Neutrinos come in three types, or flavors: electron, muon and tau. But physicists suspect that others may be out there and that they will be weird—almost never interacting with other particles. These “sterile” neutrinos may resolve some of physics' biggest mysteries. For example, they could contribute to the befuddling dark matter that apparently pervades the universe and exerts a gravitational pull on regular matter.
Despite decades of looking, however, sterile neutrinos remain elusive, and the latest attempt to catch them in action recently turned up empty, too. Physicists running the international Daya Bay Reactor Neutrino Experiment in China, which studies neutrino behavior, found no evidence for sterile neutrinos after a seven-month-long hunt.
This particular search took place underground: Daya Bay's neutrino detectors are buried at various depths below a group of nuclear power reactors in the province of Guangdong. That is because the fission reactions that take place at the plant naturally produce lots of the antimatter counterparts of electron-flavored neutrinos. Neutrinos, strangely enough, can switch flavors in a process called oscillation, so as these antimatter particles go flying, some of them change into muon or tau antineutrinos, hitting the detectors along the way. Scientists know roughly how many of the electron antineutrinos should change into the other flavors, and they use this calculation to figure out if any electron antineutrinos are missing at the deepest detectors. Missing particles would mean the originals probably turned into sterile neutrinos.
The absence of missing neutrinos at Daya Bay “leaves no room open in this particular territory for having a sterile neutrino,” says Brookhaven National Laboratory physicist Milind Diwan, a member of the experiment's team. The results, published in October in Physical Review Letters, rule out the particles only in a certain range of masses and characteristics, however, so the ultimate truth about sterile neutrinos is still out there. Physicists at the site will continue looking for the particles within a broader range of characteristics. After all, the first 30 years' worth of searches for the Higgs boson turned up nothing.