Detector spots solar chameleons. (Physics).

The ease of the missing solar neutrinos is solved--again. New data from the deep subterranean Sudbury (Ontario) Neutrino Observatory (SNO) indicate that the sun produces all the neutrinos that solar theory says it should. Even so, that spells trouble for the prevailing theory, or standard model, of particle physics.

The wispy subatomic particles known as neutrinos come in three types, or flavors--electron neutrinos, muon neutrinos, and tau neutrinos--but the sun emits only electron neutrinos. In searches for that flavor since the 1960s, solar observers have always detected fewer than expected.

The new SNO finding implies that neutrinos only seemed to be missing. What's really been going on is that electron neutrinos change, or oscillate, into one or both of the other two neutrino flavors en route from the sun's core. However, to oscillate, neutrinos must have mass and the standard model says they're massless.

"We now really know that the standard model is wrong," says John F. Wilkerson of the University of Washington in Seattle, a member of the SNO team.

Last June, SNO scientists presented suggestive evidence that solar neutrinos oscillate (SN: 6/23/01, p. 388). Since then, the researchers have transformed their observatory into the world's only detector that simultaneously counts electron neutrinos and total neutrinos. With those counts, they say, there's no reasonable doubt that solar-neutrinos change flavors.--P.W.