Bruno Zumino

Bruno Zumino, a professor emeritus of physics at the University of California, Berkeley, who was best known for developing supersymmetry, a theory now considered as a leading candidate for explaining the fundamental forces of nature, died Sunday, June 22, at his home in Berkeley, Calif. He was 91.

Supersymmetry or SUSY, developed in the early 1970s at the European Center for Nuclear Research (CERN) in Geneva, Switzerland, by Zumino and Julius Wess, was conceived to explain particle interactions involving three of the four main forces in nature – the strong, electromagnetic and weak forces. One consequence of the theory is that every particle we see today has a supersymmetric partner - the quark has an associated squark, for example, while the electron has a selectron.

Zumino and Stanley Deser, and separately Dan Freedman, Sergio Ferrara and Peter Van Nieuwenhuizen, later extended the so-called “Wess-Zumino model” of supersymmetry to include gravity, creating a theory called supergravity.

To date, none of these superpartners has been detected, though CERN’s Large Hadron Collider, which in 2012 produced evidence for the Higgs boson, a particle that endows the rest of matter with mass, is now looking for heavier particles that would be evidence of supersymmetry. Scientists even hold out hope that one of the superpartners will be the elusive dark matter particle that, despite making up one-quarter of the mass of the universe, so far has gone undetected.

“Supersymmetry is so beautiful and suggestive that most of us think it has got to show up sometime in nature, although so far it hasn’t,” said Zumino’s friend and colleague Steven Weinberg, a physics professor at the University of Texas, Austin, the 1979 Nobel Laureate in physics and the author of a major treatise on supersymmetry.

“Bruno had an uncanny ability to work on theories important both for their mathematical structure and for their connection to experiments,” said theoretician Lawrence Hall, UC Berkeley professor of physics. “When he introduced supersymmetry, he could have had no idea that 40 years later there would be a laboratory at CERN with more than 4,000 people looking for evidence of the theory.”

Symmetries in nature

Zumino was one of the first theoreticians in the 1960s and ‘70s to focus on symmetries in nature and what they might tell us about the fundamental laws of nature. Symmetries are all around us. Circles are rotationally symmetric because they look the same when rotated. Physical laws, according to Einstein’s theory of special relativity, are the same whether you are standing still or moving at a constant velocity. Zumino was seeking a mathematical description of forces that would improve on the Standard Model, which precisely describes strong and electromagnetic interactions, by including weak interactions like those involved in the decay of radioactive nuclei.

“I think Bruno understood earlier than most people how important symmetry was going to be in the development of physics,” Weinberg said.

In 1958, Zumino and Gerhart Luders proved the CPT theorem, which said that any realistic theory of nature has to look the same if the charge and parity of every particle is flipped, and time flows backwards. This theorem was incredibly important for people trying to understand why the universe contains more normal matter – the stuff from which we are made – than antimatter, Hall said.

“It really was an astonishingly important achievement,” he said.

Hall explained that requiring theories to be symmetric “narrows down the number of possible theories, to the point that eventually there are too few of them to choose from to describe nature, which is when you have to break symmetries.”

One of Zumino’s important contributions, Weinberg said, was pointing out the importance of broken symmetry, that is, “when a symmetry of the underlying equations is not reflected in the physical world,” and the real-world implications of symmetry breaking.

The Higgs boson, for example, is a byproduct of physical processes that break symmetry between the weak and electromagnetic forces, he said.

From Italy to New York, Geneva and Berkeley

Zumino was born April 28, 1923, in Rome, Italy. He obtained his DSc degree from the University