Late last year, physicists at the giant LEP accelerator in the 18-nation European Laboratory for Particle Physics (CERN) near Geneva began furrowing their brows. The four experiments set up around the 27-kilometer-long LEP tunnel had reported their values of the mass of Z°, one of the two particles that transfer the weak nuclear force. The value obtained by the OPAL experiment, in which Weizmann Institute is a major participant, deviated significantly from the average mass measured by the other three.
According to Prof. Giora Mikenberg, who leads the ten-man Weizmann team at OPAL and serves as the experiment's Senior Scientific Coordinator, the deviant results had a whole staff on edge and led to a major project to determine what was wrong. Among the factors that were checked was the measurement of accelerator beam energy.
In a finding that intrigued scientists and romanticists worldwide, beam energy was discovered to vary with the time of day and with the phase of the moon. In fact, beam energy values precisely mirrored the changes in the moon's tidal force. The researchers concluded that the gravitational tug of the moon deforms the land in which the circular tunnel is buried, shortening its circumference by about 1.5 millimeters. Even such a slight change can account for a miscalculation of 10 million electron volts of beam energy. This was the first detection of the effects of tidal forces on giant accelerator operation, a phenomenon that has attracted wide attention in the scientific and popular press and is now being taken into account by all large accelerator facilities.
However, this important astronomical effect failed to explain the skewed results of OPAL: tidal forces would affect all four LEP experiments to the same extent. Further study showed that the OPAL problem centered around an overlooked design failure.
The detector's experimental hall is located between two sets of copper cavities used to accelerate the particles. Because the cavities were not properly positioned with respect to the frequency of radio waves used in the LEP accelerator, the energies of the colliding beams were unequal at that point.
"Now, not only do we better appreciate the effects of the moon on our experiments," says Prof. Mikenberg, "but we also have the copper cavity error well under control".