A long-awaited beam of neutrinos has finally made it from Switzerland to a laboratory 730 kilometres away in central Italy. The scientists involved hope the project, first sketched out 25 years ago, will address one of the big unsolved problems in particle physics. But it may yet be upstaged by the next generation of neutrino experiments.

Massive project: the CERN particle-physics lab will beam neutrinos to Italy to see if they have mass. Credit: CERN

The project, which was switched on this week, is a collaboration between the CERN particle-physics lab, near Geneva, and the Gran Sasso National Laboratory, located 1,400 metres underground in the region of Abruzzo and run by Italy's National Institute for Nuclear Physics (INFN). A flux of neutrinos — fundamental particles that are electrically neutral — is generated by an accelerator at CERN, and beamed towards the Italian lab. Neutrinos hardly interact with matter, so they travel underground through the rock in a straight line until they hit OPERA, a 1,800-tonne detector comprising emulsion films and lead plates that will stop a handful of them.

Neutrinos are known to exist in three types, or 'flavours': muon, tau and electron. Since the late 1960s, researchers studying neutrinos reaching Earth from space have noticed that they seem to change flavour, or 'oscillate' as they travel. That is surprising because when their existence was first postulated in 1930, neutrinos were thought to have no mass; and quantum mechanics says the oscillations observed require some mass, albeit a small amount.

If neutrinos do have mass, tweaking today's standard model of the Universe to take account of it could help physicists get closer to a unified theory of fundamental forces. A neutrino with mass could also explain, at least in part, why the visible matter in the Universe forms such a tiny proportion of the total mass it is known to contain.

One way to settle the matter is to create neutrinos and fire them at distant detectors. By comparing the beam's composition at source and detector, it is possible to find out whether neutrinos oscillate along the way.

The first experiment of this kind was K2K in Japan (see 'A very lengthy business'). Between 1999 and 2004, it collected evidence that muon neutrinos — the easiest flavour to study — were disappearing between the source and final destination. A similar but more powerful experiment, MINOS, started in 2005 between Fermilab in Chicago and Soudan in Minnesota. In less than a year it saw three times as many events as K2K, and confirmed that muon neutrinos were disappearing.

We can see for the first time if new neutrinos appear.

OPERA is the third and most ambitious of these experiments: it can detect not just the absence of muon neutrinos, but the appearance of taus. “The others can only see if some neutrinos are missing and presume they have changed flavour,” says INFN president, Roberto Petronzio. “We can see for the first time if new neutrinos appear.” Next year, ICARUS, a more sensitive detector based on liquid argon, will join OPERA. Together, it is hoped the two experiments will provide definitive evidence for muon-to-tau oscillation, and determine the tau's mass.

Although the oscillation model is now widely accepted, there are still big questions. First, researchers need to rank the three flavours in order of mass. Second, they want to see whether there is a fourth kind of neutrino, which doesn't interact with matter at all. Although this type of neutrino could not be observed directly, combining results from different experiments might reveal its existence.

Great reception

For the Italian physics community, the arrival of the first neutrinos satisfies a long-held dream. As long ago as the early 1980s, Petronzio recalls, “the INFN president drew a sketch for Italian senators showing a map of Italy and Switzerland, and a line connecting CERN and Gran Sasso”. All the rooms in Gran Sasso are built so that they point to CERN.

Work on the neutrino beam got serious in the late 1990s, but funding problems slowed progress. CERN and the INFN were originally supposed to contribute equally. However, “in 2002, we decided not to have any scientific involvement in the project”, recalls Jos Engelen, CERN's chief scientific officer. CERN now acts only as 'particle provider', a role he says the lab is still happy with.

Now it is finally running, OPERA may end up squeezed between old and new generations of neutrino experiments. In a few years, two new projects, NOνA in the United States and T2K in Japan, are due to start searching for the third flavour, the electron neutrino. “That is really the missing part of the picture,” says Gary Barker, a physicist at the University of Warwick, UK, who is a member of the T2K group. “OPERA is a valuable experiment, but it will prove directly something MINOS has already shown indirectly.”

Engelen says he is confident that the Italian experiment will provide precious results, but admits that CERN's long-term commitment is not yet confirmed. “CERN will provide the beam for the next five years, then a new agreement will be made depending on the results, both from OPERA and from the other experiments,” he says. “Neutrino physics might not be among our priorities in the future.”