Tiny particles baffle physicists, again.

An experiment on neutrinos that was meant to remove a thorn from the side of fundamental physics may instead have added a new one.

Neutrinos are some of the lightest known elementary particles and among the hardest to detect, since they rarely interact with other particles. Three types are known, each coming in both matter and antimatter forms. Neutrinos and antineutrinos are also notorious flip-floppers--one type constantly shifting into another.

A 1995 experiment at the Los Alamos (N.M.) National Laboratory first showed such oscillations in a lab setting. It also revealed a much higher rate of conversion between two types of antineutrinos than standard particle physics calculations had predicted. The simplest explanation was that in addition to direct conversions, one antineutrino could turn into another by first briefly changing into a proposed seventh type of neutrino--called sterile because it can't be detected directly.

On April 11, researchers at the Fermi National Accelerator Laboratory in Batavia, Ill., announced the results of an experiment that looked for similar anomalous conversion rates in neutrinos.

The researchers had assumed that neutrinos and antineutrinos would oscillate the same way. In the energy range explored in the previous experiment, the Fermilab results showed no anomaly in their conversion rate. Either the Los Mamos results were wrong, or they must be explained without the sterile neutrino.

However, the new experiment found anomalous conversion rates at lower energies.

"We firmly stand behind the [Los Alamos] data," says Bill Louis, who was a lead researcher in that experiment and is also in the new one. Solutions of the dilemma that don't rest on sterile neutrinos range from a new and unforeseen difference in the behavior of matter and antimatter to the existence of additional dimensions of space.--D.C.