The deep inelastic collision

In the collision between an electron from the accelerator and a proton in the target, the particles affect each other through exchange of a light quantum, a photon. The wavelength of this photon determines the resolution of the "electron microscope". In this connection, scientists speak of "elastic" and "inelastic" collisions*. Inelastic collision between an electron and a proton is illustrated in figure a.
     When the wavelength of the photon is long, it "sees" the charge of the whole proton (elastic or slightly inelastic collisions), but when the wavelength becomes sufficiently short, the photon "sees" primarily any charged small constituents within the proton (deep inelastic collisions). The performance of the new accelerator, 21 GeV, guaranteed that the electrons could emit short-wave photons, but a condition for the exchange of such photons was that there should be some small "hard" receiver within the protons.
     The result of the experiment showed that when the photon wavelength is short, i.e. its momentum is great, the proton is generally smashed to pieces and a number of particles are produced (figure a). Thus there are suitable receivers! That the event in figure a looks so complex is partly because of the complicated structure of the proton and partly because its quarks must be converted into ordinary particles. The deep inelastic collision in figure a is basically a simple elastic collision between the electron and a quark (figure b).

*One way to understand these terms is to make a comparison with the collision between billiard balls. An elastic collision means that the collision does not damage either of the balls, while an inelastic collision damages one or both balls permanently.