Once a vexing mystery, the apparent vanishing of neutrinos on their way from a source to a detector arises because the three neutrino flavor states—electron, muon, and tau—are superpositions of neutrino mass eigenstates. If the detector is not sensitive to all three flavors, the in-flight transformation of one neutrino flavor into another can mimic a neutrino deficit. (See Physics Today, December 2015, page 16.) Those transformations, or flavor oscillations, are parameterized by the three mixing angles θ12, θ23, and θ13.
Early this decade, three experiments that track the disappearance of electron antineutrinos from nearby nuclear power plants—the Daya Bay Reactor Neutrino Experiment in China, the Reactor Experiment for Neutrino Oscillation in South Korea, and Double Chooz in France—definitively showed that θ13 was nonzero,1 albeit smaller than the other two mixing angles (see Physics Today, May 2012, page 13)....
