For more than a year, physicists at the largest particle accelerator in the world, Europe's LEP, have been puzzling over small but troublesome fluctuations in the beams of electrons and positrons that whip around the accelerator's 17-mile ring. Now, at last, they have identified the culprit: the moon.

"We had assumed that something in our hardware was causing these fluctuations -- a power supply, or something," said Dr. Lyn Evans, the Welsh physicist in charge of LEP. "But after Dr. Gerhard E. Fischer at the Stanford Linear Accelerator Center in California suggested that lunar tidal effects might be responsible, we conducted experiments that proved beyond doubt that he was right."

The LEP accelerator straddles the border of France and Switzerland near Geneva; its name is an acronym for Large Electron-Positron collider. It is operated by the 18-nation European Laboratory for Particle Physics, known as CERN. Since LEP began operating in 1989, its main product has been swarms of very heavy, short-lived particles called Z-zeros, the properties of which have revealed many insights into the fundamental nature of matter. One was the discovery that the universe contains no more than three families of matter.

In a telephone interview on Wednesday, Dr. Evans said that now that the effect of lunar cycles on the energies of LEP's particle beams was known, suitable corrections could be applied to all the data that the machine produces. "From now on, high-energy physicists will need to keep almanacs and tide tables handy when they do their calculations," he said.