The neutrino is the oddball of particle physics. It has no charge and rarely interacts with other particles, but it comes in three flavors—electron, muon and tau—and madly oscillates from one flavor to the next as it travels along. For the past five years, researchers at the Fermi National Accelerator Laboratory in Batavia, Ill., have been firing beams of muon neutrinos at the MiniBooNE detector, a huge spherical tank filled with 800 tons of mineral oil, to see how many of the particles changed in flight to electron neutrinos. The first results, announced in April, mostly vindicated the Standard Model—the conventional theory of particle physics—but an unexplained anomaly in the data leaves open a more exotic possibility. Some scientists speculate that the cause of the anomaly is a new kind of neutrino that can take shortcuts through the extra dimensions predicted by string theory.
The impetus behind MiniBooNE was to follow up a previous experiment, conducted at Los Alamos National Laboratory in the 1990s, which had shown evidence for a fourth type of neutrino. Called the sterile neutrino, this putative particle would be even more elusive than the three ordinary flavors because it would not be subject to the weak nuclear force as the other particles are but would interact only through gravity. Because the existence of sterile neutrinos would challenge the Standard Model, researchers were eager to run a similar experiment to confirm or refute the findings. The results from MiniBooNE, however, were a mixed bag. For neutrinos with energies ranging from 475 million to three billion electron volts, the number of flavor oscillations nicely matched the Standard Model predictions, but at lower energies investigators found a significant excess of electron neutrinos.