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Sakata, Shoichi

SAKATA, SHOICHI

(b. Tokyo. Japan, 18 January 1911; d. Nagoya, Japan, 16 October 1970)

particle physics.

Shoichi Sakata, the most important early collaborator of Hideki Yukawa on the meson theory of nuclear forces, and the founder of the Nagoya school of builders of theoretical models of elementary particles, was the eldest of the five sons and one daughter of Mikita Sakata and Tatsue Otsuka. His father was the son of a Buddhist priest from a small village on the Inland Sea near Hiroshima, but he was educated in Tokyo. At the time of Sakata’s birth, he was secretary to the prime minister of Japan, Taro Katsura, who became Sakata’s godfather. Mikita Sakata rose rapidly, being twice appointed governor of a prefecture, and in 1919 was elected mayor of Takamatsu, a city on Shikoku. After retiring from political life at the age of forty, he became a successful businessman in the Kobe area. Sakata’s mother was the daughter of Mitsugu Otsuka, who had served as governor of Ibaraki prefecture on Honshu.

Sakata spent three years in primary school in Takamatsu; the rest of his education, through high school, was obtained at Konan Gakuen (now Konan University) in Kobe, where he received an education that was both modern and informal. When he was sixteen, he joined an Esperanto club. There he met Tadashi Kato, who later translated Friedrich Engels’ book Dialectics of Nature. Kato’s influence, as well as Engels and Lenin’s Materialism and Empirio-Criticism, were, according to Sakata, lifelong guides to his thinking.

He entered the course in natural science in Konan High School, where one of his teachers was Bunsaku Arakatsu, a pioneer experimental nuclear physicist in Japan and later professor at Kyoto Imperial University. Sakata’s reading in high school included works on the history and philosophy of science by Max Planck, Henri Poincaré and Jun Ishiwara. He corresponded and talked with Ishiwara, who helped him decide to become a physicist.

After graduating from high school in 1929, Sakata audited the physics course at Tokyo Imperial University and came to know Yoshio Nishina, who had returned to Japan in 1928 after eight years abroad, six of them at the Niels Bohr Institute in Copenhagen. Sakata enrolled in the physics course at Kyoto Imperial University, attended Hideki Yukawa’s first course of lectures on quantum mechanics in 1932, and wrote a graduation thesis under Yukawa’s direction that dealt with Werner Heisenberg’s new theory of the atomic nucleus. After graduating from the university in 1933, Sakata spent a year in Nishina’s laboratory at the Institute for Physical and Chemical Research (Japanese acronym RIKEN) in Tokyo, where he collaborated with Nishina and Sin-itiro Tomonaga to produce his first scientific paper, on electron-positron pair production by gamma rays. Tomonaga, a graduate of Kyoto Imperial University who later became a Nobel laureate for his work on quantum field theory, had come to RIKEN in 1932. At RIKEN, Sakata began a lifelong association with another Kyoto graduate, Mituo Taketani.

In the early 1930’s a science faculty was established at Osaka Imperial University, whose president was the venerable physicist Hantaro Nagaoka. One of the new chairs of physics was filled by the outstanding nuclear experimentalist Seishi Kikuchi. In 1934 Yukawa was appointed lecturer in physics, and Sakata became his assistant. At the end of that year, Yukawa created his meson theory of nuclear forces, which brought him the Nobel Prize in 1949. The meson theory was applied by Yukawa and Sakata in 1935 to the process of nuclear transformation with the absorption of an atomic electron, the so-called K-capture process, later experimentally confirmed in the United States by Luis Alvarez.

The closeness of the collaboration between Sakata and Yukawa can be seen from the fact that of the fifteen papers by Sakata and the seventeen by Yukawa in the 1930’s, twelve of them bear both names. Among these joint papers are parts II-IV of the article “On the Interaction of Elementary Particles,” part I of which is Yukawa’s paper of February 1935, without collaborator, in which he proposed the existence of the meson. Part II was by Yukawa and Sakata; in part III they were joined by Taketani, who held the position of (unpaid) assistant at Osaka and Kyoto; in part IV, they added Minoru Kobayasi, who had been Yukawa’s student at Kyoto. When Sakata joined Yukawa at Osaka, Kobayasi replaced Sakata in the Nishina laboratory at RIKEN.

Prior to 1937 the meson theory had been applied only to the K-capture process, and no direct experimental confirmation of its existence had been forthcoming. But at the beginning of 1937, Yukawa tried to call attention (via a letter to Nature, which was rejected) to some “anomalous” cloud-chamber tracks of cosmic rays, observed by Carl Anderson and Seth H. Neddermeyer on Pike’s Peak, Colorado. The particles appeared to have masses between those of the electron and the proton. In the spring of 1937, Anderson and Neddermeyer identified those particles as of intermediate mass (hence the name meson), rather close to the mass Yukawa had predicted. That was the immediate stimulus for Yukawa and Sakata, joined by other theorists in Europe and the United States, to take up the theory again. The cosmic ray observations of Anderson and Neddermeyer were quickly confirmed by other groups in the United States and Japan.

Unpublished preliminary versions of parts II and III of the meson article indicate that they were worked on simultaneously. Yukawa’s first paper had been based on a meson field analogous to electromagnetism, and his meson (which he called U-quantum or heavy quantum) was to be the quantum of a field analogous to the electrostatic potential. That is similar to retaining only the Coulomb field and neglecting radiation, in the case of electromagnetism. Part II, written with Sakata, used the theory of the relativistic scalar field, developed in 1934 by Wolfgang Pauli and Victor Weisskopf, but not known to Yukawa when he wrote part I. Yukawa and Sakata derived both unlike-particle and like-particle exchange forces between nucleons (neutrons and protons) and calculated the collision probability of mesons with nuclei. They also speculated on the possible existence of a neutral meson.

Parts III and IV, which appeared in 1938, introduced the meson as a particle of spin I (vector meson) in a relativistic field theory of nuclear interaction. The revision gave a form of nuclear interaction that experiment seemed to require; it was also thought to be necessary to explain the anomalous magnetic moments of the proton and neutron. By this time, others than the Yukawa group were exploiting meson theory, including Nicholas Kemmer, Herbert Fröhlich, and Walter Heitler, who constructed a “charge-independent” vector meson theory that gave equal like- and unlike-particle forces. These three were physicists from the Continent, working in England, as was the Indian physicist Homi J. Bhabha, who pointed out that Yukawa’s meson, which functioned as an “intermediate boson” in beta decay, should on that account be radioactive and have a short lifetime. Ernst C. G. Stueckelberg worked actively on meson theory in Geneva, and Pauli, Heisenberg, and others were greatly interested.

The lifetime of charged mesons was treated in part III of the Yukawa group’s meson article, and a more accurate calculation was given in notes sent to the Physico-Mathematical Society of Japan and to Nature by Sakata and Yukawa. However, their lifetime did not agree with that of the cosmic ray meson, being a factor of about 100 times too short. Sakata also estimated, with Yasutaka Tanikawa, the lifetime of the hypothetical neutral meson.

By 1942, it had become clear that the cosmic ray meson (mesotron) did not, aside from its mass and chains, have the properties expected for the Yukawa nuclear force meson. In addition to the lifetime problem, there was no evidence that the cosmic ray meson had any nuclear interaction. As a result, some Japanese physicists began to suspect that the particle observed in cosmic rays was a secondary decay product of the Yukawa meson, the latter being produced in nuclear collisions occurring at higher altitudes.

On the basis of Yukawa’s unpublished laboratory diaries found in 1979, Michiji Konuma has concluded that the idea of a two-meson theory was first advanced in May 1942. According to Sakata, the first idea came from Tanikawa, in whose version both mesons had integral spin, and whose suggestion was inspired by the work of the Copenhagen physicists Christian Møller and Leon Rosenfeld, who pointed out that a “mixed-field” theory, containing both a vector meson and a pseudoscalar (that is odd parity, spin 0), meson would eliminate a troublesome mathematical singularity that occurs when only one type of meson is exchanged to make the nuclear force. The new theory was presented by Tanikawa and Seitaro Nakamura at a national discussion meeting held at RIKEN on 13 June 1942. At the same meeting, Sakata and Takeshi Inoue presented an alternative version in which the cosmic ray meson had half-integral spin. The Japanese text of their paper is in the Sakata Archives in Nagoya, and a paper with this content is in the Bulletin of the Physko-Mathematical Society of Japan.

In 1947, Robert Marshak and Hans Bethe proposed a different two-meson theory (not knowing of the Japanese work) in which the cosmic ray meson had zero spin and interacted only weakly with nuclei. They assumed that the meson first produced was a fermion of spin 1/2 (belonging to a theory of strong interaction that was not of the Yukawa type, but in which pairs of mesons were exchanged by nucleons). Unknown to them, a group studying cosmic ray tracks in photographic emulsion at the University of Bristol was at that time proving the existence of two mesons. In 1950, Sakata received the Imperial Award of the Japan Academy for the two-meson theory.

In 1939 Sakata married Nobuko Kakiuchi, the daughter of a biochemistry professor at the University of Tokyo, and became instructor at Kyoto Imperial University, where Yukawa had just been appointed to the chair of theoretical physics. Sakata was awarded the Ph.D. degree at Kyoto in 1941. In 1942, he was appointed professor at Nagoya University, and remained there until his death. Japanese academic physicists (who did not engage as inten sively in wartime military research as did their Western counterparts) had difficulty during the war and immediately afterward with scientific research and publication. During the war, the isolation from the scientific world outside Japan was complete, except for some scientific journals received via the Soviet Union. Between 1943 and 1946 there is a gap in Sakata’s publications corresponding to the suspension of scientific journals in Japan.

In March 1945, Sakata’s house was burned in an air raid. In April, the theoretical physics section of Nagoya Imperial University was evacuated to a primary school in Fujimi, located near Mt. Fuji, for safety and availability of food; there Sakata was visited by Taketani. In their retreat, the Nagoya group and Taketani considered the problem of the infinities encountered in quantum electrodynamics and in meson theory from the standpoint of Taketani’s methodology. Taketani argued that between the two traditionally recognized stages of development of a scientific theory—the stage of observational data and its generalization into provisional “laws” (which Taketani called phenomenological) and the stage of a formal mathematical theory (which he called essentialistic), there was an important intermediate stage (called substantialistic). Taketani supposed science to proceed in a spiral “dialectical” progression (in the Hegelian sense), passing through the three stages and emerging again in a new phenomenological stage. In the substantialistic stage, one identifies and characterizes the substances or particles that the final theory would be about. Taketani’s view on the importance of the substantialistic stage was strongly reinforced by the developments of the 1930’s, when the neutron, the positron, the neutrino, and Yukawa’s meson provided the answers to many nuclear puzzles.

The various attempts to remove the infinities of quantum field theory by arbitrary cutoff or subtraction techniques were classified by the group at Fujimi as phenomenological. Another type of quantum theory began by modifying the underlying classical electrodynamics to be nonlinear at small distances. Gustav Mie, and Max Born and Leopold Infeld, had proposed such theories. Fritz Bopp, on the other hand, had a theory that was both linear and finite. The Nagoya group considered it substantialistic, for they could show its equivalence to a theory in which the electromagnetic field was “mixed” with a neutral vector meson field, that is, a new substance.

Sakata attributed the cancellation of the infinite self-mass of the electron in Bopp’s theory to the mixing of the two fields. However, the vector meson field was not “realistic” because it had negative energy, so Sakata and Osamu Hara proposed to replace it with a neutral scalar field of positive energy. They named the new field “cohesive” and its quantum the “C meson,” since it provided the cohesive force necessary to overcome the infinite electrostatic Coulomb repulsion from which the infinite self-mass arose. (An identical theory was independently proposed by Abraham Pais in Utrecht.)

Tomonaga was interested in the Sakata-Hara result and tried to apply the C-meson hypothesis to obtain higher-order corrections to the elastic scattering of electrons; again he found it successful in removing the divergence. In so doing, he learned that the troublesome infinities could be isolated by a redefinition (renormalization) of the electron’s mass and charge; thus he was only a step away from the renormalization program for which he later shared the Nobel Prize. The mixed-field method was also used in a modified form by Pauli and F. Villars in their “regulator” method of charge renormalization.

In 1947, tracks in photographic emulsions exposed to cosmic rays at mountain altitudes clearly showed a charged particle (positive pion) decaying into another charged particle (positive muon). One could infer that an unobserved neutral particle (neutrino) was also a decay product. Further emulsion experiments showed that the muon decays into an electron and two neutrinos with the mean life of the cosmic ray meson. Negative pions brought to rest were usually captured by light nuclei, showing their strong interaction with nucleons, while many negative muons survived for a long time. Thus, the general picture envisaged by Sakata and Inoue in 1942 was largely confirmed.

Except for the muon, which behaved like a heavy electron and did not find a natural place in the apparent scheme of things, elementary particle physics seemed to have closed upon itself. No one suspected the astonishing multiplicity of unstable “elementary particles” that would soon appear, first in the cosmic rays and later at the large particle accelerators of the 1950’s. These particles participated in the strong nuclear interaction (as judged by their copious production in high-energy collisions), but some of them, christened “strange” by Murray Gell-Mann, had relatively long lifetimes, which showed that their decays occurred via an interaction that was as weak as beta decay. It was suggested that the “strange” characteristic might relate to a new quantum number that must be conserved in strong interactions (such as the production process), but not necessarily in weak interactions (such as the decay process).

But what was the nature of the new quantum number? In 1953, Kazuhiko Nishijima in Japan and Gell-Mann in the United States independently proposed identical schemes. There is a quantity called isospin that characterizes strongly interacting particles (hadrons) and that determines the number of charge states. For example, the nucleon (I=½) has two: the neutron and proton; the pion (I=1) has three: positive, negative, and neutral. The “strangeness” was proposed as a “displacement” of the average charge of a multiplet. Thus strong interactions, which conserve both charge and isospin, can produce only a set of particles whose displacements cancel each other.

To Sakata this scheme, while phenomenologically successful, appeared to be too abstract and positivistic. It reminded him of the arguments used in his student days to explain nuclear systematics (without the elementary neutron) and beta decay (without the neutrino). He wanted “strangeness” to be the property of a particular particle, the lambda hyperon. He proposed that hadrons were made up of three particles, which he took to be the neutron, proton, and lambda, and their antiparticles. (An example of the last is the antiproton, discovered in 1955, which has negative charge.)

In 1949, Enrico Fermi and Chen Ning Yang had modeled the pion as a very strongly bound anti-nucleon-nucleon system. Sakata reacted favorably to this suggestion, which corresponded to his idea of “logic of matter,” growing out of Taketani’s methodology. He assigned research problems based on Fermi-Yang; for example, he asked Shō Tanaka to see whether strangeness could be understood as the property of an excited state in a relativistic version of the model. Sakata spent the last half of 1954 at the Niels Bohr Institute in Copenhagen, and when he returned, his discussions with Tanaka and the rest of the group convinced him that strangeness could not be a dynamical property but must be a “substantialistic” one.

Thinking along these lines, Sakata wrote (in his research notebook of 1955) sets of analogies drawn from the history of atomic and nuclear physics. He started in May and reworked and enlarged them from month to month. One of the entries is marked “new law (Sept/Oct)”; it contains the trio P, N, V0 (V0 being the strange particle called lambda), and directly beneath this, a second trio: e±, ν, μ±. Next to it is the statement “The theory of structure is not to be found in field theory.” Since he had already written “A : strange neutron,” it is not surprising to find, a few pages later, that the muon is called “strange electron.”

Finding that the known hadrons could be accommodated in a generalized Fermi-Yang scheme with the A hyperon as the “strange” element, Sakata gave a talk in October 1955 at the annual meeting of the Japanese Physical Society and published his model in 1956. In the same year, he visited the Soviet Union and the People’s Republic of China and lectured on it.

The Sakata model was eventually replaced by the quark model, but in the interim it had powerful heuristic consequences. It gave a useful formula for hadronic masses, and it limited the type of hadronic decay possible via the weak interaction to that in which one of the “sakatons” (proton, neutron, lambda) changed into another. The model predicted a new meson, and the resulting octet of particles was identified in 1959 as a representation of the unitary symmetry group U(3) by Mineo Ikeda, Shuzo Ogawa, and Yoshiro Ohnuki (and independently by Yoshio Yamaguchi, and by Walter Thirring and Julius Wess). In 1964, Gell-Mann and Yuval Ne’eman proposed their “Eightfold Way” symmetry, in which the lowest-lying baryons were also an octet (not the Sakata triplet), and Gell-Mann and George Zweig, independently, proposed the fractionally charged quark triplet in 1964.

During the 1960’s, Sakata and his associates continued to develop the “logic of matter,” proposing in 1960, for example, a “unified model” in which the sakatons were to be made of the lepton triplet with the addition of positively charged “B-matter.” They generalized this, in 1962, to have two neutrinos, with the physical neutrino as a “mixed field.” In 1963, Sakata made his fundamental triplet “ur-baryons,” quarklike but without the fractional charge of the quarks that was proposed the following year. According to Yoichiro Nambu, in his popular book Quarks (1985), “It may not be an exaggeration to say that particle theory has been proceeding according to Sakata’s scenario.”

To summarize, Sakata’s main scientific achievements were work on the meson theory, including collaboration with Yukawa in the 1930’s; the two-meson theory, proposed in 1942 with Takeshi Inoue; the theory of the cohesive meson in 1946; and, perhaps most important, the composite model of hadrons (particles, such as mesons, that have strong nuclear interactions). Sakata’s composite model, which included the “strange particles” discovered in 1947; was the forerunner of the quark model.

In addition to his physics research, Sakata promoted science both nationally and internationally, and worked toward democratization of the university system in Japan after World War II. He also wrote on the history and philosophy of science and advocated Taketani’s “three-stage methodology” of science.

BIBLIOGRAPHY

I. Original Works. Almost all of Sakata’s scientific papers were written in English. They are collected in his Scientific Works (Tokyo, 1977), together with philosophical papers and reminiscences, originally written in Japanese but presented in English translation. Most of these translations are also available in Supplement of Progress of Theoretical Physics, no. 50 (1971). Unpublished Sakata papers are collected at the Sakata Archival Library. Department of Physics, Nagoya University.

II. Secondary Literature. Other material on the development of particle physics in Japan is in Shigeru Nakayama, David L. Swain, and Eri Yagi, eds., Science and Society in Modern Japan (Cambridge, Mass., 1974) : and in articles by Satio Hayakawa and Takehiko Takabayasi in Laurie M. Brown and Lillian Hoddeson, eds.. The Birth of Particle Physics (Cambridge, 1983).

Lauri M. Brown

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