Scientists in Europe say they have 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 say.
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% 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," says Professor 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 says.
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% of the universe.
"Whatever exists in the infinitely small always has repercussions in the infinitely big," Ereditato says.
"A model which could explain why the neutrino is so small without vanishing will have profound implications for the understanding of our universe - how it was, how it evolved, and how it will eventually die."
The transformation of the neutrino occurred during a programmed journey from Geneva to the Gran Sasso Laboratory near L'Aquila in central Italy.
The European Organization for Nuclear Research (CERN) provided a laser-like beam composed of billions upon billions of muon neutrinos that took only 2.4 milliseconds to make the 730-kilometre trip.
The rarity of neutrino oscillation, coupled with the fact that the particles interact only weakly with matter, bedevil scientists.
Unlike charged particles, neutrinos are not sensitive to the electromagnetic field normally used by physicists to bend the trajectory of particle beams.
They can also pass through matter, and thus keep the same direction of motion from their inception.
It took nearly four years from the time the beam was switched on to witness the muon-to-tau metamorphosis.