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

Doubt about Bhabha scattering.

  1. Dec 23, 2007 #1
    I have one doubt about Bhabha scattering which is scattering between one electron and one positron, two oppositely charged particles. Why should they scatter each other away rather than attract and eventually annihilate themselves, giving rise to the pair annihilation process? (This process is dealth with in for example, Peskin-Scroeder's book on QFT). But I do not understand when they would annihilate and when they would scatter each other. Both are described in QED, by exchange of photon. But in one case there is attraction in the other, there is repulsion. How is that possible?
  2. jcsd
  3. Dec 23, 2007 #2


    User Avatar
    Homework Helper

    Firstly, let's make one thing clear: e- and e+ always "attract" each other for they carry opposite charge. A system with an e- and e+ is basically the positronium. "scattering" is just a term to say there is an interaction. Now from QED, there are two possible diagrams that contribute to this (ie. two diagrams with the same initial and final states): one is what you call the annihilation diagram and the other one is the "direct"/"standard" one (strictly speaking you can call it what you want). So when you wish to work out the amplitude/cross-section for this interaction, QED is telling you that there are two ways via which this can happen so you need to consider them both to get the total rate. Namely, e- and e+ go close to each other an interact via a virtual photon or they annihilate each other first then reappear again. NB: you final states e- and e+ will still be attracted to each other via the electromagnetic force. So interpreting that there is a repulsion may be a bit misleading for the "direct" case.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook