For Konrad Elsener and colleagues working at the Gran Sasso National Laboratory in central Italy, the idea of looking for a needle in a haystack must seem trivial by comparison. Their aim is to detect subatomic particles known as neutrinos that are fired in a beam from the CERN particle-physics laboratory in Geneva, travel 730 km through the Earth's crust in central Europe and arrive at their detector under the Gran Sasso mountain. Making such detections will be extremely difficult because neutrinos react so weakly with other matter – the researchers expect that of the roughly 1014 neutrinos to arrive at their huge lead detector each year only one or two will interact with the nuclei in it.
The scientists working on the CERN to Gran Sasso (CNGS) project, which started up last month, are not alone in their quest. In the US, researchers working on a similar "long-baseline" neutrino experiment called the Main Injector Neutrino Oscillation Search (MINOS), some 735 km in length, have recently announced their first results. Both experiments are designed to study the mysterious phenomenon of neutrino oscillation, in which neutrinos change from one of three "flavours" – electron, muon or tau – into another as they travel through space. This phenomenon requires that neutrinos have mass, in contradiction with the Standard Model of particle physics.
Previous experiments have demonstrated the principle of neutrino oscillation. But scientists have still to prove which of the different types of oscillation are possible – i.e. which flavours can oscillate into which other flavours. They also have a very limited understanding of some of the basic properties of neutrino oscillation. In particular they would like to know whether neutrinos and antineutrinos oscillate differently. Such a difference could point to a fundamental contrast in the behaviour of matter and antimatter, which might explain why today the universe appears to consist solely of matter – even though equal quantities of matter and antimatter were believed to have been created in the Big Bang.
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