Shoichi Sakata

Sakata was born in Tokyo, Japan on January 18, 1911, to a family that held a tradition of public service. He was the eldest of six children of Tatsue Sakata and Mikita Sakata. At the time of Sakata's birth, Mikita was secretary to Prime Minister Katsura Tarō, who became Sakata's godfather. While attending Kōnan Middle School in Hyōgo Prefecture in 1924, Sakata was taught by the physicist Bunsaku Arakatsu. During his time as a student at Kōnan High School from 1926 to 1929, Sakata attended a lecture given by the influential physicist Jun Ishiwara. Sakata also became closely acquainted with Katō Tadashi, who would later co-translate Friedrich Engels's 1883 unfinished work Dialectics of Nature into Japanese. According to Sakata, Dialectics of Nature and Vladimir Lenin's 1909 work Materialism and Empirio-criticism became formative works for his thinking.[3][4]

Sakata got in to the Kyoto Imperial University in 1930. When he was a second year student, Yoshio Nishina, a granduncle-in-law of Sakata, gave a lecture on quantum mechanics at the Kyoto Imperial University. Sakata became acquainted with Hideki Yukawa and Shin'ichirō Tomonaga, the first and the second Japanese Nobel laureates, through the lecture. After graduating from the university, Sakata worked with Tomonaga and Nishina at Rikagaku Kenkyusho (RIKEN) in 1933 and moved to Osaka Imperial University in 1934 to work with Yukawa. Yukawa published his first paper on the meson theory in 1935 and Sakata closely collaborated with him for the developments of the meson theory. Possible existence of the neutral nuclear force carrier particle
π⁰
was postulated by them.[5] Accompanied by Yukawa, Sakata moved to Kyoto Imperial University as a lecturer in 1939.

Sakata and Inoue proposed their two-meson theory in 1942.[6] At the time, a charged particle discovered in the hard component cosmic rays was misidentified as the Yukawa's meson (
π^±
, nuclear force career particle). The misinterpretation led to puzzles in the discovered cosmic ray particle. Sakata and Inoue solved these puzzles by identifying the cosmic ray particle as a daughter charged fermion produced in the
π^±
decay. A new neutral fermion was also introduced to allow
π^±
decay into fermions.

We now know that these charged and neutral fermions correspond to the second generation leptons μ and
ν
_μ in the modern language. They then discussed the decay of the Yukawa particle,

π+

→

μ+

+ ν μ

Sakata and Inoue predicted correct spin assignment for the muon, and they also introduced the second neutrino. They treated it as a distinct particle from the beta decay neutrino, and anticipated correctly the three body decay of the muon. The English printing of Sakata-Inoue's two-meson theory paper was delayed until 1946,[7] one year before the experimental discovery of π → μν decay.

Sakata moved to Nagoya Imperial University as a professor in October 1942 and remained there until his death. The name of the university was changed to Nagoya University in October 1947 after the end of the Pacific War (1945). Sakata reorganized his research group in Nagoya to be administrated under the democracy principle after the War.

Sakata stayed at the Niels Bohr Institute from May to October 1954 at the invitation of N. Bohr and C. Møller. During his stay, Sakata gave a talk introducing works of young Japanese particle physics researchers, especially emphasizing an empirical relation found by Nakano and Nishijima,[8][9] which is now known as the Nakano-Nishijima-Gell-Mann (NNG) rule[8][9][10] among the strongly interacting particles (hadrons).

After Sakata returned to Nagoya, Sakata and his Nagoya group started researches trying to uncover the physics behind the NNG rule. Sakata then proposed his Sakata Model[11] in 1956, which explains the NNG rule by postulating the fundamental building blocks of all strongly interacting particles are the proton, the neutron and the lambda baryon. The positively charged pion is made out of a proton and an antineutron, in a manner similar to the Fermi-Yang composite Yukawa meson model,[12] while the positively charged kaon is composed of a proton and an anti-lambda, succeeding to explain the NNG rule in the Sakata model. Aside from the integer charges, the proton, neutron, and lambda have similar properties as the up quark, down quark, and strange quark respectively.

In 1959, Ikeda, Ogawa and Ohnuki[13][14] and, independently, Yamaguchi[15][16] found out the U(3) symmetry in the Sakata model. The U(3) symmetry provides a mathematical descriptions of hadrons in the eightfold way[17] idea (1961) of Murray Gell-Mann. Sakata's model was superseded by the quark model, proposed by Gell-Mann and George Zweig in 1964, which keeps the U(3) symmetry, but made the constituents fractionally charged and rejected the idea that they could be identified with observed particles. Still, within Japan, integer charged quark models parallel to Sakata's were used until the 1970s, and are still used as effective descriptions in certain domains.

Sakata's model was used in Harry J. Lipkin's book "Lie Groups for Pedestrians" (1965).[18] The Sakata model and its SU(3) symmetry were also explained in the textbook "Weak Interaction of Elementary Particles", L.B.Okun (1965).[19]

In 1959, Gamba, Marshak and Okubo[20] found Sakata's baryon triplet (proton, neutron and lambda baryon) bears striking similarity to the lepton triplet (neutrino, electron and muon) in the weak interaction aspects. In order to explain the physics behind this similarity in the composite model framework, in 1960, Sakata expanded his composite model to include leptons with his Nagoya University associates Maki, Nakagawa, and Ohnuki.[21] The expanded model was termed “Nagoya Model”. Shortly thereafter the existence of two kinds of neutrinos was experimentally confirmed. In 1962, Maki, Nakagawa and Sakata,[22] and also Katayama, Matumoto, Tanaka and Yamada[23] accommodated the two distinct types of neutrino into the composite model framework.

In his 1962 paper with Maki and Nakagawa, they used the Gell-Mann-Levy proposal of modified universality to define the weak mixing angle that later became known as Cabibbo angle; and extended it to the leptons, clearly distinguishing neutrino weak and mass eigenstates, thus defining the neutrino flavor mixing angle as well as predicting neutrino flavor oscillations. The neutrino flavor mixing matrix is now named Maki–Nakagawa–Sakata matrix. The nontrivial neutrino mixing as introduced in the Maki–Nakagawa–Sakata paper is now experimentally confirmed through the neutrino oscillation experiments.