The elusive third neutrino-mixing angle

The flavor oscillation that accounts for the apparent disappearance of neutrinos from the Sun and from cosmic-ray showers in the upper atmosphere is parameterized by two mixing angles, θ₁₂ and θ₂₃, that describe those oscillations. The third mixing angle, θ₁₃, which would describe the flavor oscillation of reactor neutrinos over distances of only a few kilometers, has long eluded measurement. Was that because it’s zero? The issue is of unusually wide interest because neutrino oscillation is the last hope, within the purview of standard particle theory, for explaining the manifest cosmic imbalance between matter and antimatter. But neutrinos would be capable of the requisite symmetry-breaking interactions only if all three mixing angles are nonzero. Using neutrino detectors like the three seen here, a China–US experiment set amidst six nuclear reactors on Daya Bay near Hong Kong has now measured the oscillation amplitude sin²2θ₁₃ to be 0.092 ± 0.017, a convincing five standard deviations from zero. Three nonzero mixing angles are necessary, but not sufficient, for matter–antimatter asymmetry. There’s one more free parameter, a complex phase factor in the neutrino mixing matrix, that will now have to be measured in accelerator-based neutrino experiments before one knows how strong the symmetry-breaking interactions are. (F. P. An et al., Daya Bay collaboration, Phys. Rev. Lett., in press, available at http://arxiv.org/abs/1203.1669.)—Bertram Schwarzschild