New evidence of neutrino oscillations.
Now, a 2-year experiment at the Los Alamos (N.M.) National Laboratory has produced evidence supporting the idea that neutrinos actually have a mass, albeit a small one. This finding has important implications for cosmology, because it allows neutrinos to account for much of the mass in the universe.
D. Hywel White of Los Alamos and his coworkers at 11 other institutions announced their preliminary findings last week and plan to submit them to PHYSICAL REVIEW LETTERS.
According to the standard model of particle physics, neutrinos come in three varieties: the electron neutrino, muon neutrino, and tau neutrino, along with their antimatter counterparts.
The theory says that these particles have no mass. If they do have a mass, however, neutrinos of one type would be able to transform themselves into neutrinos of another type through a process known as oscillation.
The Los Alamos experiment involved the liquid scintillator neutrino detector (LSND), which is particularly sensitive to certain neutrino transformations -- if they occur. The researchers used a particle accelerator to fire high-energy protons into water to create pions. These pions decay into muons, muon neutrinos, muon antineutrinos, and electron neutrinos, but not electron antineutrinos.
The neutrinos collide with atomic nuclei in the detector, which consists of a vat of mineral oil surrounded by an array of photodetectors. The collisions create electrons and other charged particles, which leave detectable trails of light in the liquid.
In two runs, the second completed last November, the team detected 29 events indicating the presence of electron antineutrinos, which normally would be absent. One explanation is that they were created from other types of neutrinos; this suggests that neutrinos have a mass.
Considering limits set by the LSND's sensitivity, the neutrino's mass appears to be at least 0.5 electronvolt. By comparison, an electron has a mass of about 51 1,000 electronvolts. Though small, such a neutrino mass, multiplied by the number of neutrinos in the cosmos, could contribute significantly to the universe's total mass.
At the same time, the LSND data don't necessarily rule out alternative explanations for the presence of electron antineutrinos. "With such an important result, we need more data to establish this hypothesis firmly," White says. The researchers plan to run another LSND experiment in August.