Of the 12 elementary particles thought to make up all the matter of the universe, physicists have spotted 11. Now the last hold-out, the tau neutrino, may finally be in the bag. At least three of the exotic particles appear to have left their tracks in a detector at Fermi National Accelerator Laboratory (Fermilab), in Batavia, Illinois. But researchers on the experiment, called DONUT, want to find another seven or so before popping the champagne.
Neutrinos are notoriously difficult to detect--make one in the lab, and it is likely to slip insouciantly through the table, the planet, and much of the universe. These runaways come in three flavors: the electron neutrino, the muon neutrino, and the tau neutrino. Over the years, physicists have become adept at spotting electron and muon neutrinos, but no one has ever seen a tau neutrino.
Physicists, however, are pretty sure that the tau neutrino exists. For instance, a recent experiment in Japan suggested that muon neutrinos raining down from the upper atmosphere might be changing into tau neutrinos and eluding detection. To confirm the identity shift, future experiments will try to detect the tau neutrinos directly. For that reason "it's very important to confirm that one can actually see a tau neutrino first," says Carl Albright, a physicist at Fermilab and Northern Illinois University in DeKalb.
At DONUT, a dense stream of protons from Fermilab's Tevatron accelerator smashes into a tungsten target, but fewer than one collision in 10,000 produces a tau neutrino. To detect these rare particles, the team built a stack of sheets coated with silver bromide emulsion next to the collision area. When a tau neutrino plows through these sheets it has a slight chance of bumping into an atom and creating a tau particle. The tau would leave a millimeter-long track in the emulsion before decaying to other particles. Indeed, the team found what looked like the tracks of three tau neutrinos. The odds that they were the footprints of some other particle are small, says Byron Lundberg, a Fermilab spokesperson for the DONUT experiment.
"I think it is very likely that [their observations] are correct," says Hywel White, a neutrino expert from the Los Alamos National Laboratory. "Everybody believes that there is a tau neutrino, but you have to have experimental proof of it."