In this chapter we will consider the notion of partons, in the way Feynman introduced them. The parton model (PM) corresponds to a very clever application of the concepts behind the method of virtual quanta, which we described in Chapter 2. The theoretical reasons why the PM provides a relevant description of the hadronic constituents are, however, very complicated and this chapter only contains a first introduction.

The road to the PM goes through experiment. Over many years physicists have performed in various contexts a type of experiment which can be traced back to Rutherford. They have used a charged particle to extract information on the charge and mass structure of smaller and smaller constituents of matter. Rutherford made use of α-radiation on nuclear targets and very quickly made two essential observations.

He and his assistant were able to detect the scattering of the α-particles by direct observation of the flashes that they produced on a screen. They found, firstly, that most of the beam particles simply continued through the target as if it was empty of matter. But, secondly, every now and then they found quite an appreciable deviation.

It was Rutherford's genius that not only he did take his observations seriously but also used them to provide a description of the atom. We are going to consider his result, together with the necessary corrections due to relativity, spin and the internal structure of the target.