An international team of physicists announced yesterday that it had captured bizarre behavior among some of the smallest and most elusive particles in the universe, expanding science's view of the ultimate structure of matter.

The findings, released at a scientific conference in Canada and at a seminar at the University of Pennsylvania, one of the participants in the project, help solve a 35-year-old puzzle: Why are neutrinos disappearing?

The minuscule particles, which the physicists yesterday confirmed carry a tiny amount of mass, are known to be produced in huge quantities by nuclear reactions in the sun and shot in all directions, including toward Earth. But two-thirds of the neutrinos predicted to arrive never do.

The reason appears to be that electron neutrinos - the only one of the three kinds of neutrino that the sun is capable of producing - manage to transform themselves en route into so-called muon or tau neutrinos. These more exotic varieties are more difficult to detect and had not been picked up in most earlier experiments.

The discovery that these particles can switch identities promises to lead to a deeper understanding of the cosmos. Neutrinos are one of the fundamental entities making up the universe, but are by far the least understood. Predictions of their properties fit into a number of exotic theories, including some that postulate unseen dimensions of space.

The finding "says there's interesting physics beyond our current knowledge," said physicist Eugene Beier, who led Penn's efforts in the international collaboration.

To physicists, the identity-switching that was detected at the new neutrino observatory in Sudbury, Ontario, implies that these near-nonentities have mass - but one so small, about 1/250,000th of the mass of the next-lightest particle, the electron, that the current picture of physics cannot explain it.

"The reason physicists are so excited about neutrinos is that this is the first confirmed deviation from the standard model," said John Bahcall, a physicist at the Institute for Advanced Study in Princeton. "It may point us in the right direction for the next step forward."

What neutrinos lack in mass, they make up for in numbers. So many neutrinos come from the sun that 100 billion of them pass through a thumbnail - they are not stopped by matter - every second. So many more come from other stars, and from the Big Bang, that scientists once thought neutrinos alone could carry enough mass to determine whether the universe would expand forever or collapse into a big crunch. Now, it appears they could not possibly carry enough mass to stop the universe's expansion.

Three years ago, an experiment in Japan offered the first evidence that neutrinos, in this case from cosmic rays from space, could switch identities. Physicists around the world wanted to confirm this finding as well as test whether these so-called oscillations could account for the missing neutrinos from the sun.

The new experiment, called the Sudbury Neutrino Observatory, used a 100-foot-tall chamber blasted out of a nickel mine several hundred miles northwest of Toronto. It took 10 years to build and was buried 11/4 miles below solid rock, where few particles besides neutrinos could interfere with the measurements.

The results, detailed at a physics conference in Victoria, British Columbia, depended, in part, on a comparison of the Sudbury harvest of neutrinos from the sun with a harvest from the Japanese experiment, which also collected some of the sun's neutrinos.

The Japanese experiment collected mainly electron neutrinos, as well as a few of the muon and tau neutrinos, but could not distinguish between them. The Sudbury experiment sees just the electron neutrinos - the only kind the sun should be capable of making. So by calculating and comparing the two totals, the Sudbury group showed that a few of those caught by the Japanese had apparently transformed themselves into the muon or tau neutrinos.