Neutrinos come in three flavours - electron, muon and tau neutrinos - that are assumed to have no mass in the Standard Model of particle physics. However, the evidence for neutrino mass is now so strong that this model needs to be revised.

Large numbers of electron neutrinos are produced by the Sun, and all three flavours are generated in supernova explosions. However, neutrinos are extremely difficult to detect because they are electrically neutral and only interact with other matter through the weak interaction.

Antineutrinos are the antiparticle equivalents of neutrinos and can be created in fission reactions in nuclear power plants. The Kamioka Liquid scintillator Neutrino Detector (KamLAND) is only sensitive to electron antineutrinos, and previous measurements at the facility have shown that it detects fewer of these particles than it would if they did not oscillate into other flavours.

In the current study, the KamLAND team plotted the number of antineutrinos detected as a function of L/E, where L is the distance travelled by the neutrino and E is the energy measured in the detector. According to Stuart Freedman, who is the spokesperson for KamLAND's US team, L/E can be viewed as being proportional to time in the rest frame of the antineutrino.

The oscillations in the graph show that the antineutrinos disappear, and then reappear again. Moreover, the shape of the plot is consistent with neutrino oscillation and inconsistent with a no-oscillation hypothesis and two other models that seek to explain the disappearance (see figures).

The results confirm earlier work at the Sudbury Neutrino Observatory in Canada and Super-Kamiokande in Japan that also provided strong evidence for neutrino oscillation. "We are moving away from an exploratory era of neutrino physics to an era where we actually measure detailed parameters of neutrino oscillation," Freedman told PhysicsWeb. "This is important in light of how the Standard Model will have to be amended."