Can 1 photon become 2 pairs?

1. Aug 11, 2004

Gonzolo

Can 1 photon become 2 pairs? (Edited 17 Aug 2004)

Hi, what happens if the energy of a photon is more than that of an e- and an e+, can it create two pairs instead or will the 1st pair simply have more kinetic energy? Or are other types of particles involved?

Yeah, yeah I won't forget now, you need 2 photons to make an e+ e- pair because of C. of Momentum. A stupid drawing in an otherwise good book shows one only becoming a pair and has remained in my head like the Smurf's theme song. I thus translate the question to :

Can 2 photons become 2 pairs?

Or more generally :
Is <number of photons> / <number of pairs> necessarily 1?

Or even more generally, what happens if the energy of a single photon is much more than (1/2)mc2, where m is the mass of an electron?

Last edited by a moderator: Aug 17, 2004
2. Aug 11, 2004

jcsd

A photon can't decay by itself, otherwise four momentum will be violated, but it can decay if it inetracts with other particles.

3. Aug 11, 2004

Im preety shure it can only create one pair.

4. Aug 11, 2004

jcsd

It can create just about anything as long as the fundmantal symmetries (such as the conservation of four momentum) are adhered to.

5. Aug 11, 2004

meteor

It seems to me that a photon can decays spontaneously in a process called Parametric down-conversion, but this decay produces two photons, not a pair electron-positron

6. Aug 11, 2004

jcsd

AFAIK this process involves putting a photn throuh a crystal to produce two daugter photons.

7. Aug 11, 2004

Staff Emeritus
And since the magnitude of four-momentum is mc^2, and the photon is massless, its four-momentum has magnitude zero, while any massive particle will have a four-momentum greater than zero. Therefore conservation of four-momentum means the photon can't decay into massive particles.

Once learned, never forgotten.

8. Aug 11, 2004

Gonzolo

This is non-linear optics, it relates to what happens when many photons of "regular" (visible, IR etc.) energy and crystals are involved.

What I'm questioning about is what happens at the upper "end" (?) of the energy scale of a single photon.

What it creates depends on what particle it interacted with to change?

Assuming your are serious, I'm confused. It is well known that a photon can decay into an electron and a positron, which are massive.

9. Aug 12, 2004

jtolliver

Actually that takes 2 photons.

10. Aug 12, 2004

jcsd

As I've been trying to say:

1) an isolated photon cannot decay, if you notice I said this in the second post, and it was again re-iterated by self-adjoint.

2) it may decay if it interacts with other particles, provided that the fundmantal symmetries of nature are adhered to, for example the total chrage of the daughter particles must be zero, etc. these fundamenatal symmetries are the only limits on what the daughter particles will be.

11. Aug 12, 2004

ZapperZ

Staff Emeritus
Maybe, at this point, it requires an elementary illustration.

Let's say you have a photon with energy exactly equal to the rest mass energy of an electron and a positron. So in principle, this photon has enough energy for this pair production. But while conservation of energy is maintained, there's problem with conservation of linear momentum. The photon orginally had a well-defined momentum. But after the pair production, the there's enough energy to only created the pair, but with no kinetic energy. Thus, the net momentum of the pair system is zero. Violation of conservation of momentum.

That why it has been said a few times on here that pair production cannot occur in isolation (or in vacuum). It can only occur in the vacinity of a "massive" object, such as a solid crystal. This object provides the necessary momentum conservation for the pair production to occur. The typical scenario for pair production is by passing high energy photons through something like Be crystal (or something else, I don't quite remember this).

Zz.

12. Aug 12, 2004

Nereid

Staff Emeritus
Ditto with high energy gammas in space - whether interstellar space or intergalactic space. 0.51 MeV 'line emission' is observed by satellite observatories (e.g. Compton), from objects many Mpc distant.

What happens to gammas in the TeV to PeV range? There are several astrophysical processes which can generate copious quantities of them - can they travel through 100 Mpc between superclusters?

Last edited: Aug 12, 2004
13. Aug 17, 2004

ArmoSkater87

You would need for two energetic photons (gamma) to collide head on (to conserve momentum) in order to create an electron/positron pair. The opposite happens when the pair anihhilate...two gamma rays are emitted in opposite directions.

14. Aug 17, 2004

ArmoSkater87

Decays must always conserve momentum, in fact it led to the discovery of the neutrino. In beta decay of elements with unstable proton/neutron ratios, depending on whether the ratio is higher or lower than the stable ratio, the nucleus of the element will either decay its protons or neutrons.
Proton => neutron + positron + neutrino
Neutron => proton + electron + antineutrino.
When this decay was observed, it was noticed than the neutron and positron (or proton and electron) were emitted at an angle, therefore violating conservation of momentum, which lead people to believe than there was something else there that they were not detecting.

15. Aug 17, 2004

Gonzolo

See corrected question (original post).

16. Aug 17, 2004

Nereid

Staff Emeritus
If the photon's energy is > 2 x me (i.e. >1 MeV), pair production is possible. Interesting questions: under what conditions? How does the (photon->pair) rate vary with photon energy (all other things being equal)?

Last edited: Aug 17, 2004
17. Aug 17, 2004

Staff Emeritus
Here we go again. Energy conservation isn't enough for pair production; you also need conservation of momentum. That's relativistic 4-momentum of course since we're talking about photons here.

Now the magnitude of a particle's 4-momentum is mc^2. The photon's mass is zero so its 4-momentum has zero magnitude; that's where we get all those null trajectories and so on for the photon.

But the mc^2 value for the electron is greater than zero, because it has a finite mass.

So we have impossible physics; zero magnitude momentum on one side and non-zero on the other side. Conclusion: the photon cannot spontaneously decay into massive particles. No go.

If it makes you feel better I made this same mistake about a year ago and got corrected then. Pass it on.

18. Aug 17, 2004

Nereid

Staff Emeritus
Clearly, photons of energy > 1MeV aren't like neutrinos - they can't go through a light-year of lead without being absorbed

If such a gamma enters a region with matter - a fully ionised plasma, a gas, a liquid, a solid, degenerate matter - can it undergo an interaction from which an electron/positron pair emerges? In what way does the target affect the pair production rate?

19. Aug 17, 2004

what_are_electrons

Based on my understanding, Yes, That's it. The target is the source of the pair.

20. Aug 17, 2004