New signs of shadow particles.

Physicists analyzing data from a past experiment on electronlike particles called muons suspect that something unknown affected the particles' behavior. This sign of a shadowy influence has shown up in measurements of muons racing around a large subatomic-particle storage ring at Brookhaven National Laboratory in Upton, N.Y.

Immersed in a uniform magnetic field, muons are expected to wobble like spinning tops. However, in the new data analysis, physicists found that the muons wobbled faster than predicted by the standard model, the prevailing theory of particle physics.

Many physicists regard this speed-up of muon wobbles as possible evidence for a family of hypothetical particles predicted by a theory known as supersymmetry, says B. Lee Roberts of Boston University, a spokesman for the Brookhaven research team.

According to supersymmetry, every one of the known subatomic particles has a heavier, yet so-far-undetected, companion particle. Fleeting appearances of such supersymmetric partners among the Brookhaven muons would accelerate muon wobbles, theorists say.

The Brookhaven team had found an accelerated wobble in a previous analysis of 5 billion antimatter muons, or antimuons, from the same series of experiments (SN: 9/7/02, p. 158). In a Jan. 8 announcement at the laboratory, the team reported a slightly faster wobble rate for 4 billion ordinary-matter muons.

Additional measurements are needed to determine whether the observed deviations could be due to chance fluctuations, Roberts says. He and his colleagues now plan to request funding to restart the storage ring, which was shut down in 2001.