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. The findings build on work carried out by Japanese physicists.
Neutrinos are sometimes described as "ghost particles" because they can pass through space, the Earth's atmosphere and the Earth itself with almost no interaction with normal matter. This makes studying them very difficult. 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. Fewer neutrinos arrived at the detector in Soudan than expected.
"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 added.
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.
These are the first results from the Minos experiment, which has involved scientists from 32 institutions in six countries. It confirms the earlier observations of neutrino "disappearance" found in 2002 by the Japanese K2K experiment, where scientists fired muon neutrinos at a detector situated 240km (150 miles) away.
The corroboration that the neutrino has mass has profound implications for particle physics. "In particle physics there is the Standard Model which describes how the fundamental building blocks of matter behave and interact with each other," explained Dr Falk Harris.
"And this model tells us that neutrinos should have no mass. So the fact that we have now got independent measurements of neutrinos saying that they must have mass, means that this Standard Model is going to have be revised or superceded by something else."
In the longer term, the findings may also help us to better understand the mystery of "missing mass" in the Universe.
"Various observations show there appears to be much more mass in the Universe than is visible," said Professor Jenny Thomas, a particle physicist at University College London, and a member of the Minos team.
"We are surrounded by neutrinos, so in every cubic centimetre there are hundreds at any instant. "To put it simply, if they are heavy, it means that there is a lot more mass in the Universe than we thought there was."
Neutrinos are also thought to have played an important role in the formation of the Universe. The Minos findings and future ones may help to shed light on how matter formed, and why so much of the Universe's antimatter has disappeared.
