Charm into textbooks

Charm as a further quantum number was first introduced by J. Bjorken and S. Glashow with the aim of removing the asymmetry between quarks and leptons in a previous three-quark model. The following evolution of the elementary particle theory and of experiments confirmed the justification of the existence of the fourth quark and the classification of bound states of quarks within SU(4). The introduction of charm helped resolve a number of problems of the theory of elementary particles and their interactions. (Ha)