Fresh evidence of neutrino mass.

Created by nuclear fusion reactions inside stars and left over from the Big Bang, neutrinos are both abundant in the universe and extremely difficult to detect. Usually thought to have no mass, these elusive subatomic particles barely interact with other forms of matter.

Last year, researchers using the liquid scintillator neutrino detector at the Los Alamos (N.M.) National Laboratory reported experimental data suggesting that neutrinos actually have a mass, albeit a small one (SN: 2/11/95, p. 85).

Now, the same team has obtained additional evidence that strengthens this claim.

Because neutrinos constitute a large fraction of the number of cosmic particles, these findings have important implications for understanding the composition and evolution of the universe.

According to the standard model of particle physics, neutrinos come in three varieties: the electron neutrino, the muon neutrino, and the tau neutrino.

Each of these also has an antimatter counterpart.

The Los Alamos experiment involved firing high-energy protons into a water target to produce subatomic particles called pions. Pions decay into muons and muon neutrinos, and the muons, in turn, transform into positrons, electron neutrinos, and muon antineutrinos. A tiny fraction of the neutrinos collide with atomic nuclei in a nearby detector, which consists of a large tank of mineral oil surrounded by an array of photodetectors. These rare collisions generate electrons and other charged particles, which leave trails of light in the liquid.

Carefully screening 3 years' worth of data, the researchers identified 22 events that involved electron antineutrinos, which were not predicted by standard theory. Their presence is "very strong evidence" that a muon antineutrino can transform itself into an electron antineutrino through a hypothetical process that theorists call neutrino oscillation, says Fred J.

Federspiel of Los Alamos.

According to theory, such transformations can occur only if the particles involved have a mass. In this case, the findings suggest that the muon antineutrino or the electron antineutrino has a mass of at least 0.2 electronvolt. By comparison, an electron has a mass of about 511,000 electronvolts.

If this experimental evidence proves solid, the findings indicate that neutrinos could represent a significant addition to the calculated mass of the universe, though not enough to halt its expansion.

Federspiel and his coworkers expect to collect additional data over the next 2 years. Independent experiments are also needed to confirm the neutrino oscillations. "The situation is by no means wrapped up yet," Federspiel notes.