What are Elementary Particles Made of?

Abstract

For many years scientists have been searching for the ultimate constituent of matter. At the turn of the century, the atom gave up its claim as the ultimate building block when atomic structure was understood. While we lost the heir apparent to the “fundamental building block” we got consolation prizes in terms of an understanding of the periodic system of elements and their chemical properties as well as understanding of atomic and molecular spectra. It then seemed that the photon, the electron, the proton, and the neutron had pretensions to the “throne.” However, there were vaguely disturbing features like the existence of the neutrino and postulation and the discovery of the pion and the unwelcome but persistent muon. One had not fully understood the role of these various particles as elementary constituent of matter. For many practical purposes there was a satisfactory phenomenological quantum theory of their production and interaction.