We turn now to the hadrons, bound systems of quarks which interact by the strong interaction, as well as by the weak and electromagnetic interactions. In particular we shall describe the nucleons, that is to say, the proton and the neutron, the forces between nucleons, and the effect of the weak interaction on the stability of nucleons.

3.1 Properties of the proton and the neutron

Nucleons, like leptons, are fermions with spin 1/2 The mass of the neutron is 0.14% greater than that of the proton:

Thus the mass difference mn — mp = 1.29 MeV/c2 (≈ 2 electron masses). The neutron has no net electric charge. The proton has the opposite charge to the electron: protons are responsible for exactly cancelling the charge of the electrons in electrically neutral atoms. The electric charge on a proton is not concentrated at a point, but is symmetrically distributed about the centre of the proton. By the experimental methods to be discussed in Chapter 4, the mean radius Rp of this charge distribution is found to be Rp ≈ 0.8 fm. An extended charge distribution is also found in the neutron, positive charge in the central region being cancelled by negative charge at greater distances. The matter distribution in nucleons also extends to a distance of about Rp.

Both the proton and the neutron have a magnetic dipole moment, aligned with their spin:

Clearly neither magnetic moment is simply related to the value e ft/[2(nucleon mass)] expected from a simple Dirac equation, and this is a clear indication that the nucleons are not themselves fundamental particles. Compelling evidence that the nucleons are the ground states of a composite system is given by data of which that in Fig. 3.1 is an example.