Neutrinos are one of the fundamental particles of matter. They are similar to electrons but are electrically neutral and are not therefore affected by electromagnetic forces. They can pass through large distances in matter without hindrance and are thus very difficult to detect. Three types -- or “flavours” -- of neutrino are known, the electron, muon and tau neutrinos. Anti-neutrinos are the anti-particle equivalents of neutrinos and can be created in fission reactions in nuclear power plants.

The current research was performed at the Kamioka Liquid scintillator Neutrino Detector (KamLAND), which is located in an old mine near Toyamu in Japan. The detector comprises a balloon 13 metres in diameter that contains about a 1000 tonnes of liquid scintillator. It identifies anti-neutrinos by counting the number of telltale flashes of light that are produced when anti-neutrinos collide with protons in the liquid.

The researchers recorded anti-neutrino events produced by electron anti-neutrinos from Japanese and South Korean nuclear reactors. The team, which consists of 92 scientists, detected 54 electron anti-neutrino events over a period of 145 days as opposed to the approximately 86 predicted by the Standard Model of particle physics– which assumes that neutrinos and anti-neutrinos have no mass and therefore do not oscillate.

These results confirm earlier work -- at the Sudbury Neutrino Observatory and Super-Kamiokande -- that also provided strong evidence for neutrino oscillation. The KamLAND researchers are now certain that the “solar neutrino problem” -- the fact that fewer neutrinos are observed from the Sun than predicted -- is indeed due to neutrino oscillation and not because of some interaction the neutrinos may have had with the Sun’s magnetic field, as some researchers suggested.

“The main result is that we now have the mechanism for oscillations firmly in hand,” Giorgio Gratta, the co-spokesperson for the US collaboration told Physics Web. The researchers have now turned their attention to the “mixing angle” -- a quantity that is related to physics beyond the Standard Model and determines just how the neutrinos oscillate. The mixing angle is important as neutrinos with a large mixing angle are much less affected by matter than those with a small mixing angle. KamLAND has ruled out small mixing angles -- in the context of two-flavour neutrino oscillations.

Theoretical physicists hope that this new information will help refine the Standard Model. “[It] needs to be updated to include neutrinos masses and mixing, " says Gratta.