科学美国人60秒:Neutrino Identity Switch Confirmed

Neutrinos fill the cosmos. Trillions of these subatomic particles pass clean through your body every second. That’s because they rarely interact with solid matter. Which is weird enough, but neutrinos are shifty in another way as well.

Neutrinos come in three flavors, called electron, muon and tau. But they are known to be flexible. Theory said that a muon neutrino could almost instantly change into an electron neutrino. And now the transformation has been empirically verified.

At an experiment in Japan called T2K, a particle accelerator fires a beam of muon neutrinos at a giant detector on the opposite coast, some 185 miles away.

In 2011 T2K physicists reported data suggesting that some of the particles had changed identities in flight. Now the data are conclusive. Without in-flight transformations, only about five incoming neutrinos would be the electron flavor. In reality, 28 electron neutrinos arrived. 

Next up for T2K and its competitors is exploring whether neutrinos and their antimatter counterparts change flavors in the same way. If they do not, neutrinos may help explain why our universe is made of matter and not antimatter.

—John Matson