UC Merced’s Roland Winston Resolves 40-Year Particle Physics Problem

MERCED — UC Merced Professor Roland Winston and two colleagues have published a solution to a 40-year-old quandary in particle physics. They reanalyzed many years of measurements of mu meson and neutron decay to obtain a new result that satisfies mathematical requirements and confirms the accuracy of previous theories. This work relates to the matter-antimatter imbalance in the universe and its coding in the description of the weak force that holds together subatomic particles.

The paper, "Semileptonic Hyperon Decays and Cabibbo-Kobayashi-Maskawa Unitarity," was published in June 2004 in Physical Review Letters.Winston's collaborators on the discovery were Nicola Cabibbo of the University of Rome - La Sapienza and Earl C. Swallow of Elmhurst College in Illinois and the Enrico Fermi Institute at the University of Chicago.

"Professor Winston's latest discovery is just one among many in his distinguished career, but it exemplifies the excellence of UC faculty in making major discoveries that change the way we view the world," says Dean of Natural Sciences Maria Pallavicini. "His work is at the highest level and carried out with patience, exactness and a collaborative spirit. We were very proud to see 'The University of California, Merced,' published beneath the name of Roland Winston on this paper."

The story began when Winston was a graduate student at the University of Chicago in 1963 and read a Physical Review Letterthat would greatly influence the course of his career. It built on a discovery by Caltech's Sam Berman, identifying the small difference between the strengths of two processes in particle physics, mu meson decay and neutron decay.

Mu mesons, also known as muons, are copiously produced by cosmic rays high in the atmosphere, striking the earth's surface at a rate of about 1 per minute per square centimeter. Mu mesons decay to form either an electron, or a positron with a neutrino and an anti-neutrino.

The author of the 1963 letter, Nicola Cabibbo, interpreted the difference between the strengths of mu meson decay and neutron decay as the cosine, or Vud, of an angle that became known as the Cabibbo Angle. In the ensuing years, Cabibbo's work contributed to the formation of the overall theory called the Standard Model, which describes the interactions of the four forces that affect particles - gravity, electromagnetic force, the strong force and the weak force.

Winston focused his research on Cabibbo's paper after he graduated from Chicago. First at the University of Pennsylvania, then back at Chicago, he and his students and colleagues measured each particle decay process. As the years passed, their measurements confirmed the overall integrity of the picture.

In the meantime, the 1960s passed into the 1970s. Two Japanese physicists, Kobayashi and Maskawa, created a 3x3 matrix based on Cabibbo's work. The matrix described how the weak force operates in quarks, which also experience strong-force and electromagnetic interacions. It indicated that Cabibbo's work had correctly predicted the discovery of entire new families of quarks and leptons.

In 1979, the Nobel Prize in Physics was awarded to Glashow, Salaam and Weinberg, the physicists who had led the way in configuring the overall theory of the Standard Model. The Standard Model was gaining validation. But a small discrepancy worried physicists.

"The sine squared and the cosine squared of the Cabibbo angle didn't quite add up to 1, as every student of Pythagoras knows they must," Winston explains. "People worried that something was missing, that the picture was incomplete." If the description didn't add up mathematically, it could have meant that physicists were overlooking another entire class of quarks and leptons.

The Annual Reviews of Nuclear and Particle Science commissioned a review of the situation. Cabibbo, Winston, and Earl Swallow, a longtime colleague, used an innovative approach to reanalyze the data and obtained a new result for Vus that was somewhat larger, satisfying Pythagoras. The results of the new analysis were submitted to Physical Review Lettersin June 2003 and published in the summer of 2004.

Supporting these results, an independent analysis from different processes (K meson decays) was submitted to Physical Review Letters in June 2004. This analysis obtained a value for the angle that agrees with the Cabibbo-Swallow-Winston result to four places, indicating that the Cabibbo picture is very consistent.

"Professor Winston, who has had a long-standing involvement with hyperon physics, has recently managed to put to rest a troubling question about the validity of the standard model," said Professor Robert Littlejohn, a physicist at UC Berkeley, in response to the June paper. "While previous data, based on kaon physics, suggested a violation of unitarity, his new analysis of hyperon data, done in collaboration with Cabibbo and Swallow, produces values of the Cabibbo-Kobayashi-Maskawa matrix that are completely in accord with the requirements of unitarity. Winston himself has been active for years in the gathering of this hyperon data. The result is a satisfying confirmation of the standard model."

Another Berkeley physicist, Professor Eugene Commins, concurs. "The overall theory of the Standard Model was the crowning glory of the field when it was developed in the 1960s," he says. "But it remained for scientists to validate the model with experimental data. The work recently published by Roland Winston and his colleagues contributes to that effort with very precise measurements of an element of the Cabibbo-Kobayashi-Maskawa mixing matrix, which describes weak interactions among quarks. Roland has worked a very long time on this issue and is a real expert on it. He is an excellent physicist who has completed a very impressive work in this field."

The overall picture built through this work also helps explain the preponderance of matter over anti-matter, which Winston calls "one of the more intriguing characteristics of our world." The matter and antimatter of the universe are thought to have mostly expired in a release of energy just after the time of the Big Bang. However, there was an imbalance between the two; matter still outnumbers antimatter by about one part in a billion. This excess of matter makes up all known objects in the universe today.

That alone is significant, not to mention the fact that this achievement results from more than 40 years of painstaking research and collaboration.

And, Winston adds, "Now Pythagoras can rest easy."

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