June 22, 1999

Physicists Zero In on Ghostly Neutrinos

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    Scientists operating huge underground detectors in Japan and Canada are racing to obtain independent proofs that the elusive neutrino, a ghostly particle whose vast family may constitute a large part of the mass of the universe, changes form as it flies through matter or space.

    At least some neutrinos are now believed to have some mass, and physicists would love to learn how much, a goal that may be reached by studying the changes in form a traveling neutrino undergoes.

    A race to prove that a pervasive particle changes its form.

    At issue is the effect of neutrinos, which pervade every cubic inch of the universe, on the rate at which the universe expands. Physicists also hope that further neutrino discoveries will account for the Sun's "missing neutrinos," neutrinos predicted by theory but not yet detected by observation.

    A year ago at an international meeting at Takayama, Japan, a Japanese-American team of physicists disclosed the first clear evidence that neutrinos oscillate from one "flavor" to another as they pass through Earth. The discovery, made with the help of a cathedral-size underground water tank called Super Kamiokande near the city of Kamioka, Japan, may shed light not only on neutrinos but on the future expansion rate and behavior of galaxies and the universe.

    In the past year physicists in North America, Europe and Japan have been racing each other to find independent evidence confirming or refuting the finding of neutrino oscillation, and several large laboratories have left the starting gate in the quest.

    The existence of neutrinos was predicted in 1930 and the particles were actually discovered in 1956, but they remain cloaked in enigmas. Physicists have determined that in particle reactions, separate neutrino types are associated with three other types of lepton particles: electrons, muons and tau particles. Thus, electron neutrinos (the kind emitted by the Sun's nuclear reactions), muon neutrinos (produced by laboratory accelerators) and tau electrons (known by inference but never directly detected) interact with matter in different ways.

    Any experimental investigation of neutrinos is extraordinarily difficult because these particles have no electric charge and their mass, if not zero, must be vanishingly small. This means that a typical neutrino could pass through a six-trillion-mile thickness of solid lead with only a negligible chance of hitting anything.

    Sudbury Neutrino Observatory
    The vessel above holds heavy water at the the Sudbury Neutrino Observatory, 6,800 feet deep in a Canadian nickel mine.

    Fortunately for scientists, however, an occasional neutrino defies the astronomical odds and hits a bit of matter in some detector -- a tank of water, an atom of air or a huge block of Antarctic ice, for example -- yielding a shower of debris. Some of the debris leaves trails of blue light as it streaks through transparent objects or fluids. Measurements of this light, called Cherenkov radiation, allows scientists to reconstruct the events that caused it.

    Experiments under way promise to shed light on the neutrino's apparent ability to transform itself into different forms, a phenomenon that could explain why the Sun's nuclear furnace seems to emit many fewer neutrinos than the process should produce. It may simply be that the solar neutrinos are reaching Earth in the predicted numbers, but that many of them have turned themselves into forms that are not revealed using conventional detectors.

    On June 9 scientists announced that the first neutrinos had been captured by a uniquely sensitive new detector at the Sudbury Neutrino Observatory, known by the acronym SNO, 6,800 feet below ground in the Creighton nickel mine near Sudbury, Ontario.

    Like other large neutrino detectors, SNO mainly consists of a huge water tank. What gives it greater sensitivity for corralling neutrinos than any similar detector is that in place of ordinary water, it is filled with 10,000 metric tons of heavy water, water in which each ordinary hydrogen atom with a lone proton in its nucleus is replaced by a hydrogen isotope, called a deuteron, containing a proton and a neutron.

    Unlike ordinary water, which can snag only one "flavor," or type of neutrino, heavy water is sensitive to all three neutrino flavors: electron, muon and tau. In theory, heavy water should register the arrival of all three flavors with equal probability, regardless of how they try to hide by oscillating from one flavor to another.

    Building SNO took 15 years, and its heavy water alone, lent for the experiment by Atomic Energy of Canada Ltd., is valued at $210 million. There were many delaying snags along the way, and yet it took only a few days to begin seeing the first neutrinos after the detector was inaugurated in April, say the Canadian, American and British scientists who built it.

    Dr. Arthur B. McDonald, the director of the observatory, said in his announcement on June 9 that "to see such clear examples of neutrino interactions within days of finally turning on was a real triumph for the entire SNO team."

    Dr. Eugene Beier of the University of Pennsylvania, a spokesman for the United States contingent of the multinational observatory team, said it would probably take about two years before the detector yielded "definitive" evidence of having spotted all three neutrino flavors.

    Meanwhile, the detector may have to be modified to improve its sensitivity. In one experiment, magnesium chloride would be mixed with the heavy water, because chloride is particularly effective in capturing neutrons emitted by neutrino interactions. In another modification, detectors containing the rare isotope helium-3 would be added to the water tank; helium-3 offers neutrons an even fatter target.

    At stake is a hoped-for confirmation that neutrinos in flight oscillate between flavors. Under the rules of quantum mechanics, this implies that neutrinos have mass, a situation with enormous implications for astrophysics and cosmology because neutrino mass could affect the rate of expansion of the universe and influence the formation of galaxies and large-scale structure in the universe.

    Two new developments in Japan are narrowing the search for confirmation of neutrino oscillation.

    In a paper to be published next week, a large Japanese and American team reports the discovery of a nonsymmetric effect in the arrival of neutrinos at the Super Kamiokande detector. More neutrinos were found hitting the detector from the western direction than from the east, an effect predicted from Earth's magnetic field.

    Neutrinos themselves are not influenced by magnetic fields, but in this case they were created by the annihilation of positively charged cosmic ray particles drawn toward Earth by the planet's geomagnetic field. Neutrinos created by the impact of cosmic-ray particles on upper atmospheric atoms continue along roughly the same trajectories as the original cosmic rays.

    The asymmetric east-west neutrino result came as no great surprise to the experimenters, "but it was an extremely important result," Dr. Beier said, "because it showed by independent means that the measurements of neutrino flow reported in Japan last year were made correctly."

    Another important neutrino experiment making use of a powerful proton accelerator in Japan is expected to begin in a few days, said Dr. Lawrence R. Sulak of Boston University, a member of the Super Kamiokande research team.

    In the experiment, a proton beam from Japan's KEK (a Japanese acronym meaning High Energy Accelerator Research Organization) is to be fired into an aluminum target producing an intense beam of pi-meson particles, which decay rapidly into neutrinos and muon particles. The resulting neutrino beam will first pass through a nearby water detector and then hit the ground at a downward angle of 1.5 degrees, re-emerging 155 miles to the west at the Super Kamiokande detector. (The experiment is known as K2K, for KEK to Kamiokande.)

    The 155 miles of Earth through which the beam passes offers no appreciable barrier to the neutrinos. If the same neutrino flux is found at the nearby detector as at the distant Kamiokande detector, the experiment will offer no proof of oscillation. But if, as expected, fewer neutrinos are found at the end of the line than at the beginning, it will mean that detectable neutrinos have transformed themselves into an undetectable form on their way through Earth's crust.

    The experiment is expected to take about two years to yield solid results, Dr. Sulak said.

    Meanwhile, somewhat similar experiments are planned in Europe, where a beam of neutrinos from the Fermi National Accelerator Laboratory near Chicago will slice through Earth to a neutrino detector 460 miles away in an iron mine near Duluth, Minn.

    In a third project, a neutrino beam from the European Laboratory for Particle Physics (CERN) near Geneva would be directed through the ground to detectors within the Gran Sasso Tunnel under the Italian Alps 455 miles away.

    "Many experiments are closing in on the neutrino from many different angles," Dr. Sulak said. "With luck, it won't be too long before we penetrate the particle's secrets."

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