CERN* is collaborating with the National Institute of Nuclear Physics (INFN) in Italy to send a beam of neutrinos through the earth, under the mountains from Geneva in Switzerland to the Gran Sasso laboratory in central Italy, 730 km away. The experiments will shed light on the possibility that neutrinos have mass and exhibit the exotic property of transforming from one kind into another. The go-ahead for the CERN Neutrinos to Gran Sasso Facility was given at the CERN Council Meeting on Friday 17th December following approval of the project by INFN. Prof. Luciano Maiani, CERN Director General said,"The approval of the CERN Neutrinos to Gran Sasso Facility is a tremendous boost to Europe's strong community of neutrino physicists." The data taking for the experiments at Gran Sasso is planned to begin on 15th May 2005. Over half of the 71 million Swiss francs required for the project, in addition to the equipment valued at 22 million Swiss francs already existing at CERN, are being provided by INFN. Voluntary contributions from Belgium, France, Germany and Spain complete the funding for the new facility.
Neutrinos are elusive particles. They travel at the speed of light, interact hardly at all with matter, and for many years it was believed that they have no mass. There are three kinds of neutrinos, this was precisely determined at CERN's Large Electron-Positron Collider (LEP) in 1989. There is the electron neutrino, associated with the electron, the muon neutrino, associated with the muon (a muon is a heavier version of the electron), and the tau neutrino, associated with the tau (a tau is like the muon and electron, only heavier still). The electron neutrino and muon neutrino have both been discovered, but the tau neutrino has never been seen. However, the existence of an associated neutrino can be inferred from the known properties of the tau.
Wolfgang Pauli first predicted the existence of neutrinos in 1930, and even before they were discovered in 1955, it was being postulated by Bruno Pontecorvo that they might transform, or "oscillate", from one type into another. Indications of this phenomenon first came from the observation of the deficit of neutrinos coming from the sun. The sun produces electron neutrinos, which perhaps are not disappearing, but transforming into other types of neutrinos that escape detection, for example muon or tau neutrinos.
When cosmic rays pass through the Earth's atmosphere, a known ratio of electron and muon neutrinos is formed. The SuperKamiokande experiment in Japan measured the change in this ratio depending on the distance that the neutrinos had travelled before detection. It was found that the further the neutrinos had travelled through the Earth before detection, the lower was the proportion of muon neutrinos compared to electron neutrinos. This important result provided a clue to what might be happening. If the oscillation was occurring only over very long distances, then more of the neutrinos would have a chance to transform, perhaps into tau neutrinos, thus seeming to disappear, when they travelled a long distance through the planet.
If neutrinos really are oscillating, this could go a long way to explaining another mystery that the mass of all the visible matter in the Universe only adds up to a small proportion of the total mass value derived from the observed dynamics at work in galaxies. Physicists have long been wondering where the majority of the mass in the Universe could be. All of our standard physics theories so far assume that neutrinos have no mass, since they hardly interact with matter, and nobody has ever been able to measure a mass for them. But if they are oscillating, then theory dictates that they must have mass. It is probably only a very tiny mass, but there are so many neutrinos around in the universe (almost a billion times as many as there are protons!) that together they could account for at least as much mass as exists in visible stars.
The next step is to observe the behaviour of neutrinos over a long flight path under the controlled conditions of an accelerated beam. The Japanese are conducting such an experiment over 250 km from the KEK laboratory to the SuperKamiokande detector, and the Fermi National Accelerator Laboratory (Fermilab) in the United States are well advanced in preparations for a long range beam of neutrinos, to travel 730 km from their laboratory in Illinois to the Soudan Underground Laboratory in Minnesota. What these projects have in common is that they both have a beam and an experiment optimised to look for the disappearance of muon neutrinos in controlled conditions.
The CERN Neutrinos to Gran Sasso facility is complementary to these experiments but is optimised to the identification of tau neutrinos into which the muon neutrinos are supposed to change. If tau neutrinos are detected, this would not only show directly what the neutrinos are oscillating into, but would also be the first time that this kind of neutrino has been seen. At the moment there are two experiments in preparation at Gran Sasso, OPERA and ICANOE, which will enable the start of data taking in 2005.
CERN makes the neutrinos by smashing high energy protons against a target. The large amount of energy produced in the crash results in the formation of multitudes of particles, including particles called pions. The pions are focused by magnets and allowed to decay in a beam pipe which points towards Gran Sasso. Pions decay into muons and neutrinos, which both come out of the end of the pipe. The muons, whose in flight decay provides more neutrinos, are stopped within 800 metres by iron and earth shields, leaving only the neutrinos to carry on to reach the detector. The resulting neutrino beam is about 1 km wide on arrival at Gran Sasso, while the detectors of OPERA and ICANOE are a few metres wide. The pointing accuracy which can be obtained with well established methods already in use at CERN, is such that the beam on arrival at Gran Sasso can be positioned to within 40 metres.
The facility has been specially designed to maximise the intensity of the beam - it will send around 1018 muon neutrinos from CERN to Gran Sasso every year. This is necessary because neutrinos interact so weakly with matter that the vast majority of them will fly straight through the detectors in the same way as they travelled unimpeded from CERN to Gran Sasso. Only about 2500 of those neutrinos will interact with a 1000 ton target. If the results from SuperKamiokande are correct, tens of the neutrinos interacting with the Gran Sasso detectors are expected to have transformed into tau neutrinos, although the exact number depends on what the neutrino mass turns out to be. For this signal to be seen clearly, the background (events which look like a tau neutrino from CERN interacting with the detector, but are actually a false alarm) needs to be very low. Consequently, the experiments have been optimised to give an expected background of only one event every two years. This is one advantage of the location of the experiments deep under the mountains at Gran Sasso, as it provides extra protection against cosmic rays.
CERN is sending the neutrinos to Gran Sasso in pulses. There is one pulse every few seconds, each pulse lasts 13 microseconds and contains in the order of 1012 neutrinos. From Geneva, the neutrinos will travel in a straight line through the earth, below Mont Blanc, Aosta, Alessandria (where they cross the river Po), the Appenine Chain (where they reach their maximum depth of about 8 km), Florence (at a depth of about 4 km) and Assisi, before finally emerging at Gran Sasso. This is truly the quickest method of international travel, at the speed of light it takes the neutrinos only 2.5 milliseconds to get from CERN to Gran Sasso!
Hopefully this long distance project will provide answers that are similarly far-reaching. Detecting the tau neutrino and measuring its mass would surely open a new era of particle physics.
For further information from INFN contact :
Prof. Alessandro Pascolini
Tel +39 049 827 7201
Fax +39 049 827 7208