Departments
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26 June 1998 |
Vol. II, No. 1 |
Feature
Article
BU researchers contribute to massive discovery
by Eric McHenry
Neutrinos are the universe's tiniest, most numerous, and -- until now -- most misunderstood constituents. A research team assembled by physicists from BU, the University of California-Irvine, and the University of Tokyo has determined beyond doubt that neutrinos have the property of mass, a discovery that will shake contemporary particle physics to its foundations. The finding was announced June 5 at Neutrino '98, an international conference in Takayama, Japan.
"This is the first true indication that there is more physics, new physics, beyond what we call the standard model of particle physics," says CAS Professor of Physics James Stone, one of the lead researchers. The evidence justifies attaching such big claims to such small particles. Massive neutrinos are not accounted for in traditional theories of physics. Some scientists have suspected, but not been able to prove, that neutrinos possess an infinitesimal mass. Many have maintained that they have none, like the photons that carry beams of light. The finding, Stone says, necessitates nothing less than a re-envisioning of the entire framework within which physicists operate. It also makes the question 'What is the origin of mass?' more germane than ever before.
"The question has always been there, but it's been avoidable up until now," explains Stone, who is the U.S. co-spokesperson for the project. "It can't be avoided any longer. And one of the reasons is that there are more neutrinos wandering around our universe than anything else. Even if the neutrino mass is extremely small, neutrinos have as a minimum the same amount of total mass as all the stars and planets and galaxies that we currently see and add up and account for, because they're so plentiful. So neutrino mass has implications not only in particle physics, but in cosmology, and in particular, in the study of the early universe. As you look at the evolution of the early universe to the present, you now have to allow for part of the energy, part of the mass, part of the dynamics of the universe to include the massive neutrino."
Looking into the Super-Kamiokande detector at Japan's Kamioka Neutrino Observatory, the researchers saw incontrovertible evidence of a phenomenon known as oscillation. This was the clue they needed to confirm that neutrinos have mass, because only massive particles, according to a rudimentary principle of physics, can oscillate.
A 50,000-ton tank of highly purified water held in a mine 1,000 meters underground, the detector is lined with 13,000 light-sensitive photomultiplier tubes. Neutrinos -- created when energetic cosmic rays produced by supernovae, the sun, and other sources bombard the upper atmosphere -- are constantly showering and moving through the earth. Super-Kamiokande's photo detectors recognize tiny light flashes that occur when passing neutrinos interact with protons or neutrons in the water. Observing these, scientists could determine the "flavor" of each neutrino -- electron or muon.
"An electron neutrino, in a charged-current interaction, will produce an electron plus some other low-energy debris that won't be seen by our detector," says Stone. "A muon neutrino will produce a muon. And the light from an electron has an identifiable pattern that it makes, which is different than the pattern that the light from a muon makes. So our detector looks for electrons and muons, separates them, counts them, and categorizes them.
"We can predict very accurately the ratio of the types of neutrinos in those secondary particle showers from the top of the earth's atmosphere," he says, "and we know that the ratio of muon neutrinos to electron neutrinos should be 2:1."
But that wasn't the ratio Super-Kamiokande detected. Its photomultiplier tubes caught the expected number of electron neutrinos, but only half of the expected muon neutrinos. The researchers deduced that those not accounted for had oscillated -- turned into other sorts of neutrinos that are known to exist but are not detectable -- as they traveled from their source to the tank.
"It certainly was not a surprise," Stone says of the finding. "Neutrino mass has been a burning question in physics for the last 30 years, and there have been many reported discoveries of neutrino mass, all of which up until now have turned out to be flawed in some way. So although we've known for probably a year that we were seeing evidence of neutrino mass, we were trying to be extremely careful. Once we make the claim that we've observed it, our data and our analysis techniques are going to be open to the scrutiny of the worldwide scientific community. And they have to hold up."
Stone points to the statistical rigor with which the data were treated. Conclusions drawn from the Super-Kamiokande water, he says with pride, are absolutely watertight.
"It's not as if this is on the hairy edge of a discovery and barely statistically relevant. It's greater than a 6 standard deviation effect. That means that the probability of its being an error or a statistical fluke is immeasurably small. When we presented this at the conference, the skeptics were silenced. There was nothing they could say."
"It's irrefutable evidence," Lawrence Sulak, professor and chairman of the CAS department of physics, told the Boston Globe. "All the unknowns that we were concerned about have been laid to rest."
The partnership between American and Japanese investigators began, Stone says, as a friendly rivalry. Both countries had been conducting neutrino research using smaller detectors for many years, and had made some incremental advances in the field. When Stone and some colleagues approached Japanese scientists in 1991, the circumstances were ideal for collaboration.
"We had competed with each other and published papers back and forth for 10 years," recalls Stone. "So we were aware that they were still asking themselves the same questions we were.
"The funding climate was more favorable in Japan at the time. It looked like they were probably going to get money for building a very large detector of this type, and so we contacted them and opened up some discussion about collaborating on a big project. And they were actually quite thrilled that we were interested in working with them. They had most of the money, but we had a lot of expertise and manpower."
Research proceeded, Stone says, with the U.S. contributing about half of the intellectual input but underwriting only about a tenth of the $100 million project.
"We're very fortunate," he says, "that our Japanese colleagues have been so generous with the construction funds."
Continued financing is currently a question mark, although the neutrino mass discovery has laid groundwork for many new experiments; only 24 hours after its announcement, the New York Times reported that the Kamioka Neutrino Observatory's budget could be pared by as much as 30 percent next year.
But the project has generated a good deal of praise, and there have been rumblings in the major media about its strength as a Nobel Prize candidate. Stone hopes all the positive attention will help his group leverage the Japanese government for further support.
"We're working on that aspect of it," he says. "We're making it very difficult for the Japanese government not to fund this experiment. You can't take the number one discovery in science this year and shut it off."