Physicists have found the first direct proof of oscillation between two of the three known types of neutrinos by discovering a tau neutrino in a muon neutrino beam. In this photo, a women passes a poster explaining the OPERA experiment at the Laboratory for High Energy Physics at the University of Bern September 23, 2011. Reuters/Pascal Lauener

Of all the known subatomic particles, neutrinos are perhaps one of the most exotic and least understood. Scientists have long suspected that these elementary particles, which are produced by the decay of radioactive elements, have a unique trait -- they can change, or “oscillate,” between their three known types, or “flavors” -- the electron neutrino, the muon neutrino and the tau neutrino.

Now, physicists have found the first direct proof of oscillation between two of the three types by discovering a tau neutrino in a muon neutrino beam produced at the European Organization for Nuclear Research (CERN). This metamorphosis was discovered by physicists working on the OPERA (Oscillation Project with Emulsion-tracking Apparatus) experiment at an underground laboratory in Italy, where the beam was captured after its 450-mile journey through the Earth’s crust.

Despite being one of the most abundant particles in the universe, neutrinos hardly interact with matter. While this characteristic allows neutrinos produced at CERN to be sent directly through the rocks below the Earth’s surface, it also means that only a very tiny fraction of the incoming particles interact with the OPERA detector. Moreover, because neutrinos have no charge, they only interact with matter through the weak force.

“The direct observation of the transition from muon neutrinos to tau has now reached for the first time the statistical accuracy of 5-sigma -- the level required for a discovery in particle physics,” Giovanni De Lellis, spokesman for Italy's National Institute for Nuclear Physics (INFN), said, in a statement released Tuesday.

Under particle physics’ stringent criteria to assess a discovery, one sigma can be a random statistical fluctuation in the data, 3-sigma counts as evidence, while a result of 5-sigma or more is ranked as a clear observation, meaning that the probability of a 5-sigma result being wrong is less than one in a million.

“So we can finally announce the discovery of the appearance of tau neutrinos in a muon neutrino beam,” De Lellis added.

Neutrinos oscillating between various flavors have an important implication -- they hint at the existence of physics beyond the Standard Model, which has so far failed to incorporate one of the four fundamental forces -- gravity.

In addition, scientists also hope that the observation of these oscillations would help them determine which type of neutrino is the heaviest and explain the current matter-antimatter imbalance in the cosmos.