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

I electron and positron annihilate

  1. Jul 4, 2017 #1

    dyn

    User Avatar

    Hi.
    I am just starting to self-study particle physics. Came across the following in some notes -
    Electron and positron annihilate to form a photon with zero momentum. I thought all photons had momentum due to the de Broglie equation ?
    Also 2 electrons scatter to form a photon with zero energy. Again I can't see how this can be true.
    Any help would be appreciated.
    Thanks
     
  2. jcsd
  3. Jul 4, 2017 #2

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Please provide proper references to where you have read this. Without proper references it is impossible to tell what the source has said and what you might have misunderstood from the source. In addition, we need to have more context to figure out what exactly you are looking at.
     
  4. Jul 4, 2017 #3

    dyn

    User Avatar

    I can't provide references. It came from some handwritten lecture notes. Is it possible for a photon to have zero momentum or energy ?
     
  5. Jul 4, 2017 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Explain to me something I'm not going to show you is a hard question to answer.
     
  6. Jul 4, 2017 #5
    It's probably part of an explanation why the annihilation always produces 2 photons.
     
  7. Jul 4, 2017 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    No. But without any context it is impossible to tell what exactly went wrong.

    You could write the relevant part of the notes here in the forum.
     
  8. Jul 4, 2017 #7

    dyn

    User Avatar

    The Feynman diagram shows an electron and positron annihilating to form a photon which then decays into an electron and positron. The notes say " if an electron and a positron annihilate a photon is formed with zero charge , zero momentum and energy 2Ee and hence an apparent mass of 2Ee/c2 "
     
  9. Jul 4, 2017 #8

    jtbell

    User Avatar

    Staff: Mentor

    This is one of the two leading-order Feynman diagrams for electron-positron scattering (Bhabha scattering).

    https://en.wikipedia.org/wiki/Bhabha_scattering

    The photon is virtual, not real, so it need not obey the relationship ##E^2 - (pc)^2 = (mc^2)^2 = 0##.
     
  10. Jul 5, 2017 #9

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    If this was really stated in these lecture notes they are for sure not a "proper reference". So rather get a good textbook.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: electron and positron annihilate
Loading...