CERN is set to send a beam of neutrinos through the Earth to Italy's Gran Sasso laboratory 730 kilometres away, and it will take just 2.5 milliseconds to get there. The CERN Neutrinos to Gran Sasso project is a collaboration with the Italian national institute of nuclear physics (INFN) and was approved by CERN Council in December. The INFN will bear two thirds of the 71 million Swiss franc price tag, with special contributions from CERN Member States expected to make up the balance. Scheduled to start up in 2005 - at 8 a.m. on 15 May, to be precise! - the project will shed light on subjects as diverse as how the Sun works and where some of the Universe's missing mass is hiding.

Neutrinos are elusive particles. They hardly interact with matter at all and for many years they were believed to have no mass and thus to travel the Universe at the speed of light. Recent experiments at the Japanese SuperKamiokande laboratory and elsewhere, however, have shown that they probably do have a very tiny mass. The consequences of this observation are profound. Neutrinos have been puzzling physicists for decades. Experiments designed to detect neutrinos coming from the Sun, for example, only see about half the number expected. Neutrino mass could explain why. Neutrinos come in three types, electron, muon, and tau neutrinos, and if they have mass, then theory tells us that they could change - or oscillate - from one kind to another. If neutrinos oscillate on their way from the Sun, that would explain why the solar-neutrino experiments, which are designed to look for the electron-neutrinos produced in nuclear reactions in the Sun, don't see as many as expected.

Many experiments in the past have looked in vain for neutrino oscillations, including CERN's recent CHORUS and NOMAD, but they have always looked at neutrino beams close to the source. The idea behind this new project is to give neutrinos from CERN longer to oscillate by sending the beam to detectors far away at Gran Sasso. Similar projects are also under way in the USA and Japan, but the CERN Neutrinos to Gran Sasso project is unique in that it will look for the appearance of tau neutrinos in a muon neutrino beam. The US and Japanese projects, on the other hand, will look for the disappearance of muon neutrinos. It is a subtle difference, but an important one. If tau neutrinos are detected, it will not only show directly that neutrinos oscillate, but it will also show into what new kinds of neutrinos they are transformed. Two experiments, OPERA and ICANOE, are being evaluated for Gran Sasso.

If neutrinos really do oscillate, they could go a long way towards explaining another of nature's mysteries - missing mass. When scientists count up the mass of all the visible matter in the Universe, they arrive at a total of just 10% of what we know must exist. If neutrino oscillation can clearly be observed, it will provide compelling evidence that neutrinos really do have mass. And since there are so many neutrinos around, their mass, however small, would account for at least part of the Universe's missing mass. With weighty questions such as these at stake, the CERN Neutrinos to Gran Sasso project promises to open up an exciting new era in physics research, at 8 a.m. on 15 May 2005 - plus 2.5 milliseconds !