An experiment on neutrinos that was meant to remove a thorn from the side of fundamental physics may instead have added a new one.
Neutrinos are some of the lightest known elementary particles and among the hardest to detect, since they rarely interact with other particles. Three types are known, each coming in both matter and antimatter forms. Neutrinos and antineutrinos are also notorious flip-floppers—one type constantly shifting into another.
A 1995 experiment at the Los Alamos (N.M.) National Laboratory first showed such oscillations in a lab setting. It also revealed a much higher rate of conversion between two types of antineutrinos than standard particle physics calculations had predicted. The simplest explanation was that in addition to direct conversions, one antineutrino could turn into another by first briefly changing into a proposed seventh type of neutrino—called sterile because it can't be detected directly.