October 11, 2015

UW physicists celebrate contribution to Nobel-winning neutrino discoveries

At 2:45 a.m. on Oct. 6, bleary-eyed Nobel Prize enthusiasts on the West Coast were treated to an unexpected lesson about fundamental particles and forces in our universe. Across the globe in Stockholm, a panel of scientists announced that the 2015 Nobel Prize in Physics would honor two scientists who led international collaborations to understand mysterious particles called neutrinos.

The prize recipients, Takaaki Kajita and Arthur McDonald, respectively led the Super-Kamiokande Collaboration in Japan and the Sudbury Neutrino Observatory Collaboration in Canada. As the Emerald City awoke to the news, the two teams of University of Washington researchers who were members of these multinational, decades-long scientific groups began to celebrate.

“It feels great,” said UW physics professor Jeffrey Wilkes. “We’re glad the recognition came for the hard work everyone has done.”

Meanwhile, physics professor Hamish Robertson has led the UW team working with the Sudbury Neutrino Observatory, or SNO, which includes over thirty professors, students and other researchers from the UW on experiments that were completed in 2006.The endeavors honored by the 2015 Nobel Prize in Physics were large-scale, multiyear experiments to measure the fundamental properties of neutrinos. Wilkes and the late UW physics professor Kenneth Young began the UW’s involvement with the experiments at Super-Kamiokande, or Super-K, when the facility was still under construction in the early 1990s. A team of about a dozen UW postdoctoral researchers, graduate students and engineers later joined them on the Super-K Collaboration.

“Before these experiments, people used to think neutrinos had no mass, and there were many other unanswered questions about their behavior,” said UW physics professor Jason Detwiler, who worked on the SNO collaboration and still conducts neutrino research.

As early risers for the Oct. 6 announcement learned, neutrinos are the second most common type of particle in our universe — after light. Yet neutrinos are difficult to detect and measure with precision. They are ubiquitous and easily form when other types of particles collide or undergo radioactive decay. But they are tiny, electrically neutral — neutrino means “little neutral one” — and can easily pass through light years of matter without interacting with it. Neutrinos also come in three “flavors,” but the differences between flavors have proven difficult to tease out. At times neutrinos, especially those generated by the sun, seemed to disappear as they traveled to Earth.

In the 1990s, two international groups of researchers constructed elaborate facilities to learn some basic facts about neutrino behavior. Both teams wanted to detect neutrinos that form far above our heads — Super-K focused on neutrinos generated in Earth’s atmosphere while SNO sought out neutrinos from the sun. But to do this, they had to conduct their experiments in underground mines.

“Cosmic rays sometimes hit the upper atmosphere, creating a shower of particles that you and I don’t notice, but they completely block out any signal from neutrinos on the surface,” said Detwiler. “So you have to take your neutrino detectors far underground.”

Both Super-K and SNO used massive tanks of ultrapure liquids to detect neutrinos. In the SNO facility, “heavy” water in a spherical tank made of clear acrylic served as the detection medium.

“When a neutrino comes in and interacts with the water, it makes little flashes of light,” said Detwiler. “Outside this huge spherical tank, they mounted almost 10,000 light detectors to measure these flashes.”

Super-K used a tank of purified water to detect neutrinos. Both facilities required years of construction and calibration before experiments could begin. UW teams pored over and analyzed data along with their collaborators at other institutions. But within just a few years of each other, both international teams announced major discoveries.

The Super-K collaboration discovered that atmospheric neutrinos can “oscillate” between at least two flavors. In addition, months before Young’s death in 1998, the group announced that neutrinos do indeed have mass. Three years later, SNO confirmed neutrino oscillation among all three flavors, which explained why solar neutrinos seemed to vanish by the time they reach Earth. On Oct. 6, the Nobel committee specifically cited these groundbreaking discoveries during the prize announcement.

UW researchers are not yet finished with the subject.

“There are still many questions about neutrinos,” said Wilkes. “For example, we don’t know their mass — just that they have mass, it’s very small and we don’t know why.”

Neutrinos may also be a route toward detecting new types of physical interactions, added Detwiler.

Unanswered questions aside, the UW Department of Physics plans to celebrate the SNO and Super-K experiments. On Monday Oct. 12 at 4:00 p.m., the department will hold a colloquium in the Physics/Astronomy Building in room A102 to revisit the contributions of UW researchers.

###

For more information, contact Wilkes at wilkes@u.washington.edu or 206-543-4232 and Robertson at rghr@uw.edu or 206-616-2745.