SOMETIMES the smallest of signs heralds the greatest of changes. Last June, the Japanese-American SuperKamio-kande team announced that at least one of the three ghostly particles known as neutrinos must have some mass-perhaps less than a millionth that of an electron. Yet this tiny quantity has profound consequences.
"It's a fantastically important result, the biggest breakthrough for particle physics in over a decade," says Joel Primack, a physicist from the University of California, Santa Cruz. In the Standard Model of particle physics, none of the three varieties-the electron neutrino, the muon neutrino, and the tau neutrino-has any mass at all.
But if the Standard Model is wrong, what will replace it? The answer depends on exactly how hefty each kind of neutrino is. Their masses will tell us much about the next generation of fundamental theories. And they are also important for cosmology, as different neutrino masses would have had ...
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