Research team member James P. Miller of Boston University says the difference between the standard model's prediction and the new measurement is akin to a pair of kilometer-long strings differing by the length of a bacterium. One possible explanation is that the deviation indicates some new physics. Although a cousin to the electron, the muon is about 200 times as heavy. Accelerators and cosmic-ray collisions produce muons, but the particles quickly decay. That makes muons rare in nature.
Muons possess a quantum mechanical property called spin, analogous to the twirling of a top.
Because of this trait, muons behave like tiny bar magnets. Their spin also makes them into minute gyroscopes, responding to upward or downward forces by swinging the axes of spin around horizontally. To measure muon properties, the researchers at Brookhaven fired an intense muon beam into an extraordinarily uniform magnetic field. Because muons are electrically charged, the field made them travel in a circle. As speeding muons entered the magnetic field, the axes of their spins had initially pointed in the direction of the particles' overall motion.