Regenerating neutrinos.

Regenerating neutrinos

Neutrinos are another of the hard-to-detect particles ofphysics. They come in three varieties of "flavors": electron neutrinos, muon neutrinos and tau neutrinos. One of the questions before physicists in recent years is whether the flavor of a given neutrino is constant or whether it can oscillate from one flavor to another.

Neutrinos are made in the sun, in supernovas and presumablyin other astrophysical events. The number of neutrinos from the sun that are deteced on earth is only about a third of what the most widely accepted theory of the sun's energy-producing processes says there ought to be. To solve the discrepancy, some theorists have proposed that, as the neutrinos pass through the outer layers of the sun, conditions there trigger a massive flavor change. The one experiment now capable of detecting solar neutrinos, located in a mine near Lead, S.D., and operated by Raymond Davis of the University of Pennsylvania in Philadelphia can detect only electron neutrinos.

Now come Anthony J. Baltz and Joseph Weneser ofBrookhaven National Laboratory in Upton, N.Y., to complicate the matter further. They calculate that electron neutrinos that have been turned into muon neutrinos by their interactions with electrons on their way out of the sun could be turned back into electron neutrinos by similar interactions with electrons as they pass through the earth. This, Baltz told physicists at the meeting, could show up in the detectors as a day/night shutter effect.

In the daytime a detector should record few regeneratedelectron neutrinos, but at night they have to pass through the bulk of the earth and so have more chances to regenerate before reaching the detector. Davis's experiment does not seem to show such a diurnal effect, but Baltz says he doubts it is sensitive enough.

He suggests that new experiments, which will look for lower-energyneutrinos than Davis's does, may be able to see the effect.