Like the Cheshire cat, the elusive neutrino particle can appear and disappear, seemingly at will. And like the smile on the face of the Cheshire cat, the neutrino may be a mere wisp, or have actual substance.
But evidence is growing that this ghostly subatomic particle actually has a corpus. If that is so, the particle may not have been disappearing at all, but simply cloaking itself with another form.
"There are still many puzzles," says University of Washington physics professor Kenneth Young. "But experiments are giving us something much stronger than hints that neutrinos have mass." Young is reporting on the international quest to unravel the nature of the neutrino at the spring meeting of the American Physical Society and the American Association of Physics Teachers in Washington, D.C.
Young and his colleagues are preparing to release the first year's results from a gargantuan Japanese laboratory designed to probe the mysteries of the neutrino. It is called Super-Kamiokande, and it detects the particles in a massive tank, containing 50,000 metric tons of water, buried more than a half-mile deep in a mine outside Kamioka in the Japanese Alps.
The University of Washington is one of nine institutions supporting the $100 million laboratory, which began its research last April.
Based on the first 100 days of research in Japan, Young says there are tantalizing hints that not only does the neutrino have mass and can change its form (or what researchers call "flavor"), but it may also be more abundant at night than during the day, and more plentiful during certain times of the year.
The importance of the neutrino is far more than an intellectual exercise. A puzzle of astrophysics is that much of the universe -- perhaps 90 percent -- is seemingly hidden from view. Researchers postulate that much of this so-called dark matter is actually composed of neutrinos, which are clearly abundant in nature. If the neutrino has mass, then it could be part of "the omnipresent dark matter," says Young.
To date, the results from Japan are largely "the inference of statistics," he notes. It will take another two years to provide evidence that the particle may have mass. Along the way, researchers also hope to solve the myth-like question of how a subatomic particle can suddenly disappear, then reappear.
Super-Kam (as physicists have dubbed the laboratory) tracks neutrinos from two sources, the sun and the Earth's atmosphere, where they are created from the reaction of proton bombardment. Every day, says Young, the laboratory registers about one million particle reactions in the water. Most of these reactions are the result of background radiation, such as that produced by rocks surrounding the water tank. Only 30 reactions are separated out as solar neutrinos, and just 10 are identified as atmospheric neutrinos. Because the neutrinos are so shadowy they cannot be tracked directly, but are registered through their collisions with atoms in the ultra-pure water, which is constantly filtered to remove dust and debris.
Although Super-Kam's measurement of solar neutrinos striking the Earth confirms previous experiments, the central mystery still remains: theoretical predictions of the sun's emission of neutrinos calls for twice as many solar neutrinos as are being recorded. The suggestion that previous experiments have simply had a low efficiency in measuring solar neutrinos is discounted by researchers. They have proved their case by firing electrons into the water, and accurately counting their numbers.
So where are the missing neutrinos? The answer, says Young, could be that the neutrinos are there all along, but are changing flavor. Specifically, the laboratory tracks just one type of neutrino, called the electron neutrino. But there are also two other types: the muon neutrino (muons are massive electrons) and the tau neutrino (tau particles are very heavy). Each of these is called a different flavor of neutrino.
Says Young: "The question is, do these neutrinos stay with a particular flavor all their lives, or do they change? If they do indeed have mass, it is possible they can change so that the electron neutrino becomes the muon or the tau neutrino? In the case of the solar neutrino we would not be able to see it anymore, because this experiment is at an energy level that can only see the electron neutrino."
This changing form from one type of neutrino to another is called oscillation, and it could partially explain why so many solar neutrinos appear to be missing. But there may also be other explanations, says Young. One is "the suspicion" that there may be a detectable drop in electron neutrino population during the day, and an increase at night. During the day, the neutrinos have only to pass through the Earth's atmosphere and the mine rock face to reach Super-Kam. But at night the neutrinos pass completely through the Earth, because the sun is below the horizon. "Oscillation may be far greater if the electron neutrinos have to go through the Earth's core," Young theorizes. In fact, this effect could be so great that just another year of results from Super-Kam "could be enough to make a definitive statement."
The electron neutrino population, he says, may also be greater at certain times of the year. That is because of the Earth's elliptical orbit, which changes its distance from the sun by 5 percent during the year.
Until Super-Kam can provide statistical evidence for these theories over the next year or two, the greatest frustration of neutrino hunting may lie with the researchers themselves. Says Young: "They can invent theories faster than they can improve their measurements."
Young can be reached through the American Physical Society newsroom at the
Renaissance Hotel in Washington D.C., (202) 898-9000, or at
The U.S. Super-Kamiokande home page is at http://www.
A cutaway sketch of the laboratory is at http://www.