Opening Nature's Tiniest Window
MOZUMI, JAPAN — For 400 years, miners have burrowed deep into the cedar-cloaked flanks of Japan's Mt. Ikenoyama in search of silver, lead, and zinc.
Today, miners of another sort spend eight-hour shifts 2 kilometers (five-eighths of a mile) from daylight and a kilometer beneath the mountain's peak. In an underground control center, they keep round-the-clock watch on a row of computer screens and a wall display, waiting patiently for the rare, telltale signs from one of nature's most elusive fundamental particles - the neutrino.
The feeble ring of light marking each neutrino's passage through an enormous water-filled tank acts as another waymark pointing toward answers to some of the most basic questions physicists are asking about the origin, earliest stages, and, perhaps, the future of the universe.
The underground detector also is opening a window on the processes that take place during the life and death of stars. And when upgrades are complete on an older, smaller detector in a cavern 150 meters away, the observatory could open a similar window on the processes stoking the furnace at the earth's core - a source of heat that ultimately propels the crust's vast plates, sculpting continents and sea floors.
The range of exploration, and the results, bring a smile to Yoji Totsuka's face. "We are very excited these days," acknowledges the director of the University of Tokyo's Kamioka Observatory here and of the university's Institute for Cosmic Ray Research. Indeed, this is a big year for the lab and the scientists worldwide who collaborate.
In June, the large detector here, known as Super Kamiokande, made international headlines when a team of scientists announced that the facility had gathered the first solid evidence that neutrinos change forms as they travel, something they can do only if they have mass. Establishing that these subatomic no-see-ums have mass could lead to "new physics beyond the standard model," says Boston University physicist Chris Walter, referring to the family tree physicists have built to link hundreds of subatomic particles and identify their roles.
Says another United States physicist, "This will go down as the major [physics] discovery of the decade. This will send Yoji Totsuka to Stockholm."
Now, the observatory is preparing a follow-up experiment to gather more precise details on neutrinos as quick-change artists. June's results relied on neutrinos produced when cosmic rays collide with Earth's atmosphere - a random process.
In January, physicists at the National High Energy Physics Accelerator Research Organization in Tsukuba City, some 260 kilometers southeast of here, are scheduled to fire up an accelerator to produce neutrinos on demand and aim them at Super Kamiokande.
By measuring the energies and types of neutrinos as they leave the accelerator and comparing those results with what scientists see as the neutrinos pass through Super K's tank, researchers not only could provide solid confirmation of June's neutrino-oscillation results, but they also will be able to measure several other neutrino parameters.
Why go to all this trouble to eke information from a particle that carries no electrical charge and so rarely interacts with matter that hundreds of billions of neutrinos pass unnoticed through us every second?
From the standpoint of particle physics, "the neutrino represents one of the soft underbellies of the standard model," says Harvard University physicist and Nobel laureate Sheldon Glashow. In the simplest versions of the standard model, neutrinos have no mass, he explains, adding that June's results will help researchers paint a more accurate picture of the subatomic world.
Super K's primary purpose is to monitor the sun in the hopes of solving a long-standing riddle about Earth's nearest star - and in the process make basic measurements of neutrinos. The nuclear reactions taking place in the sun produce copious amounts of neutrinos, providing physicists with a direct view into the processes at the heart of the sun. In 1968, Brookhaven National Laboratory's Ray Davis reported results from an experiment that confirmed the existence of solar neutrinos. But his results also showed a serious shortfall in the expected number, says Kamioka's Dr. Totsuka.
That left two possibilities, he continues. Either he did the wrong thing, or the theories are wrong. In 1988, the old Kamiokande detector beautifully confirmed the solar neutrino deficit. And theorists reviewed their standard solar model to the point where people grew increasingly confident that the model is correct. But the deficit remains.
That leaves the possibility that neutrinos either change their form - or oscillate - as they travel, appearing as one of three types. The distance a neutrino travels before it changes from one type to another depends on its mass and energy, says University of Tokyo physicist Tuneyashi Kamae. Thus, to spot the change, the observer has to be in the right place.
Last June's results showing evidence of oscillation by atmospheric neutrinos as they travel the diameter of the earth gave weight to the notion that solar neutrinos oscillate. Super K may confirm that notion.
"This is very preliminary," Totsuka says, "but we are beginning to believe that the deficit is caused by oscillation." As Super K continues to collect data on solar neutrinos passing through the detector, scientists are measuring their energy spectra and comparing them with straightforward predictions about what that spectra should look like.
Researchers are noting slight differences that "can't be explained away by altering conditions inside the sun," Totsuka says. "People are being very cautions, but I think in the next year or two we can announce something new in the solar-neutrino field."
The issue of mass that oscillation raises may have a bearing on understanding the future of the universe. Some astrophysicists have put neutrinos forward as a candidate for dark matter, a reference to 90 percent of the universe's mass, which so far has eluded detection. Dark matter plays a critical role in calculations about whether the universe continues to expand or collapse back on itself.
Neutrinos are attractive dark-matter candidates, in part because physicists calculate that so many of them would have been released in the titanic explosion that gave rise to the universe - and would still be flitting largely unhindered around the cosmos.
The situation is up in the air, Dr. Glashow cautions. "First, you have to know if neutrinos are required to explain part of the dark-matter problem. If they are, then they need masses considerably above those suggested by experiments so far," he says. Still, he adds, "it's an interesting and open question" as to whether neutrinos are critical pieces in cosmology's puzzle.
In addition, researchers are laying plans to have Super K help anchor a worldwide network of underground detectors to provide early warning of nearby supernovae. These are immense explosions that snuff out massive stars and seed the galaxy with chemical elements, such as carbon and iron, which are produced as the star exhausts the last of its fuel. The vast majority of the energy released in the explosions is carried off by neutrinos.
In 1987, when a supernova exploded 170,000 light-years from Earth in a satellite galaxy known as the Large Magellanic Cloud, the old Kamiokande detector spotted the neutrinos at least an hour before its light arrived.