Phys. Rev. Lett. (in the press); preprint at http://arxiv.org/abs/1310.6732 (2013)

The three flavours of neutrino have mass and their mass eigenstates mix, the mixing being defined by a set of 'mixing angles'. The trickiest of these to pin down was the so-called θ13 mixing angle, but in 2012 the Daya Bay collaboration, based at a nuclear reactor complex in China, achieved the first measurement of this parameter, which has since been backed up by other experiments including T2K in Japan and MINOS in the USA.

Now, with the addition of more data and a refreshed analysis that exploits the three 'near' and three 'far' antineutrino detectors positioned around the reactor complex, the Daya Bay collaboration has a refined measurement of θ13, the most precise yet. Moreover, their study of the oscillation patterns of the electron antineutrinos emitted by the reactors has made possible the first direct measurement of a quantity known as the 'mass-squared difference'. Its consistency with a similar quantity measured for muon neutrinos (by MINOS) supports the three-flavour mixing model for neutrino oscillations.