Archive of historical materials
On the Theory of the New Particle in Cosmic Ray
OU1937-C3 (4 pages) Date: October 4, 1937
Yukawa, Shoichi Sakata and Mitsuo Taketani sent their co-authored paper to Physical Review Letters, the journal published by the American Physical Society. It arrived at the office of Physical Review Letters on October 22.
In 1936, new particles with the mass of halfway between the mass of an electron and that of a proton were discovered during the observation of cosmic rays, but they could not be identified at that time. The particles observed in the cosmic rays were later determined as muons, but Yukawa et. al. formulated a hypothesis that the new particles found in the cosmic rays were the same as the particles (called U particles) responsible for the nuclear force that he proposed in 1934 and attempted to discern their properties. Based on the examination of both the cosmic ray observation results and the nuclear force, they estimated the mass of the new particle to be about 200 times the mass of an electron. Because it was assumed that the U particle had either a positive or negative charge, it was possible to explain that the interchange of proton and neutron causes the nuclear force; however, Yukawa et. al. pointed out that the predicted force between protons or between neutrons was only about 1/10 of the observed force. On page 2, they commented that a new neutral particle seemed to be necessary for proper explanation. Modern physics tells us that there are uncharged pi-mesons (neutral pi-mesons) in addition to positively charged pi-mesons. When writing this paper, Yukawa et. al. believed that pi-mesons appeared as triplets. It was an important development in physical theories.
In the paper, Yukawa et. al. began explaining that the anomalous magnetic moments of the protons and neutrons resulted from the change of a proton (or neutron) to a neutron (or proton) and a U particle due to the quantum effect. It was a very novel idea, but that explanation was incomplete. Today, it is known that the fact that the proton and neutron being composite particles are made up of three quarks is important consideration. Yukawa et. al. did not know that back then. Yukawa et. al.’s idea of using the quantum effect for explaining the “anomalous” characteristic became a model in elementary particle physics. Yukawa et. al. also assessed the energy loss in cosmic rays and concluded that the issues concerning the atomic nucleus and the issues related to cosmic rays were intertwined. This paper was never published in Physical Review. Also, see the explanation in OU1937-C2. (Written by Yutaka Hosotani)
