Quarks [theory of quark production in the collision of energetic particles and in cosmological conditions]

The paper gives a general account of the theories in which elementary particles are regarded as systems consisting of simpler 'sub-elementary' units. After a historical survey, the quark hypothesis, according to which such sub-elementary units have fractional charges, is expounded. It is shown how this hypothesis, in agreement with experiment, explains the structure of the families of strongly interacting particles known at present, including the family of the baryons, to which the proton and the neutron belong. The electromagnetic properties and the decay and scattering of particles are also shown to be satisfactorily explained. A review of the experimental search for free quarks, with accelerators, in cosmic rays and in ordinary matter is given. It is pointed out that this work has not yet led to any definite conclusions about the existence of quarks. The theory of quark production in the collision of energetic particles and in cosmological conditions is described.