Yukawa’s Prediction of the Meson

In October 1934, at a meeting of the Osaka Branch of the PhysicoMathematical Society of Japan, Hideki Yukawa proposed a new theory of nuclear forces involving the exchange between neutron and proton of an electrically charged “heavy quantum”.l Yukawa’s theory is known today as the meson theory of nuclear forces and his “heavy quantum” is called the meson.2 The theory provided a fundamental explanation for the charge-exchange nuclear force that Werner Heisenberg had proposed in 1932 and had associated with the exchange of an electron between a neutron and a p r ~ t o n . ~ In the same paper Yukawa suggested an alternative form of Enrico Fermi’s betadecay theory and showed that the two forms were phenomenologically equivalent in the treatment of nuclear beta decay, although Yukawa used the meson as an intermediary in beta decay and clearly distinguished the “strong” nuclear binding force from the “weak” force responsible for radioactivebeta d e ~ a y . ~ * ~ Yukawa’s meson thus played a dual role: it carried energy, momentum, and electric charge between neutron and proton, producing a strong nuclear force, and it decayed weakly (i.e., with small probability) into electron and antineutrino, or positron and neutrino, providing a mechanism for nuclear beta decay. Yukawa showed that mesons would never be emitted in ordinary nuclear transformations (radioactive decays and low-energy nuclear reactions), but that they could materialize in nuclear processes involving sufficient energy release, such as those occurring in the interactions of cosmic rays. Produced this way, the mesons would either be absorbed by matter, or they would themselves decay by the beta-decay process.