Number 213 (Story #2), February 7, 1995 by Phillip F. Schewe and Ben Stein INDIRECT EVIDENCE FOR NEUTRINO MASS comes from a Los Alamos experiment in which muon antineutrinos are perhaps transmuting into electron antineutrinos in a process called "neutrino oscillation." Los Alamos uses a proton beam to produce pions whose decays result in streams of various daughter particles, including muon antineutrinos. The pion decay process does not produce any electron antineutrinos, so any that turn up further downstream must, the researchers believe, come from the metamorphosis of another neutrino type, probably muon antineutrinos. Neutrinos, regardless of their type, interact very feebly. During the five months of data taking, the Los Alamos scientists looked for rare interactions in which the newly minted electron antineutrino enters the reaction vessel (filled with 180 tons of mineral oil) and collides with a proton, creating a positron and a neutron. The apparatus is designed to search for characteristic light (Cerenkov radiation) from the positron; meanwhile, the 2-MeV neutron eventually combines with a proton to make a deuteron and a gamma ray. From the sample size one can calculate the oscillation rate. From that, one can infer not a value for neutrino mass directly but rather the difference of the squares of the masses for the two neutrino species. Current theoretical models hold that if oscillation is occurring, at least one of the neutrino types has mass. According to D. Hywel White and William Louis of Los Alamos, the observed rate of electron antineutrino interactions suggests a neutrino mass range of 0.5 and 5 eV. The results are not statistically sufficient to settle the issue of neutrino mass and more tests are needed. The issue is important for particle physicists and for cosmologists, who suspect that neutrinos with even a very small mass may play a role in organizing matter into galaxies.