Scientists in Europe announced they had likely solved the case of the missing neutrinos, one of the enduring mysteries in the subatomic universe of particle physics.

If confirmed in subsequent experiments, the findings challenge core precepts of the so-called Standard Model of physics, and could have major implications for our understanding of matter in the universe, the researchers said.

For decades physicists had observed that fewer neutrinos - electrically neutral particles that travel close to the speed of light - arrived at Earth from the Sun than solar models predicted.

That meant one of two things: Either the models were wrong, or something was happening to the neutrinos along the way.

At least one variety called a muon-neutrino was actually seen to disappear, lending credence to a Nobel-winning 1969 hypothesis that the miniscule particles were shape-shifting into a new and unseen form.

Now scientists at Italy's National Institute for Nuclear Physics have for the first time observed - with 98 percent certainty - what they change into during a process called neutrino oscillation: Another type of particle known as tau.

"This will be the long-awaited proof of this process. It was a missing piece of the puzzle," said Antonio Ereditato, a researcher at the Institute and spokesman for the Opera group that carried out the study.

"If true, it means that new physics will be required to explain this fact," he said by phone.

Under the prevailing Standard Model, neutrinos cannot have mass. But the new experiments prove that they do.

One implication is the existence of other, as yet unobserved types of neutrinos that could help clarify the nature of Dark Matter, which is believed to make up about 25 per cent of the universe. "Whatever exists in the infinitely small always has repercussions in the infinitely big," Mr Ereditato said.