Exotic Particle Caught After It Changes Flavor

For the first time, physicists have caught a sneaky particle called a neutrino after the act of changing from one flavor to another.

Neutrinos are elementary particles that come in three types, or flavors: electron neutrinos, muon neutrinos and tau neutrinos. In the new study researchers observed a single neutrino that had transformed from muon-type to tau.

The changeling neutrino appeared in a beam of muon neutrinos after over three years of steady emission. The spray of neutrinos was created at the European Organization for Nuclear Research (CERN)'s Super Proton Synchrotron particle accelerator in Geneva, Switzerland, and sent to the Gran Sasso laboratory, run by Italy's National Institute for Nuclear Physics, about 450 miles (730 kilometers) away.

Neutrinos are created inside the sun and in radioactive decay reactions. Because they don't often interact with other particles, neutrinos constantly rush through our bodies and the Earth in a straight path from the sun every second.

Since the particles contain no electric charge and pass through matter unaffected, they are nearly impossible to detect (of course they are too miniscule to see).

An additional challenge, unachieved until now, has been observing the appearance of a neutrino that has changed from one family to another, a process that occurs naturally as they propagate through space.

"This is like a sinusoidal curve, it goes up down, up down, up down, etcetera," researcher Antonio Ereditato, of the Italian National Institute for Nuclear Physics, told Livescience. "So you have to be clever in catching them in the right place, where they most[ly] are tau neutrinos. Otherwise if you wait too much, then they will become muon neutrinos again."

The experiment was part of a project called OPERA (Oscillation Project with Emulsion-tRacking Apparatus). From its initial source in Switzerland, a pulse of neutrinos took 2.4 milliseconds to travel to a detector in central Italy buried in a cavern thousands of feet below ground.

When a neutrino interacts with a detector, it usually transforms into the particle that matches its flavor, explained Ereditato, also affiliated with the University of Bern in Switzerland. For example, a muon neutrino transforms into a plain muon, a particle that travels for several meters before it decays. Where muon neutrinos are nearly mass-less, muons have about 200 times the mass of an electron.   

In contrast, a tau particle, the end product of a tau neutrino interaction with the detector, travels only 0.08 inches (2 millimeters) before it vanishes.  

"Detecting a particle which leaves a track of only two millimeters is a nightmare," Ereditato said. "We managed to do this."

Despite solid acceptance by scientists, the ability of neutrinos to shift in and out of different flavors contradicts the overarching theory, which explains the relationship between fundamental forces and particles. According to the theory of quantum mechanics, neutrinos can only oscillate and change flavors if they have a mass, but the so-called Standard Model theory requires that neutrinos do not have a mass, Ereditato explained.   

Out of a thousand "normal" muon-neutrino interactions analyzed in detail, the researchers found only one tau interaction. Observing this interaction is very rare, Ereditato explained, because it relies on "threefold good chance," including: the chance of being in the right place for the oscillation; the chance of that neutrino, out of many billions, being one of the few that interacts with the detector; and the chance that the detector is efficient enough to notice the interaction.

Of the many billions of neutrinos that were sent from the Super Proton Synchrotron, 5,000 interacted with the OPERA detector, 1,000 were studied in detail, and only one has a 98-percent chance of being a real tau neutrino.

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