Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Pair Production

  1. Apr 6, 2009 #1
    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. jcsd
  3. Apr 6, 2009 #2
    I think it is correct but the probability of a photon/photon encounter is very small.
     
  4. Apr 6, 2009 #3

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Two separate processes.
     
  5. Apr 6, 2009 #4

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    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.
     
  6. Apr 6, 2009 #5

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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. [itex]e^+ + e^- \rightarrow e^+ + e^- + e^+ + e^- [/itex]. The momentum of each photon is "tagged" by measuring the recoil electron/positron. See Z.Phys.C30:545,1986 (as an example).
     
  7. Apr 6, 2009 #6
    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?
     
  8. Apr 6, 2009 #7
    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.
     
  9. Apr 6, 2009 #8

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    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.
     
  10. Apr 7, 2009 #9

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
     
  11. Apr 7, 2009 #10

    mathman

    User Avatar
    Science Advisor
    Gold Member

    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.
     
  12. Apr 8, 2009 #11
    Interesting but it is easy to think that it raises more difficult questions than it answer such as the perennial .......Where is the antimatter?
     
  13. Apr 8, 2009 #12

    mathman

    User Avatar
    Science Advisor
    Gold Member

    That is one of the unsolved problems of modern physics. Qualitatively it appears to be related to CP violation (matter and anti-matter behave differently), but a quantitative theory has not been found.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Pair Production
  1. Pair production (Replies: 2)

  2. Pair production (Replies: 11)

  3. Pair production (Replies: 12)

Loading...