Bruno Zumino, a physicist who proposed a theory called supersymmetry that promised to help tie together the fundamental laws of the universe but that has yet to be borne out in experiments, died June 21 at his home in Berkeley, Calif. He was 91.
His death was confirmed by the University of California, Berkeley, where he was an emeritus professor.
Dr. Zumino developed supersymmetry with Julius Wess, a colleague at the CERN particle physics laboratory in Switzerland. It predicted the discovery of elementary particles that have yet to be observed in particle collision experiments at the Large Hadron Collider at CERN.
Nonetheless, “supersymmetry is still an extremely attractive idea,” said Steven Weinberg, a professor of physics at the University of Texas.
Fundamental particles fall into one of two groups: fermions and bosons. The familiar constituents of matter like electrons, quarks and neutrinos are fermions. Those that carry fundamental forces like photons, which are particles of light, are bosons. In the Wess-Zumino model, the distinction between fermions and bosons is blurred, with every fermion having a boson “superpartner,” and vice versa.
The boson equivalent of an electron, for example, would be called a selectron. The superpartners would be much heavier than the known particles.
While supersymmetry doubles the number of particles, it simplifies the underlying equations, providing a way to unify three of nature’s fundamental forces: the strong nuclear force that holds atomic nuclei together, the weak nuclear force responsible for radioactive decay, and electromagnetism.
“The thing that was so attractive about it is that there’s essentially only one way to do it,” Dr. Weinberg said. “They weren’t just opening a huge variety of possibilities, which we would go looking for forever. There was a very specific possibility. It couldn’t be changed. It had to be that or nothing.”
Dr. Zumino and other physicists later expanded the equations to include gravity and Einstein’s theory of general relativity; that version was called supergravity.
Some physicists were hopeful that supersymmetric partners would be found in experiments at the Large Hadron Collider, pointing the way toward a new physics that could help fill gaps in the understanding of the universe.
The CERN experiments did turn up the long-sought Higgs boson, which explains why matter has mass, but there were no signs of the supersymmetric partners.
The partners may be so heavy that they could never be detected in particle accelerators. Or they might not exist at all.
“Right now, we’re in a quandary,” Dr. Weinberg said “We don’t know whether there is any truth to supersymmetry at all.”
Still, supersymmetric particles are a leading candidate for solving the riddle of dark matter, a component of the universe that is five times as plentiful as ordinary matter but still not yet identified.
Bruno Zumino was born in Rome on April 28, 1923. He received his doctorate from the University of Rome in 1945, joined New York University in 1951 and moved to CERN in 1968.
In 1981, he and his wife, Mary K. Gaillard, were both hired as physics professors at Berkeley. He retired in 1994.
Dr. Zumino was a member of the National Academy of Sciences.
He is survived by his wife and three stepchildren: Alain, Dominique and Bruno Gaillard.
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