# Is there an intermediate 'particle' to pair production?

1. Jul 6, 2009

### Rymer

This question was brought to mind while reading other threads. I didn't think it was appropriate to diverge those threads off their subjects, so I started a new one. (hope this is right).

I'm specifically thinking about the extremely rare event of the interaction of two gamma rays as the
pair production event. It would seem that there should be an intermediate 'combined' particle since the event can't be instantaneous. Does this particle have a name? What would its properties have to be?
(Is it spin 2 like a graviton?)

This is not my field, just interested. Thank you.

2. Jul 6, 2009

### malawi_glenn

in terms of Feynman diagrams, the intermediate particles are virtual electrons/positrons, but in the REAL interaction (Feynman diagrams are just mathematical remnants when we do perturbation theory - I don't think that Nature DOES perturbation theory), just as middle steps in our calculations when we calculate work etc.

Now, ask yourself WHY the event CAN NOT be instantaneous, why this bias?

3. Jul 6, 2009

### Rymer

The process involves finite constrained interaction -- as all must to some degree -- it cannot be instantaneous. Uncertainty Principle I thought. So what are the properties of the intermediate?

4. Jul 6, 2009

### malawi_glenn

You have confused with what we can now with a statistical certainty and how nature works. Also, when we approach nature, we can only apply our models and see if they make sense, we can never go "down there" and see how these things occur with our own eyes. In the language of QFT with perturbation theory, I don't think it makes sense to ask how long time it took for the intermediate virtual particle to propagate etc.

Also, why is the forming of this intermediate state NON-instantaneous?

Last edited: Jul 6, 2009
5. Jul 6, 2009

### Bob_for_short

You can think of a superposition of positronium states (positronium is a neutral system which can have spin 2 in excited states).

Last edited: Jul 6, 2009
6. Jul 6, 2009

Why??

7. Jul 6, 2009

### Rymer

I have not calculated it -- so I guess I don't understand it.

To me instantaneous and infinitesimal seem to be mathematical conveniences -- not scientific possibilities.

8. Jul 6, 2009

### malawi_glenn

But you argued that 2gamma's -> 1electron + 1positron can not be a instantaneous process, but that it should go though an intermediate state [with spin 2?? why not spin 1 or 0? ;-) ]

thus
2gamma's -> intermediate state ->1electron + 1positron

why is this cancelling the "instantaneous" "problem"?

9. Jul 6, 2009

### Rymer

Don't know. Only mentioned spin 2 because I understood spin 0 would be disallowed and spin 1 would not balance.

I was wondering if this could be related to the pair production reported near super-massive objects in the galactic core.

10. Jul 6, 2009

### malawi_glenn

why is spin0 dissalowed and spin1 not "balance"? define "balance"

We, as far as I know, do not know so much of that ep-prodution yet, but what is related to it? The "intermediate step mechanicsm" or?? why should it?

11. Jul 6, 2009

### Rymer

No idea why ... that was why the question.

I don't know the basics of how this interaction occurs. Are these photons both spin-up or down?
How are they 'approaching' each other ... a 'head-on' or at an angle?

How is the interaction occurring?

12. Jul 6, 2009

### malawi_glenn

do you even know ANY interaction in quantum field theory? "how" they occur, as I wrote, it all depends on what model you are using.

You can, in the lowest order in perturbation theory, write it as 2 real photons, exhanging virtual electrons/positrons with a spectator nucleus, giving rise to 1 real electron and 1 real positron. Then we can derive angular- and spin dependent cross section (probability function that the process occur - recall that we are in the quantum realm).

13. Jul 6, 2009

### Rymer

I know very little about it -- but have read that there is another reaction -- far more rare -- that does not require the presence of a 'spectator nucleus'. It was that interaction to which I was referring.

14. Jul 6, 2009

### malawi_glenn

But where did you read it? Maybe if you show us we can help you more :-)

Saying things like "I have read/heard" is not so good :-(

15. Jul 6, 2009

### Rymer

More than one place -- sorry I had thought it was 'common knowledge' in the field. But since, I know little about this field, I guess I'm wrong.

I'll do some checking and track it down.

16. Jul 6, 2009

### malawi_glenn

Well since you said you had little knowledge about this field, you should not assume what is common knowledge and not, just an advice for the future ;-)

17. Jul 6, 2009

### Rymer

My knowledge -- what little there is of it -- is 40 years old.

A quick internet search gave a reference to: http://prola.aps.org/abstract/PR/v155/i5/p1404_1

It seems to be from 'my era' so it may be passe now.

18. Jul 6, 2009

### malawi_glenn

ok it should just be a virtual particle exchange of an electron or a positron in lowest order perturbation theory.