GHOST HUNTERS

By MICHAEL D. LEMONICK

IN THE TIME IT TAKES TO READ THIS SENtence, millions upon millions of neutrinos, pouring in from outer space, will zip through the body of every human being on earth. It's like a never-ending barrage of subatomic bullets, but with one important difference: unlike bullets, neutrinos are so ethereal that they pass through ordinary matter as though it were not there at all. Unless a neutrino scores a direct hit on an atomic nucleus, it leaves no hint of its passage. And such hits are so unlikely that the average neutrino can easily penetrate a slab of lead a trillion miles thick without grazing a single atom.

Yet for all their elusiveness, neutrinos are among the most important particles in the universe. Because they emerge unscathed from the core of the sun and from the explosive death throes of stars, they carry information about processes that are otherwise unobservable. And because the cosmos is packed with neutrinos--so many that there is no "illion" large enough to count them--their combined mass could be the dominant force in the evolution and eventual fate of the universe.

It could, that is, if neutrinos have any mass. Nobody knows if they do or if they're massless, like the photons that carry electromagnetic force. That is one of the reasons Canada, the U.S. and the United Kingdom are combining their scientific resources and building the world's most powerful neutrino catcher near the shores of Lake Huron in northern Ontario. When the multimillion-dollar Sudbury Neutrino Observatory goes into operation sometime next year, it should settle some of the thorniest questions in the universe. Says John Bahcall of the Institute for Advanced Study in Princeton, New Jersey, a pioneer in modern neutrino theory: "This is the kind of experiment you do every few decades that can literally set the course of physics for many years to come."

The word observatory conjures up images of white domes silhouetted against a mountain sky. Forget them. S.N.O. is buried more than a mile underground, in a tunnel branching off from a working nickel mine. On Earth's surface, cosmic rays and other stray subatomic particles would trigger false signals in sensitive detectors. Nothing but a neutrino, however, can get through 6,000 ft. of earth and rock. The detector is a gigantic spherical vat containing 1,000 tons of heavy water--a form of H2O in which the H, or hydrogen atom, has an extra neutron in its core. The vat in turn is surrounded by a 7,000-ton jacket of ordinary water, which shields it from trace amounts of naturally occurring radiation in the environment.

A neutrino that laughs at lead won't be stopped by a few drops of water, of course. But the awesome penetrating power of these particles applies to the average neutrino, not to all of them. A very tiny percentage of neutrinos do slam into water nuclei. The number is so vanishingly small that even with quadrillions of neutrinos passing through millions of pounds of water every second, only a few thousand collisions will take place every year. That's still 50 times the rate at existing observatories. When one of these rare collisions happens, the impact can force the nucleus to emit an electron. That in turn generates a tiny flash of light that passes through the clear acrylic walls of the containment vessel and is recorded on electronic sensors.

This is how existing neutrino detectors work in Japan, Italy, Russia and the U.S. What makes S.N.O. different is its exclusive use of heavy water, abundantly available in Canada because it is stockpiled for use in a type of nuclear reactor Canadians favor. Says Barry Robertson, S.N.O.'s associate director: "It's the heavy water that makes this project worth the trouble." An extra neutron in the nucleus doesn't make the water's appearance, chemistry or taste any different from ordinary water used in other detectors. It does, however, change its nuclear structure enough to make this observatory sensitive not just to one but to all three known varieties of neutrinos. Only electron neutrinos--one of the three types--generate flying electrons. But all three will knock the extra neutron from heavy-water molecules.

This property has Bahcall and other physicists speaking in superlatives about S.N.O., because it will allow the device to solve one of the enduring mysteries of astrophysics. Known as the solar neutrino problem, it was discovered back in the 1960s. According to calculations originally made by Bahcall, the nuclear fusion reactions at the sun's core should be generating about 200 trillion trillion trillion electron neutrinos every second. But when physicists set out to find them, they were shocked to see evidence of only about a third that number. Among the possible explanations: perhaps scientists didn't understand nuclear physics as well as they thought, or maybe some unknown factor was cooling the sun's core and thus keeping a lid on neutrino production.

But physicists eventually became intrigued with a third idea. Perhaps some electron neutrinos were switching identities, changing by a process called oscillation into muon or tau neutrinos (the two other varieties) en route to Earth. If so, existing detectors could never see them. And while some of the fine print in the laws of physics says that a massless neutrino can't change its stripes, a neutrino with even a tiny bit of mass might. If neutrinos have mass, they can change; conversely, if they can change, they must have mass, despite what textbooks have been saying for decades.

And if neutrinos do have even a tiny bit of mass, their great numbers would make them a major component of the so-called dark matter that astronomers believe constitutes most of the substance of the universe. Dark matter has never been seen directly, but its powerful gravitational influence is evident from the way galaxies spin on their axes and orbit one another. There is at least 10 times as much dark matter as visible matter, in fact, and perhaps as much as 100 times as much--which would be enough eventually to halt and reverse the universe's headlong expansion. Even at the low end, dark matter's dominant gravity largely accounts for the existence of galaxies and their assemblage into huge structures known as the Great Wall and the Great Attractor. Give neutrinos as little as a few ten-thousandths of the mass of electrons, the lightest known particles, and they could account for all this dark matter. Indeed, physicists at Los Alamos National Laboratory announced last year that neutrinos do have a tiny mass. But that result still isn't considered definitive.

Thanks to its exquisite sensitivity, however, S.N.O. may be able to settle the question of whether the sun's deficit in electron neutrinos is offset by a previously undetected flood of the other kinds. If this works as expected, it should determine once and for all whether neutrinos oscillate. If they don't, solar physics will have to be revised; if they do, particle physics will be turned on its head. "I'd say our solar models are quite reliable," says Bahcall. "But that's why you do experiments. Because what you think you know might turn out to be completely wrong."