Image credit: Daya Bay group, University of Wisconsin.
The international Daya Bay collaboration has announced new results, including their first data on how neutrino oscillations vary with neutrino energy, which allows them to measure mass splitting between different neutrino types. Mass splitting represents the frequency of neutrino oscillation while mixing angles represent the amplitude and both are crucial for understanding the nature of neutrinos.
The Daya Bay experiment, which is run by a collaboration of more than 200 scientists from six regions and countries, is located close to the Daya Bay and Ling Ao nuclear power plants, 55 km north-east of Hong Kong. It measures neutrino oscillation using electron antineutrinos created by six powerful nuclear reactors. Because the antineutrinos travel up to 2 km to underground detectors, some transform to another type and therefore apparently disappear. The rate at which they transform is the basis for measuring the mixing angle, while the mass splitting is determined by studying how the rate of transformation depends on the antineutrino energy.
Daya Bay’s first results were announced in March 2012 and established an unexpectedly large value for the mixing angle θ13 – the last of three long-sought neutrino mixing angles. The new results, which were announced at the XVth International Workshop on Neutrino Factories, Super Beams and Beta Beams (NuFact2013) in Beijing, give a more precise value – sin2 2θ13 = 0.090±0.009. The improvement in precision is a result both of having more data to analyse and of having the additional measurements on how the oscillation process varies with neutrino energy.
The KamLAND experiment in Japan and other solar neutrino experiments have previously measured the mass splitting Δm221 by observing the disappearance of electron antineutrinos from reactors some 160 km from the detector and the disappearance of electron neutrinos from the Sun. The long-baseline experiments MINOS in the US and Super-Kamiokande and T2K in Japan have determined the effective mass splitting |Δm2μμ| using muon neutrinos. The Daya Bay collaboration has now measured the magnitude of the mass splitting |Δm2ee| to be (2.54±0.20) × 10–3 eV2.
The result establishes that the electron neutrino has all three mass states and is consistent with that from muon neutrinos measured by MINOS. Precision measurements of the energy dependence should further the goal of establishing a hierarchy of the three mass states for each neutrino flavour.
