For decades, physicists were stumped as to why neutrinos (one of the fundamental particles that make up the universe), seemed to disappear into thin air when traveling from space to Earth. Until now. With the help of CAS professors, postdocs, and graduate students from the Boston University Neutrino Group, a 20-year experiment solved the mystery—and won a Nobel Prize.
Neutrinos are an important part of particle physics’ “Standard Model,” which explains what the world is and what holds it together. For decades, the neutrino had been taken to be massless. This was cast into doubt when studies of the sun and cosmic rays in the 1970s and ‘80s showed neutrinos were missing in measurements performed on Earth compared to theoretical calculations of how many should exist.
The Nobel winning experiments, one in Japan, one in Canada, were designed to clarify the neutrino mystery. The data analysis team of the Japanese Super-Kamiokande experiment, co-lead by Nobel laureate Takaaki Kajita of the University of Tokyo and CAS Professor Ed Kearns, showed that neutrinos from cosmic rays were switching identities as they traveled through the earth. The Super-Kamiokande collaboration showed that neutrinos must have some mass in order to transform through space and matter, thus explaining why so many particles were missing in the decade-old studies. CAS Professor of Physics Ed Kearns, who was involved with the experiment, describes it as “seeing a blurry picture become clear, pixel by pixel, until the subject matter was obvious and recognizable.”
The discovery of “neutrino oscillations” won Kajita and Arthur McDonald, the leader of the Canadian experiment, the 2015 Nobel Prize in Physics. According to the Nobel Prize organization, the Super-Kamiokande experiment “has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe.”
BU’s Neutrino group, supported by the Department of Energy Office of Science, played a large role in Super-Kamiokande’s discovery. At the time, the group was comprised of Professors of Physics Ed Kearns, Jim Stone, and Larry Sulak, as well as two graduate students and three post-docs. Graduate student Mark Messier (GRS’99) wrote his PhD thesis on the discovery. Part of the experiment was built at BU with the help of undergraduates and engineering/technical staff, and the Neutrino Group operated the detector and analyze data once the experiment began, an activity that continues to this day.
“It was a tremendous experience as a scientist to help build and operate this magnificent detector…and see the [resulting] data,” says Kearns. “Within two years of operation we had put to rest decades of questions about the nature of the neutrino, and lit a path to the work that has followed.”
Not only does the discovery redefine the Standard Model, but it also leaves open a variety of questions that could only be answered if neutrinos have mass. A wide range of experiments, including T2K and DUNE (both of which the current BU Neutrino Group is involved in), are pursuing these queries.
Kearns and Stone, U.S. co-spokesperson, will attend the Nobel Prize award ceremony in Stockholm, Sweden, on December 10, 2015.