Asian experiments unlock neutrino oscillation mystery

Two reactor experiments, China’s Daya Bay and Korea’s RENO, have made the best measurement of the neutrino mixing angle, θ₁₃, an essential property for neutrino research. The discovery of a non-zero θ₁₃ at approximately 9˚ – which was published in March¹ and April² this year – completes our picture of neutrino mixing. This quite large value for the mixing angle will make it easier to conduct future long baseline neutrino experiments. This, in turn, may lead to a better understanding of the matter-antimatter asymmetry seen in the Universe.

Neutrino oscillations – the change in flavour seen as neutrinos move – were discovered in Japan in 1998. The three mixing angles – θ₁₂, θ₂₃, and θ₁₃ – describe the relationship between certain flavour and mass states of neutrinos. They can be seen as the Euler angles between flavour (_e, _μ, _τ) and mass (₁, ₂, ₃) states, considered as two sets of orthogonal axes. θ₁₂ and θ₂₃ were known – and large – but before now, we had only hints at the smallest value, θ₁₃³.

Reactor experiments measure the flux near and at a distance of around 2 km, this being its disappearance probability proportional to θ²₁₃. Accuracy and sensitivity demand a redundant number of source-detector pairs: six reactors and six detectors for Daya Bay, and six reactors with two detectors at RENO.

In addition, the T2K experiment in Japan is searching for the “appearance” of _e, at a distance of 295 km, from an accelerator-produced _μ beam. The probability is also, in the first order, proportional to θ²₁₃. A 2.5-sigma evidence was obtained in 2011⁴, before the Tōhoku earthquake brought the experiment to halt. Having now been fully repaired, T2K is expected to produce results later this year.

1. An, F. (2012-4) Observation of Electron-Antineutrino Disappearance at Daya Bay. Physical Review Letters, 23(17), 26. DOI: 10.1103/PhysRevLett.108.171803 [↩]
2. RENO Collaboration, & J. K. Ahn. (2010) RENO: An Experiment for Neutrino Oscillation Parameter theta_13 Using Reactor Neutrinos at Yonggwang. . arXiv: 1003.1391v1 [↩]
3. Fogli, G., Lisi, E., Marrone, A., Palazzo, A., & Rotunno, A. (2011) Evidence of θ_{13}>0 from global neutrino data analysis. Physical Review D, 84(5). DOI: 10.1103/PhysRevD.84.053007 [↩]
4. T2K Collaboration, K. Abe, N. Abgrall, Y. Ajima, H. Aihara, J. B. Albert, C. Andreopoulos, B. Andrieu, S. Aoki, O. Araoka…. (2011) Indication of Electron Neutrino Appearance from an Accelerator-produced Off-axis Muon Neutrino Beam. Phys.Rev.Lett.107:041801,2011. arXiv: 1106.2822v2 [↩]

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