Yukawa's Heavy Quantum and the Mesotron (1935-1937)

Abstract

The unified “standard model’ of strong and weak nuclear forces of the mid-1930s was based on the emission and/or absorption of an electron and a neutrino, the so-called Fermi field, by the proton and the neutron. However, theorists could never explain simultaneously the required nuclear properties, such as saturation, strength, range, and charge independence of nuclear forces, while also accounting for β-decay. Yukawa proposed to exchange, instead of the Fermi field, the massive charged quanta of a new field which he invented, the U-field. His theory, however, languished for about two years. In this paper we describe this central episode in the history of elementary particle physics from Yukawa's proposal, published in 1935, to the year 1937 when particles resembling the quanta of the U-field were detected in the cosmic rays.