Yukawa received his degree (Doctor of Science) from Osaka Imperial University on April 5, 1938. To further advance his research activities, in that year Yukawa started to take notes of what were discussed in the study meetings and seminars held in the laboratory. “Laboratory Journal – Theory Colloquia 1938” shown in OU1938-X1 was written at about that time. On its cover, Yukawa wrote “Laboratory of Theoretical Physics (Kyoto Imperial University)” and “Theory Colloquium Record (Osaka Imperial University 1938 – 1939 I.” The first half of the notes are records of 31 laboratory activities conducted from April 21 to December 23, 1938 when Yukawa was an associate professor at Osaka Imperial University. Yukawa became a professor of Kyoto Imperial University on May 26, 1939 and took a post in the Faculty of Physics, Division of Science, Kyoto Imperial University. The second half of the notes were written after he went to Kyoto Imperial University.

(The first three lines and “Osaka Imperial University,” “1939” and “I” on the lower section were added after Yukawa became a professor of Kyoto Imperial University.)

Record of the 8th Colloquium (page 13 in the journal):

The 8th Colloquium on May 19 (Thursday), 1938, in the cafeteria on the 1st floor (“Cafeteria” referred to the room in the Kikuchi Laboratory that was used as an eating room by the members of the group.)

The note says Shoichi Sakata explained the theory of beta decay. The beta decay is a phenomenon expressed by the following:

Neutron (N) -> Proton (P) + Electron (e) + Neutrino
He explained the beta decay by separating the process into the following two stages of disintegration [equation above (56)].

First stage: Disintegration of neutron (N) to proton (P) + U particle (U-)

Second stage: Disintegration of U particle (U-) to electron (e) + neutrino

The U particle (U-) is the meson (known today as pi-meson) proposed by Yukawa.

According to today’s understanding, the beta decay occurs due to the weak interaction (force). In 1938, however, the difference between the nuclear force and weak force was not known very well, and the pi-meson had not been discovered at that time. Yukawa explained that the first-stage process was triggered by the nuclear force and the second-stage process occurred due to the weak force. We now know that the weak force mediated by W boson directly causes the following disintegration:

Neutron (N) -> Proton (P) + Electron (e) + Neutrino

Yukawa and Sakata attempted to explain this phenomenon by trials and errors. Their activities eventually led to the fourth paper on the mesons co-authored by Yukawa, Sakata, Kobayashi and Taketani.

Written by Yutaka Hosotani (Osaka University)

“Laboratory Journal – Theory Colloquia 1938”