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 Inside the World of Neutrinos
By Monte Basgall
Two new Duke faculty physicists are exploring the realm of neutrinos, ghostly subatomic particles that have received an upsurge of scientific attention in recent years.
Kate Scholberg and her partner Chris Walter, each assistant professors of physics, spend much of their non-teaching time at Duke probing the mysteries of these neutrino particles at both subterranean and aboveground research facilities in Japan.
Neutrinos have been called "ghostly" because they can pass through the entire mass of the Earth without colliding with any other particles. Until the late 1990s scientists thought that neutrinos lacked any mass. But experiments at neutrino observatories, including one called Super-Kamiokande -- or "Super-K" -- where Scholberg and Walter both spend some of their time inside a Japanese zinc mine, proved otherwise.
That Super-K facility in which the couple works resembles a giant high-tech movie set, with banks of blown-glass neutrino detector tubes lining the inner walls of a 135-foot-tall tank filled with 50,000 tons of ultra-pure water.
Neutrinos are known to arise from an array of sources -- including nuclear reactions in active stars and exploding stars, the collision of cosmic rays with Earth's atmosphere, and, on Earth, nuclear power plants and specialized research accelerators.
However, for decades scientists studying neutrino emanations from our own Sun were puzzled because they detected far fewer of the particles than theories predicted that the Sun should emit.
Coordinated studies at many neutrino observatories revealed that those "missing" neutrinos were there all the time. They had simply "oscillated" into a different form that could not be detected by the kind of detectors being used.
Studies using different kinds of detectors at many facilities have shown that neutrinos from nuclear reactors and atmospheric cosmic ray interactions also undergo such oscillations. In fact, Scholberg said "the first convincing evidence of oscillations" came from studying atmospheric neutrinos, an area in which the Super-K observatory team is "really the leader."
If neutrinos can change forms, theory requires them to have at least a little bit of mass, which raises new questions for physicists. "We don't really have a good model for why neutrinos have mass and why that mass is so small," said Walter.
Scholberg, who was an assistant professor at MIT before coming to Duke, has won a five-year CAREER award for young faculty from the National Science Foundation. The award is to study oscillations in a beam of neutrinos produced by a special research accelerator and aimed at Super-K from about 290 kilometers distance.
Walter, previously an assistant research professor at Boston University, will do experiments at that new high intensity accelerator. That will continue work he is now doing at a less powerful research neutrino maker -- the first of its kind -- also aimed at Super-K from 250 kilometers away.
He is also in charge of part of the computer code associated with the Super-K detector system. After Super-K suffered a massive accident in 2001 that caused much of its detector array to implode and shatter, Walter had to change that code "so that it would work with half as many tubes," he said.
Meanwhile, Scholberg also helped write a computer code enable the interconnection of neutrino observatories so that they can serve as an early warning system to alert astronomers about the explosions of large stars.
The first evidence for such "supernovas" is not the flash of light but rather a burst of neutrinos, she said (for more, see this site).
Moving from the Boston-area to Duke solved a problem of juggling two careers for the couple, who met at Caltech where they were graduate students. "Something we'd been worrying about for a long time is how to find two academic positions in the same city," said Scholberg, who was raised in Montreal, Canada.
Their Duke jobs give them both tenure-track positions of the same rank in the same place. And "the opportunity to start a new neutrino group here was very appealing," added Walter, who grew up in Southern California.
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