In what colleagues hailed as a historic landmark, 120 physicists from 23 research institutions in Japan and the United States announced on Friday that they had found the existence of mass in a notoriously elusive subatomic particle called the neutrino.

The neutrino, a particle that carries no electric charge, is so light that it was assumed for many years to have no mass at all. After Friday's announcement, cosmologists will have to take account of the possibility that much of the mass of the universe is in the form of neutrinos. The discovery will also compel scientists to revise a highly successful theory of the composition of matter known as the "standard model."Word of the discovery had drawn some 300 physicists gathered here to discuss neutrino research. Among other things, they said, the finding of neutrino mass might affect theories about the formation and evolution of galaxies and the ultimate fate of the universe. If the neutrino has sufficient mass, their presence throughout the universe might create enough gravity to slow down its present expansion.

Speaking for the collaboration of scientists who discovered the existence of neutrino mass using a huge underground detector called Super-Kamiokande, Takaaki Kajita of the Institute for Cosmic Ray Research of Tokyo University said that all explanations of the data collected by the detector except the existence of neutrino mass had been essentially ruled out.

But because the elusive particles cannot be seen, the evidence that they have mass is indirect. Neutrinos come in three types or "flavors." The data gathered by the Super-Kamiokande team during the two years the detector has operated indicate that at least one of these three known "flavors" or types can "oscillate" into one of the other flavors as it travels along at nearly the speed of light. According to the theories of quantum mechanics, any particle capable of transforming itself in this way must have mass.

Study of the neutrino particle has been glacially slow since its existence was hypothesized in 1931 by the Austrian physicist Wolfgang Pauli as a way to explain the mysterious loss of energy in certain nuclear reactions. The particle was finally discovered in 1956 by two physicists at the Los Alamos National Laboratory, Frederick Reines (for which he was awarded a Nobel Prize) and the late Clyde Cowan.