For two years, a vast water tank deep beneath the Japanese Alps has been trapping some of nature's most elusive particles.
Now, a team of US and Japanese researchers at the facility say, the experiment has yielded definitive evidence that these particles, called neutrinos, change identities as they travel - and thus have mass.
If confirmed, the results offer a significant clue toward solving a fundamental mystery about the sun. They also could boost efforts to explain the origin and future of the universe, and help complete a family tree that physicists have drawn of subatomic particles.
"This is very exciting," says Columbia University physicist Janet Conrad, who conducts neutrino research at the Fermi National Accelerator Laboratory in Batavia, Ill.
The implications are so profound that "we need to stop, take a deep breath, and look for more evidence," adds Michael Turner, chairman of the astrophysics department at the University of Chicago.
The subject of the research, reported today at an international meeting in Tokyo, is a tiny particle first theorized about in 1931 by physicist Wolfgang Pauli. He merely saw the neutrino as a bookkeeping device to balance an equation about a neutron decaying into a proton and electron. Neutrinos weren't discovered in the lab until 1956. Since then, physicists have verified that the neutrino comes in two "flavors" and possibly a third.
Nature creates neutrinos in at least three ways. For example, they are formed by nuclear explosions taking place in the center of stars. And staggering numbers of neutrinos were apparently a byproduct of the Big Bang, the explosion that may have spawned the universe. Neutrinos also result from collisions between cosmic rays and Earth's atmosphere.
Detecting these miniscule particles is difficult, because they rarely interact with other particles. To spot enough interactions to make meaningful measurements, data must be gathered over along period of time using detectors that present the biggest target. In this case, the target was 12.5 million gallons of purified water in a cave lined with stainless steel. Detectors recorded the brief bursts of light given off by the rare interactions.
UNTIL now, atmospheric neutrinos had presented physicists with a conundrum. They knew how these neutrinos were produced and knew what mix of "flavors" they should see, but these researchers failed to get the expected mix of neutrino types.
The Japanese-US neutrino observatory, Super Kamiokande, also spotted the discrepancy. But the data also showed that the shortfall in the expected number of muon "flavored" neutrinos was most pronounced when the particles passed all the way through Earth, instead of coming from directly overhead. The team concluded that muon neutrinos change types as they travel. Called neutrino oscillation, "the effect we're seeing is big," Dr. Learned says.
The news is likely to be welcome among scientists who have been puzzling over a substantial shortfall observed in the number of neutrinos the sun ought to produce.
Neutrino oscillation has been proposed as one explanation for the shortfall. To Victor Stenger, another University of Hawaii physicist and Super Kamiokande collaborator, helping explain the solar-neutrino deficit may be the most important result from this work. "It doesn't prove that solar neutrinos oscillate, but it makes that explanation more viable," he says.
Yet to oscillate, theories say, neutrinos must have mass. This is the prospect that tantalizes many researchers.
Within what physicists call the standard model of particles and their interactions, "neutrinos don't have mass," Dr. Turner says. If they do, it means the standard model needs some fill-ins. Neutrino mass is one of the "smoking guns" predicted by grand-unification theories, he adds. These theories hold that gravity and other forces are descendants of one unified force that existed in the universe's earliest moments.
If neutrinos have mass, Turner continues, "they will contribute to the comic mass density" - an attribute that helps determine if the universe continues to expand forever or eventually will collapse in the Big Crunch. Since their mass is likely to be very small, neutrinos may account for only 10 to 20 percent of the universe's total mass. Even so, he says, "that would explain a number of puzzles."