Neutrinos come in three flavours - electron, muon and tau neutrinos. According to the Standard Model of particle physics they have zero mass and only interact weakly with matter, which makes them very difficult to detect. However, muon neutrinos can change or "oscillate" into electron or tau neutrinos, and so on. Last year the Super-Kamiokande team announced that they had strong evidence to show that atmospheric muon neutrinos - which are created when cosmic rays collide with nuclei in the atmosphere - can oscillate into tau neutrinos as they pass trough the Earth (see Super-Kamiokande finds neutrino mass).

The advantage of accelerator-based experiments, such as K2K, is that the initial flux of neutrinos can be measured at the accelerator and then much further away. This is not possible in experiments with atmospheric neutrinos. The neutrino source at KEK produces muon neutrinos. If the flux detected at Super-Kamiokande is lower than that at KEK, that is evidence for neutrino oscillations.

The Super-Kamiokande experiment consists of 50000 tons of water surrounded by hundreds of photon detectors. It is placed 1000 metres below ground in a lead and zinc mine. A tiny fraction of electron and muon neutrinos give off faint flashes of light known as Cerenkov radiation when they interact with electrons in the water molecules. This allows them to be distinguished from tau neutrinos.

There are also plans to send neutrino beams from the CERN particle physics lab in Geneva to the Gran Sasso underground lab some 730 km away in Italy, and from Fermilab near Chicago to the Soudan experiment, 710 km away in Minnesota.