In order to formulate a fundamental quantum field theory of nuclear forces that explains their strength, range, and exchange character, while at the same time accounting for the weak β-decay interaction, Hideki Yukawa introduced a new kind of quantum field. In contrast to the real field of quantum electrodynamics (QED), which he took as his model, Yukawa's U-field was complex, and in contrast to the neutral massless photon of QED, the U-field's ‘heavy’ (i.e. massive) quanta were charged, carrying the electronic charge (positive and negative). The theory was proposed in November 1934 and published a few months later; however, its advantages were ignored, and for more than two years it went unnoticed, probably because there was no direct experimental evidence for the existence of U-quanta.
The Method of Unitary Clothing Transformations in Quantum Field Theory: Applications in the Theory of Nuclear Forces and Reactions