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Electron-positron collisions

  1. Mar 18, 2010 #1
    I'm trying to understand electron-positron collisions and I've got a few questions if you don't mind :)

    I know hadrons can be created through electron-positron annihilation. Can protons be produced like this? Has it been done before? How much energy would be required? How do you figure the amount of energy needed?

    Also, does annihilation occur due to scattering? I think I'm confused on that terminology.

  2. jcsd
  3. Mar 19, 2010 #2
    proton is a hadron
  4. Mar 19, 2010 #3
    I know, but I can't find any information on if it's been done before.
  5. Mar 20, 2010 #4
    in high e+e- collisions one produces quark- antiquark pais, which hadronizes. The probability that only a proton and an antiproton is produced is close to 0 since they consists together of 3 valance quarks and of equal flavour.
  6. Mar 20, 2010 #5
    Yes, protons are among the measured products in high energy electron-positron collisions. An internet search shows, for example, the DELPHI experiment at LEP that among other things measured identified hadrons (including protons):

  7. Mar 20, 2010 #6
    The leading factor in the cross section for electron-positron (pair) production by photons (electromagnetic) is

    σ0 = (1/137) (e2/mec2)2 = 0.58 x 10-27 cm2 = 0.58 millibarns

    where me is the electron mass, so by substuting the proton mass Mp, the cross section for pbar-p (proton anti-proton) production in electron positron collisions is probably down by 18362, so

    σ0 = ~2 x 10-34 cm2 = 200 picobarns for pbar-p production,

    at collision energies well above 2 GeV (threshold to produce one p and one p-bar) in the center of mass. Equation 37.3 in


    gives the transformation from the energy in the lab frame to the energy in the center of mass frame.

    Bob S

    [added] The above very crude pbar-p production cross section estimate is probably missing an extra factor of 1/137. Also see page 10 of


    for differential pbar-p cross section yields (units in picobarns) in the L3 CERN-LEP experiment.
    Last edited: Mar 21, 2010
  8. Mar 24, 2010 #7
    what is position of the electron in the both magnetic field and elect. fild will be parellel ?
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