NEW YORK — Yoichiro Nambu, a particle physicist at The University of Chicago whose mathematical description of the phenomenon known as spontaneous symmetry breaking helped explain the interaction of subatomic particles, contributed to the prediction of the Higgs boson, or “God particle,” and earned him the Nobel Prize in Physics in 2008, died July 5 in Osaka, Japan. He was 94.
The University of Osaka, where he was a distinguished professor, announced his death Friday.
In the late 1950s, Dr. Nambu began investigating superconductivity, the process by which, at very low temperatures, electric current suddenly flows without any resistance.
He decided that spontaneous symmetry breaking, or SSB — the change from a symmetric to asymmetric state that scientists were just beginning to observe at the subatomic level — might better explain how substances become superconducting.
He developed a mathematical model to describe this phenomenon and quickly turned his attention to the world of subatomic particles. In 1960, he published a mathematical description of spontaneous symmetry breaking that became a cornerstone of the Standard Model, the unified theory that physicists use to explain three of the four fundamental forces in nature: strong, weak, and electromagnetic. The fourth, gravity, has not yet been incorporated into the standard model.
“SSB arises from a kind of group mentality, group psychology, among the constituents,” Dr. Nambu said in a speech in Chicago after winning the Nobel Prize. He offered an analogy.
When a group gathers in a large, open area, he explained, people usually look in a variety of directions. But sometimes, when one person begins looking in one direction, others in the crowd do likewise. “That’s a broken symmetry,” he said.
His theory helped explain, for example, how particles that carry the weak nuclear force obtain mass, a central insight in developing the unified theory of weak and electromagnetic forces.
Peter Higgs, François Englert, and others used his theories in the 1960s to predict the Higgs boson, which was discovered in 2012. In an interview with Physics World magazine in 2004, Higgs said, “Although my name gets thrown around in this context, it was Nambu who showed how fermion masses would be generated in a way that was analogous to the formation of the energy gap in a superconductor.”
Dr. Nambu shared the Nobel Prize in Physics with Makoto Kobayashi and Toshihide Maskawa, who were recognized for their work on quarks.
He received half the prize money. The rest was divided equally between his cowinners.
“There are scientists who set the direction in which the whole field moves,” Peter Freund, a retired professor of physics at The University of Chicago, said in an interview. “He was one of those.”
Yoichiro Nambu was born Jan. 18, 1921, in Tokyo and grew up in Fukui, northwest of Kyoto. His interest in science was inspired by science magazines that his father bought. “I liked physics because it was a way of studying fundamental problems in natural science,” he told The University of Chicago’s news service when he won the Nobel Prize.
He earned a master’s degree in physics at the Imperial University of Tokyo in 1942, when he was drafted into the army and assigned to a radar laboratory.
After the war, he married Chieko Hida, whom he leaves as well as their son, Jun-ichi, and a sister.
He returned to the Imperial University in 1946 as a researcher and earned his doctorate in 1952. From 1950 to 1956, he was a professor at the newly created Osaka City University. In that period he spent two years with nuclear physicist J. Robert Oppenheimer at the Institute for Advanced Study in Princeton, N.J., and, on one occasion, he summoned the courage to introduce himself to Albert Einstein, a faculty member at the institute.
His association with The University of Chicago began in 1954, when he was hired as a research associate. He became a professor in 1958 and from 1974 to 1977 he was chairman of the physics department. He retired in 1991.
His Nobel Prize struck many physicists as long overdue. “Part of it had to do with the fact that he was very modest and did not cheerlead for himself,” said Jeffrey Harvey, the Enrico Fermi distinguished service professor in physics at The University of Chicago. “In the real world, people campaign for the Nobel. He was not like that. He thought the work should speak for itself.”
Dr. Nambu continued working in particle physics for the rest of his career, making vital contributions.
In 1965, working with Moo-Young Han, now at Duke University, he developed the forerunner of the modern theory of quantum chromodynamics, which accounts for the nuclear forces that bind protons and neutrons into atomic nuclei.
At the time he received the Nobel, Dr. Nambu was exploring what he called “one of the last unsolved puzzles of the Standard Model”: why different quarks have different masses, possibly a result of spontaneous symmetry breaking.
“He was a magician,” Freund said. “He would pull one rabbit out of the hat and another, and then suddenly the rabbits would arrange themselves in a pattern and start dancing in a way you’d never seen before. Where he got the idea, you could never imagine.”