Quarks for hadrons and leptons
J. Leite Lopes · 1974
The simplest, naive, model for a unified description of leptons and hadrons consists in postulating, besides the usual quarks p, n, h, a fourth quark, with very heavy mass and very high binding to pairs like pn and p3.. In a SU(4) scheme the fourth quark has a quantum number charm which may be taken as proportional to the lepton number. Muons would be distinguished from electrons by the occurrence of a i-quark instead of a n-quark in their structure. The forces among these quarks would have to be such as to give leptons an almost point-like structure at the experimentally known energies as well as absence of strong interactions at these energies. However, one would expect the display of strong interactions by leptons at extremely high energies.