Is the neutrino as changeable as a weather vane?

We conclude the first part of our feature on the CNGS project with a sneak preview of next week's articles.

The neutrino is something of a headache for physicists, who have come to wonder whether the muon neutrino is capable of changing into a tau neutrino. This hypothesis would explain the deficit of muon neutrinos in the atmosphere.
When cosmic rays interact with the nuclei of atoms from the upper atmosphere, two kinds of neutrino are produced: muon neutrinos and electron neutrinos. Measurements have shown that there are fewer muon neutrinos than would normally have been expected. In 1998, the Super Kamiokande experiment in Japan revealed that the oscillation (or transformation) of muon neutrinos into tau neutrinos could be responsible for this shortfall, an idea which was supported, shortly afterwards, by the K2K (KEK to Kamioka) experiment.
The main purpose of the experiments at the CNGS (CERN Neutrinos to Gran Sasso) project is to demonstrate this oscillation, which is thought to occur over long distances. This explains why the 732 kilometres separating CERN and Gran Sasso are needed to detect it.
A high-energy beam will be sent from CERN to the detectors of the two CNGS experiments, OPERA and ICARUS, which will look for the appearance of tau neutrinos as the muon neutrinos travel from CERN to Gran Sasso.
If this oscillation is confirmed, it calls into question the very nature of the neutrino, which until only a few years ago was thought to be immutable. We know that there are three types of neutrino: the electron, the muon and the tau neutrino, each of which has a charged partner particle (lepton). We also now know that neutrinos have a mass, allowing them to change from one type into another.
Physicists have determined the minute difference in mass between the three types of neutrino but the exact mass of each neutrino is still not known at this point. However, it seems very likely that the electron neutrino is the lightest of the three. In any case, neutrinos are too light to make a meaningful contribution to the total mass of the Universe and influence its future.
As weakly interacting and uncharged (or neutral, as their name suggests) particles, neutrinos interact very little with matter. This means that they are capable of travelling the 730 km between CERN and Gran Sasso, through the Earth's crust, at almost the speed of light, without their trajectory or properties being modified.
The neutrinos' very weak interaction with matter means that large and heavy detectors are required. Those of OPERA and ICARUS will weigh 1800 and 3000 tonnes respectively. Among the billions of billions of muon neutrinos produced from the beam every year, only a little under twenty tau neutrinos are likely to be detected in five years... if the oscillation hypothesis proves correct. Patience is thus the name of the game!