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Subatomic particle Neutrino Has Mass, Researchers Say

By Thomas H. Maugh II
Los Angeles Times

In a finding that is being called the physics discovery of the decade, an international team of researchers reports today that the elusive, ghostlike subatomic particle known as the neutrino has a mass. This suggests, among other things, that they represent at least part of the so-called missing or dark matter necessary to explain how large-scale structures in the universe came into being.

Neutrinos - formed primarily in the Big Bang that created the universe and in the fiery nuclear furnaces of stars - are by far the most common elementary particles. The discovery that they have mass, just like the more substantial particles known as electrons and protons, was announced at a Takayama, Japan, meeting called Neutrino '98.

The discovery also will force revisions in what physicists call the Standard Model - the set of esoteric rules that describe how particles interact with each other to form larger particles, atoms and everything else in the universe.

That Standard Model now says that neutrinos have no mass, just like the photons that carry light energy.

The finding that they do have mass should help resolve some inconsistencies in the theory.

For two decades, physicists have been puzzled by results of experiments to detect neutrinos. Large detectors built to measure neutrinos produced by the sun and those produced when cosmic rays strike atoms in the upper atmosphere have found far fewer neutrinos than theory predicted.

That failure was initially attributed to defects in the detectors, but the new results provide "very strong evidence" that the problem lies in the nature of neutrinos themselves, said physicist William Louis of the Los Alamos National Laboratory.

The key is the demonstration that neutrinos have the ability to repeatedly undergo a Jekyll-and-Hyde transformation from one type of neutrino to another - for example, from an electron neutrino to a tau neutrino and back again. Physicists call this process oscillation.