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Winter/Spring 2003 Vol. 3 No. 1

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Colored photograph of a neutrino-proton interaction event in a bubble chamber at the Brookhaven National Laboratory, ©Science Photo Library/Photo Researchers Inc.

Tracking the
Elusive Neutrino

Tiny Particles May Help Answer Big Questions

What is the nature of the dark matter that makes up most of the universe? What laws govern all of the forces and particles that exist? For clues that could help answer weighty questions like these, scientists are increasingly turning to the least substantial of objects, the neutrino.

S maller than atoms and having almost no mass, neutrinos are emitted by explosions within the sun, supernovas, and other cosmic phenomena. These invisible particles rarely interact with other matter -- a quality that makes them extremely hard to detect. Each second, trillions of neutrinos glide through our bodies and almost all other things on Earth as if they weren't even there.

For astronomers, this ability to pass through matter unscathed is the neutrino's greatest appeal. Neutrinos emitted by distant exploding stars and supermassive black holes can travel through space without being absorbed or altered by objects en route. They reach the Earth as evidence that hasn't been tampered with -- giving astronomers a "direct line of sight" into phenomena that occur at the edges of the universe.

The tiny neutrino might also ultimately challenge the "standard model" of physics -- the currently accepted description of the building blocks of matter and how these particles interact. The recent discovery that neutrinos have mass showed the standard model to be partly incorrect, and a better understanding of neutrinos may alter the model in other ways as well.

Cosmologists hope that studying these particles may shed light on the dark matter that holds the universe together. A small portion of this matter is made up of neutrinos, but the remainder is still a mystery. The most tantalizing possibility is that if a powerful new detector is built, it may spot more than neutrinos and reveal the other particles that make up dark matter as well.

But before new knowledge can be gained or paradigms challenged, scientists first need to be able to study neutrinos more closely. And this isn't easy, because the Earth is constantly bombarded with cosmic rays -- protons, photons, electrons, and other particles -- from space. These create a distracting "background" against which neutrinos are hard to detect. To elude this interference, scientists have begun taking their observations below the Earth's surface, where it is more difficult for cosmic rays to reach. Two new initiatives are being considered for future funding, and the Bush administration asked the National Research Council to evaluate their scientific merits.

One proposed laboratory would be built deep underground. So far, underground experiments have been conducted in mines, but there isn't enough space available to meet scientific demand. A laboratory built from scratch in the United States could be deeper, larger, and more versatile, allowing a broader range of experiments, a Research Council committee concluded in a new report.

National Science Foundation-funded AMANDA Telescope project at the South Pole, where neutrino detectors are located 1.5 kilometers beneath the surface, photo by Robert Morse

Also of strong scientific promise for the detection and observation of neutrinos, the report says, is the proposed international collaboration called IceCube. This project would place instruments in a cubic kilometer of ice far beneath the surface at the South Pole. Scientists hope that the neutrinos which shower the Earth will interact with the ice, and produce particles called muons; these release light, which could be seen and measured through the ice. The path and brightness of the light would reveal the neutrinos' direction and level of energy, and the light's pattern would indicate which of the three known types of neutrino is present.

The two facilities' capabilities would complement -- not duplicate -- each other, the report says. And though the projects are ambitious, at least some of the rewards would be swift. IceCube could be completed in five or six years, and experiments could begin even during construction.   -- Sara Frueh

Neutrinos and Beyond: New Windows on Nature. Neutrino Facilities Assessment Committee, Board on Physics and Astronomy, Division on Engineering and Physical Sciences (2002, approx. 95 pp.; ISBN 0-309-08716-3; available from the National Academies Press, tel. 1-800-624-6242; $18.00 plus $4.50 shipping for single copies).

The committee was chaired by Barry C. Barish, Ronald and Maxine Linde Professor of Physics, California Institute of Technology, Pasadena. The study was funded by the National Science Foundation.

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Copyright 2003 by the National Academy of Sciences