By Ron Cowen
In a truly transformative event, physicists have for the first time found direct evidence that a neutrino, a ghostly elementary particle that barely interacts with matter, morphs from one type into another. The finding, announced May 31 in a news release, provides additional support for the notion that neutrinos have mass, a property that requires an explanation beyond the realm of the standard model of particle physics.
Since the late 1990s, experiments such as Super-Kamiokande in Japan have indicated that neutrinos spontaneously transform themselves, or oscillate, among three varieties or “flavors”: the electron neutrino, the muon neutrino and the tau neutrino. Such oscillations indicate that neutrinos, long thought to be weightless, must have some small amount of mass.
Those experiments revealed a lower-than-predicted abundance of a certain type of neutrino compared with the number produced at the neutrinos’ source. The abundance of that type had clearly declined, but it wasn’t clear which type of neutrino they had transformed into.