New evidence of a heavy neutrino.

New Evidence of a Heavy Neutrino 

Sometimes you see it; sometimes you don't. The ghostly neutrino has again brought its puzzling vanishing act to center stage.

New experimental data, obtained independently at three laboratories, hint at the existence of a type of neutrino with a mass isgnificantly greater than theorists had anticipated. Yet other research teams conducting similar searches have so far uncovered no such evidence.

Despite this controversial uncertainly in the data, the mere possibility that a subatomic particle with such surprising characteristics may exist has sent tremors racing through the particle physics and astrophysics communities. Theorists are already hard at work studying ways of modifying the "standard model" of particle physics to accommodate a heavy neutrino and exploring how its existence would fit into current models of how the universe evolved to its present state.

"There are many problems in astro-physics having to do with neutrinos," says physicist Eric B. Norman of the Lawrence Berkeley (Calif.) Laboratory. If the heavy neutrino exists, "there must be some new physics involved."

Norman, who heads one of the teams that saw hints of this mysterious particle's existence, described his group's findings this week at a meeting of the American Physical Society in Washington, D.C.

The controversy started in 1985 when John J. Simpson of the University of Guelph in Ontario reported a tiny anomaly in his measurements of the energies of beta particles, or electrons, emitted during the radioactive decay of tritium atoms. He attributed this slight deviation from the expected result to the presence of a subatomic particle -- possibly a neutrino -- with a mass (expressed in energy units) of 17 kiloelectron-volts (keV). Although small compared with the 511-keV mass of an electron, this was substantially larger than expected for a neutrino.

Nine subsequent experiments by other research groups failed to confirm Simpson's result. But Simpson continued his work, and in 1989 he reported results from two new experiments -- one involving tritium and the other the radioactive isotope sulfur-35 -- that again pointed to a 17-keV particle.

These findings caught the attention of Norman and his co-workers. In an attempt to settle the question, the California team designed an experiment in which they measured the energy released by beta particles spit out by decaying carbon-14 atoms embedded in a germanium crystal. By housing the carbon-14 beta-particle sources inside the germanium detector, they hoped to get a cleaner, less ambiguous signal than that attainable in experiments in which the source and detector are separated.

To their surprise, the researchers detected an effect remarkably similar to that reported by Simpson. They observed a tiny "kink" in the beta-particle energy spectrum -- a deviation invisible to the eye but made apparent by painstaking statistical analysis. "As soon as we had the first month's worth of data, the effect was there," Norman says. And as the data continued to come in, the effect became more evident.

This led to a lengthy search for an explanation. "Like almost emeryone, we were extremely skepticla," Norman says. "We examined maybe two dozen possible, conventional explanations for this effect, but none of them could explain it. We [now] believe the effect is real."

Similar results with different equipment and isotopes come from Andrew Hime and N.A. Jelley of the University of Oxford in England and from physcists at the Rudjer Boskovic Institute in Zagreb, Yugoslavia. At the same time, researchers at Caltech in Pasadena and other investigators have again failed to detect the anomaly in the beta-particle energy spectrum.

Even the positive reports furnish little information about thje identity of the particle causing the apparent kink in the spectrum. If it exists, the particle must be electrically neutral, have a mass of roughly 17 keV and interact weakly with ordinary matter.

One possibility is a heavy neutrino -- perhaps even the neutrino thought to be associated with the tau particle, a massive cousin of the electron. Alternatively, the particle may be one unknown to physics. Both of these options present difficulties. Because the existence of a totally new fundamental particle has such serious ramifications, physicists have so far tended to concentrate on the neutrino hypothesis.

The particle itself remains elusive. "We are going to be doing a lot more epxeriments," Norman says, "and so are others."