For the first time, physicists say they have witnessed a subatomic particle change its “flavor.” Physicists at OPERA, run by Italy’s national nuclear physics institute, announced yesterday that they have observed one neutrino change its type, or flavor, spontaneously. The experiment solves a 50-year-old physics mystery, and may uncover some of the universe’s hidden mass.

The Mystery

Neutrinos, which come in three different flavors, can have fairly violent births: they can come into the world via nuclear reactions in the sun, particle decay, or collisions in particle accelerators. But, once formed, they seem to ignore almost everything around them, including magnetic fields, electric fields, and matter. In fact, there are trillions of them zipping through each of us every second; they go right through our bodies and keep on moving through the planet itself.

The mystery of “neutrino oscillations” began with the number of neutrinos that should be coming from the sun. Theory predicted a certain number of various flavors to arrive, but observation showed much less:

The neutrino puzzle began with a pioneering and ultimately Nobel Prize winning experiment conducted by US scientist Ray Davis beginning in the 1960s. He observed far fewer neutrinos arriving at the Earth from the Sun than solar models predicted: either solar models were wrong, or something was happening to the neutrinos on their way. [CERN]

In 1969, Bruno Pontecorvo and Vladimir Gribov theorized that the neutrinos weren’t disappearing, they were changing their flavors mid-journey. Though physicists were looking for one type, they weren’t finding what they ordered.

The Experiment

For the past three years, CERN has fired a beam of muon neutrinos (one of the kinds that fall out of decaying muons) from Geneva, Switzerland to the OPERA experiment in a laboratory near Italy’s Gran Sasso mountain. After three years and billions of billions of muon neutrinos in and muon neutrinos out, for the first time, the OPERA physicists saw something else arrive in Italy. After the 450-mile, 2.4-millisecond voyage, a muon neutrino appears to have transformed into a tau neutrino. Researchers are 98 percent sure that this is true neutrino presto-chango:

“You have to be sensitive to even one single neutrino that has been transformed,” says OPERA spokesman Antonio Ereditato at the University of Bern in Switzerland…. “It’s like a murder. You have the murder scene, but now we have found the first part of the body.” [New Scientist]

The Meaning

If this is true neutrino oscillation, this experiment does more than confirms Pontecorvo and Gribov’s theory. It means changes for the organizing theory of particle physics, called the Standard Model. The nitty-gritty of neutrino oscillation theory requires that neutrinos have mass, but the Standard Model assumes that they have none. If neutrinos do have mass, this could explain some of what cosmologists call dark matter, the 25 percent of the universe’s mass that we believe should exist, but can’t seem to find.

“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…. “Whatever exists in the infinitely small always has repercussions in the infinitely big.” [AFP]

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Image: The Detector OPERA