# Pair production: Has it been observed experimentally?

1. Jul 20, 2011

### damianpaz

I was reading about electron-positron annihilation and I got to the part where it says that this is a reversible process, meaning that an electron and a positron could be created from photons.

About this reverse process the article says "In nuclear physics, this occurs when a high-energy photon interacts with a nucleus".

But that doesn't sound like the reverse of annihilating an electron and a positron to produce photons.

Has ever been observed in the laboratory the creation of a "pair" only from photons?

If no, why not? If yes any source to the experiment would be appreciated.

2. Jul 20, 2011

### triclon

I work with gamma ray astronomers and they see it all the time, in fact it creates an annoying background when trying to observe very high energy gamma rays from space. Basically a cosmic ray, a proton or atomic nucleus moving near the speed of light through space strikes the upper atmosphere. It his the nucleus of one one of the atoms in our atmosphere and leads to pair production of an electron and positron (and perhaps other particles as well) which then in turn strike other nuclei creating a shower of electrons and positrons. Even more cool is that these particle showers created by cosmic rays they think seed clouds in thunderstorms with strong enough electrical fields to trigger lightning. So basically:

particle + nucleus --> electron + positron + nucleus

You need the nucleus to conserve energy and momentum, although only it's motion is altered in this reaction usually (ie. it's momentum and kenetic energy).

3. Jul 20, 2011

### ZapperZ

Staff Emeritus
This is so well-known and verified that they are being used as positron sources for accelerators. The ILC design used this concept, for example.

Here's an example of the same pair production principle being used to produced polarized positrons:

http://www.slac.stanford.edu/exp/e166/

Zz.

4. Jul 20, 2011

### LostConjugate

So WAIT.

Fermions are made up of bosons? Why didn't anyone say so?

5. Jul 20, 2011

### Drakkith

Staff Emeritus
Sir you must be joking!

6. Jul 20, 2011

### LostConjugate

Your saying that you can make electrons out of photons and you can make photons out of electrons?

7. Jul 20, 2011

### ZapperZ

Staff Emeritus
Huh?

Pair production is electron-positron pairs out of photons, with some momentum transfer! That's why they passed gamma photon through high-Z crystals to get e-p pairs! No one said anything about fermions being made out of bosons!

Zz.

8. Jul 20, 2011

### LostConjugate

The reverse of this is annihilating a photon to produce electrons and positrons.

Once again mention of fermions (electrons / positrons) being created from bosons (photons).

Now your response to these statements was:

This is so well-known and verified

It is so well-known and verified that electrons and positrons are created from photons? This is new to me.

Last edited: Jul 20, 2011
9. Jul 20, 2011

### ZapperZ

Staff Emeritus
Could you show where, for example, in that experiment that I cited that they "annihilate" a photon?

Yes, but created using something does NOT mean that fermions are MADE of it! There are EXTRA INGREDIENTS required beyond JUST photons (momentum transfer). And how many bosons can you add and subtract to give you a net spin of 1/2?

Zz.

10. Jul 20, 2011

### LostConjugate

I just made that up to keep in reference with the "opposite" of. I don't really know what they do.

What in the world does that mean?

I understand that positrons / electrons are permutations in an electron field and photons are obviously permutations in the electromagnetic field. Two different things entirely.

I also understand that an electron and a proton can become a neutron in the construction of a neutron star, however both those particles are bosons.

11. Jul 20, 2011

### ZapperZ

Staff Emeritus

I converted $4 today into a burrito. You're saying that my burrito is made up of$4 bills?

Zz.

12. Jul 20, 2011

### LostConjugate

Zapper your the one that replied to this post and stated that it has been verified. This post is speaking of annihilation of photon, creating electrons and positrons from photons, and the creation of a "pair" ONLY from photons.

I have never heard of such a thing, so I am confused.

13. Jul 20, 2011

### ZapperZ

Staff Emeritus
I have no idea what you read in that post that caused you to conclude that fermions are made of bosons.

The "reverse" means that it must be photon-photon annihilation? How about reverse as in "run the movie backwards"?

Zz.

14. Jul 20, 2011

### LostConjugate

meaning that an electron[fermion] and a positron could be created from photons[boson]

15. Jul 20, 2011

### ZapperZ

Staff Emeritus
... and other stuff, such as a momentum transfer.

But is an electron made up of photons? Nope! Nothing in the conversion process implies that one is made up of the other.

Zz.

16. Jul 20, 2011

### LostConjugate

Ok now we are on the right path.

What do you mean by momentum transfer, certainly electrons are not made up of photons and momentum. What other stuff?

17. Jul 20, 2011

### Drakkith

Staff Emeritus
No, he means that electrons can be created by momentum transfer. For example, colliding two protons together at high energies results in the creation of many particles, including electrons, positrons, photons, ETC.

18. Jul 20, 2011

### LostConjugate

Where to they come from?

19. Jul 20, 2011

### vkroom

I think we are messing up something here.

$\gamma \rightarrow e^- + e^+$ is mass creation from energy: $E=m_0 c^2 + p^2c^2$. If the photons have less energy than twice the rest mass of electrons this creation process will not occur except maybe as vacuum fluctuations.

20. Jul 20, 2011

### Drakkith

Staff Emeritus
The energy of the collisions!