UC Physicists Play Important Role in Experiments That Provided New Understanding of Neutrinos

Three physicists at the University of Cincinnati played a key role in recent experiments which provided a surprising new understanding of a tiny subatomic particle known as the neutrino. The UC researchers are Professor Randy Johnson, former graduate student Masoud Valiki, and current graduate student Narumon Suwonjandee. They were part of a 45-member team working at the Department of Energy's Fermi National Accelerator Laboratory (Fermilab) near Chicago. The elusive neutrinos carry no electric charge and "feel" only the weak force, which is 100 times weaker than the electromagnetic force. As a result, neutrinos rarely interact with each other or with other particles, making them extremely hard to detect. Physicists designed the NuTeV experiment in order to observe the interactions of millions of the highest-energy, highest-intensity neutrinos ever produced. Starting with a proton beam from Fermilab's Tevatron, the world's highest-energy particle accelerator, experimenters created a beam of neutrinos directed at a giant particle detector. The detector itself was a 700-ton sandwich of alternating slices of steel and detector. As the beam passed from the first to the last slice, one in a billion neutrinos collided with a target nucleus, breaking it apart. "Cincinnati was instrumental in designing the beam, the test beam, and the slow monitoring system in the experiment," said Johnson. "These are three of the improvements that made the increased precision of the experiment and our ability to see such a small deviation from the "Standard Model" possible. As a result, the scientists found a surprising discrepancy between predictions for the behavior of neutrinos and the way the subatomic particles actually behave. Although the difference is tiny, it could be the first sign of something that profoundly changes our picture of nature. "This wouldn't be the first time that neutrinos have surprised us," said Sam Zeller, a graduate student at Northwestern University. "We looked at a quantity that physicists call 'sine squared theta W,'" Zeller explained. "It tells us the strength of the interaction of neutrinos with the Z boson, one of the carriers of the weak force. The predicted value was 0.2227. The value we found was 0.2277, a difference of 0.0050. It might not sound like much, but the room full of physicists fell silent when we first revealed the result." In particle physics, such "misfit" results are often the harbinger of new particles, new forces and new ways of seeing nature. The discrepancy translates to a 99.75 percent probability that the neutrinos are not behaving like other particles. "Our picture of matter has held true for thirty years of experimental results," said Fermilab Associate Director Michael Shaevitz, a cospokesperson for the NuTeV (Neutrinos at the Tevatron) experiment. "With the NuTeV result, it's possible we may have stumbled across a crack in the [Standard] model. As yet, we don't know the explanation, but we believe it may foreshadow discoveries just ahead at accelerator laboratories." The 45-member NuTeV collaboration-small on the scale of today's particle physics experiments-operated for 15 months in 1996 and 1997. Rochester's McFarland presented the measurement at an October 26 seminar at Fermilab. The collaboration has submitted the results to Physical Review Letters for publication. The collaboration included physicists from the University of Cincinnati, Columbia University, Fermilab, Kansas State University, Northwestern University, the University of Oregon, the University of Pittsburgh and the University of Rochester. The research was supported by the National Science Foundation, the U.S. Department of Energy and the Alfred P. Sloan Foundation.