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Proton and anti-proton annihilation can produce extra pions

  1. Jun 15, 2005 #1
    Why do proton and anti-proton annihilation can produce extra pions, in addition to the photons; whereas the electron and positron interaction only gives photons?
  2. jcsd
  3. Jun 15, 2005 #2


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    Think about the rest masses before employing the machinery of Feynman diagrams.Apart from the neutrinos,the electron & its antiparticle are the lightest massive particles.

    Then you could employ Feynman calculus and assert that the proton is a composite particle and several quark processes take place,whivh could result in bound states like pions.

  4. Jun 15, 2005 #3
  5. Jun 15, 2005 #4


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    One has to consider the kinetic energy of the incident (mostly likely anti-proton) particle in order to understand the possible annihilation products.

    Annihilation of colliding positron-electron pairs will produce particles other than photons provided the kinetic energy is above certain thresholds.

    In a collision, the additional kinetic energy allows for other particles to be formed as demonstrated in the link provided by juvenal.
  6. Jun 15, 2005 #5
    Juvenal, it looks, there are lots of different possibilities relating to the outcome products. Are they all by chance or is there a mechanism involved?
  7. Jun 15, 2005 #6


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    The products will be determined by the total energy available and the other conservation laws.
  8. Jun 15, 2005 #7
    One calculates the differential cross section for a particular scattering process using the standard model. This essentially gives the probability distribution for that process.

    If the process is mostly QED - e+e- -> e+e-, mu+mu-, etc, then the cross section is well known. (In fact it's one of the first calculations one does in a particle physics class). If QCD is involved, it can get ugly.
  9. Jun 15, 2005 #8


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    Your question about proton / antiproton collisions mostly making pions, while electron / positron collisions mostly making photons deals with the fascinating subject of the difference betweeen "electromagnetic" showers and "hadronic" showers. The differences in rest mass is not the answer. You can accelerate electrons and positrons up to energies far in excess of the rest mass of the proton, and still the results of the collision will give fewer hadrons than a proton / antiproton collision at the same energy.

    This is a fact that is well known to people who read papers on cosmic rays because one can distinguish the primary particles in cosmic rays by looking at the ratio of hadronic (i.e. pions) to electromagnetic (i.e. photons, electrons and positrons) in their showers. I just finished an outlandish paper that touches on this subject:

    An intuitive explanation (that is wrong in that it ignores gluons and stuff) I've seen for the extra pions produced in hadronic interactions, is to say that at short distances, the quarks act like free particles. So when you have a collision between protons, the 6 quarks involved are very likely to end up split in ways that are not color neutral. For example, three quarks might go one way, one another way, and the other two in a third direction.

    There is no problem with such a collision at first, but if the debris is not color neutral, and since the color force increases with distance, the separating quarks cause the vacuum to make more quark antiquark pairs. So all those extra pions get extracted from the vacuum by the color force.

    Since leptons don't have color charge, their collisions don't make nearly as many pions.

    Last edited: Jun 15, 2005
  10. Jun 16, 2005 #9

    Meir Achuz

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    Protons and antiprotons have strong interactions with pions, but electrons and positrons do not. Most p-pbar annihilations do not produce photons, because that requires the weaker EM interaction.
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