Proton-Antiproton Collisions

If collisions occur by the exchange of a single intermediary particle of zero mass between point particles, the $q^{-4}$ dependence of the collision probability on momentum transfer will occur in proton-antiproton collisions as in the Geiger-Marsden experiment. However, if the colliding particles are not point particles, a form factor which decreases for increasing momentum transfer will occur as with the Hofstadter experiments.

When collisions between protons and antiprotons of a few hundred GeV are arranged, certain types of events called two jet events are recorded. In these events, two jets, each containing many particles, are emitted in opposite directions at wide angles (i. e., with large momentum transfer) from the colliding beams. Furthermore, these jets show a probability distribution as a function of momentum transfer very close to $q^{-4}$. This indicates that the colliding particles are point-like, at least down to the minimum spatial resolutions available to today's accelerators.

Figure 18.8: Illustration of what happens in a high energy collision between a proton and an antiproton according to the Bjorken-Feynman parton model.

According to the Bjorken-Feynman parton model of the proton, the collision between highly energetic protons and antiprotons should operate as shown in figure 18.8. The actual collision is between individual partons. Figure 18.8 illustrates the collision between a quark in the proton and an antiquark in the antiproton. The result of this interaction is the scattering of these particles out of the incident particles, resulting ultimately in a two jet event as described above.