For particles with almost no mass, neutrinos are making quite a splash. On 18 June, scientists from Canada, the United States, and the United Kingdom announced that they had spotted neutrinos that had been missing for 3 decades.
The mystery of the missing neutrinos emerged in the late 1960s, when physicists calculated the number of relatively energetic neutrinos that should be streaming from the sun--due to the decay of boron-8 cooked up in the solar furnace--but detection experiments came up short. There were too few neutrinos.
Now experiments from the Sudbury Neutrino Observatory (SNO) have cleared up the mystery, validating a solution that earlier experiments had pointed to: flavor changing. Neutrinos come in three flavors, named after the particles they are linked with. Electron neutrinos are the type produced by the sun; muon and tau neutrinos exist as well, though they are harder to detect. In the late 1990s, experiments provided fairly strong evidence that electron neutrinos turn into muon and tau neutrinos as they stream away from the sun--something that can only happen if the particles have mass (Science, 4 July 1997, p. 30). The "missing" neutrinos from the sun had merely transformed into muon and tau neutrinos and escaped detection.
Buried 2 kilometers underground in a nickel mine in Ontario, SNO has just given a resounding confirmation to this picture. The detector measures the neutrinos coming from the sun in two ways. The first method spots the recoil of a neutrino off of an electron. Any of the three flavors of neutrino could potentially cause such a recoil and be detected. However, the second method detects when an electron neutrino strikes a neutron inside a 1000-ton sphere of heavy water. Only an electron neutrino can make the neutron spit out an electron, triggering the detector. The two methods, combined with another detector's results, reveal that muon and tau neutrinos emerge en route from the sun to the Earth, says SNO project director Art McDonald, of Queen's University in Kingston, Ontario--enough to account for the missing electron neutrinos with a 99% confidence level.
"I'm thrilled by the precision of the result; I'm thrilled it agrees with the solar model calculations; I'm thrilled we have an answer to the problem," says John Bahcall, a physicist at the Institute for Advanced Study in Princeton, who is not a member of the SNO team. "I'm thrilled like someone who was accused of a heinous crime, always insisted on his innocence, and now a new DNA technique proves it."