Physicists have confirmed that neutrinos, which are thought to have played a key role during the creation of the Universe, have mass. This is the first major finding of the US-based Main Injector Neutrino Oscillation Search (Minos) experiment.  The findings suggest that the Standard Model, which describes how the building blocks of the Universe behave and interact, needs a revision. Neutrinos are believed to be vital to our understanding of the Universe. But scientists know frustratingly little about these fundamental particles.

There are three kinds - or "flavours" - of neutrinos: muon, tau and electron. To examine their properties, scientists created muon neutrinos in a particle accelerator at the Fermi National Accelerator Laboratory (Fermilab) in Illinois, US. A high intensity beam of these particles was fired through a particle detector at Fermilab, and then to another particle detector 724km (450 miles) away in a disused mine in Soudan, US.

Because they so rarely interact with matter we can shoot them straight through the Earth, and most will travel through without doing anything," explained Dr Lisa Falk Harris, a particle physicist at the University of Sussex, and a member of the Minos team.  "Of course, most of them travel right through our detectors as well, but once in a blue moon one of them will interact - about one or so per day."

The scientists' set up established that fewer particles were being detected at the Soudan site than had been sent. They had effectively "disappeared".  "What they have done is to convert into another type of neutrino," Dr Falk Harris told the BBC News website. Physicists call the process of transforming from one type of neutrino into another flavour oscillation. And to be able to perform this transformation, particle physics theory states that the particles need mass. "The fact that we see them 'disappear' and they do this little transmutation, means that they must have mass," said Dr Falk Harris.