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1.8 Electron scattering from nucleons
There is a clear advantage in using electrons to probe The proton and neutron, since electrons interact with quarks primarily through electromagnetic forces which are well understood; the weak interaction is negligible in the scattering process, except at very high energy and large scattering angle, and the strong interaction is not directly involved.
In the 1950s, experiments at Stanford on nucleon targets at rest in the laboratory revealed the electric charge distribution in the proton and (using scattering data from deuterium targets) the neutron. These early experiments were performed at electron energies 500 MeV (Hofstadter et al., 1958). Scattering at higher energies has thrown more light on the behaviour of quarks in the proton. At these energies inelastic electron scattering, which involves meson production, becomes the dominant mode.
At the electron-proton collider HERA at Hamburg, a beam of 30 GeV electrons meets a beam of 820 GeV protons head on. Many features of the ensuing electron-proton collisions are well described by the parton model, which was introduced by Feynman in 1969. In the parton model each proton in the beam is regarded as a system of sub-particles called partons. These are quarks, antiquarks and gluons. Quarks and antiquarks are the particles which carry electric charge. The basic idea of the parton model is that at high energy-momentum transfer Q2, an electron scatters from an effectively free quark or antiquark and the scattering process is completed before the recoiling quark or antiquark has time to interact with its environment of quarks, antiquarks and gluons. Thus in the calculation of the inclusive cross-section the final hadronic states do not appear.
Figure 1.10. This figure, illustrating particle tracks, is taken from an event in the ZEUS detector at HERA, DESY. Figure 1.10(a) is the event projected onto a plane perpendicular to the axis of the beams. Figure 1.10(b) is the event projected onto a plane passing through the axis of the beams.
A hadron jet has been ejected from the proton by an electron. The track of the recoiling electron is marked e. The initiating beams and the proton remnant jet are confined to the beam pipes and are not detected.
In the model, at large Q2 both the electron and the struck quark are deflected through large angles. Figure 1.10 shows an example of an event from the ZEUS detector at HERA. The transverse momentum of the scattered electron is balanced by a jet of hadrons which can be associated with the recoiling quark. Another jet, the ``proton remnant'' jet is confined to small angles with respect to the proton beam. Events like these give further strong support to the parton model.
The success of the parton model in interpreting the data gives added support to the concept of quarks. The parton model is not strictly part of our main theme, but in view of its interest, and importance in particle physics, a simple account of the model and its relation to experiment is given in Appendix D.