritus
I don't think that's true. You have tagged two-photon events: a colliding beam of electrons and positrons where each radiates a photon and the photons merge: e.g. $e^+ + e^- \rightarrow e^+ + e^- + e^+ + e^-$. The momentum of each photon is "tagged" by measuring the recoil electron/positron. See Z.Phys.C30:545,1986 (as an example).

6. Apr 6, 2009

### Zman

From the ref Z.Phys.C30:545,1986 it seems that there is good evidence for photon-photon collisions.
Then surely low energy photons can also collide. If they don’t produce particles do they just deflect off each other? If they deflect off each other to any extent, wouldn’t that produce a lot of noise in vision?

Does this visual noise exist?

7. Apr 6, 2009

### hamster143

If you want to produce an electron and a positron, input photons must carry enough energy (511 kev each). That's far beyond visible light.

Low energy photons can scatter off each other too, but, firstly, that's a higher-order process, secondly, the likelihood of interaction goes down as the sixth power of energy, and it's so tiny at visible light energies that it's almost impossible to observe, even with high-powered lasers.

8. Apr 6, 2009

### ZapperZ

Staff Emeritus
Oops.. you are right. I forgot about the stuff that was done at TESLA.

I suppose I was actually looking for a direct gamma-gamma collision rather than by photons that were produced internally at the interaction point itself. What I had in mind was more along what was published by J. Gronberg Nuc. Phys. B v.126, p.375 (2004).

Oh well...

Zz.

9. Apr 7, 2009

Staff Emeritus
As you know, it is much easier to produce a high brightness electron beam (at least at GeV energies) than a high brightness photon beam. So the easiest way to get at this kind of physics is to make the photons just before you need them: at the collision point. Also, there are other reasons to do e+ e- collisions, so this measurement ends up being a bonus on top of the mainline physics program.

Indeed, the JADE experiment was more interested in discovering the gluon.

10. Apr 7, 2009

### mathman

Although photon-photon collisions are quite rare these days, immediately after the big bang they were quite common, being responsible for the creation of matter.

11. Apr 8, 2009