Muon orbits may defy main physics theory.

By measuring a magnetic trait of electronlike muon particles, scientists have uncovered evidence of a possible crack in the prevailing theory of physics.

For 30 years, the theory called the standard model has withstood all challenges to its predictions of the properties and interactions of elementary particles (SN: 7/1/95, p. 10). Despite the theory's successes, physicists have long suspected that it is an incomplete picture of the subatomic world.

In the dogma-rocking experiment at Brookhaven National Laboratory in Upton, N.Y., an international team of researchers detected a minuscule deviation from the standard model's prediction for the muon's magnetic strength. The findings were announced during a colloquium at the lab on Feb. 8.

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.

Nonetheless, says research leader Vernon W. Hughes of Yale University, "any deviation indicates either there is something missing in the experiment or in the standard theory... 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. These axes then veered away from dead ahead. In fact, during every 29 muon orbits around the accelerator, the particles' spin axes swept through 360[degree].

A quantum phenomenon causes this precession, which is related to the muon's magnetic strength, physicists say. As the muons circle the accelerator, they repeatedly transform into heavier, so-called virtual particles, and back again.

This cycling influences measurements of the muon, so the results differ from those expected if the muon remained stable throughout.

The standard model predicts the influences of known virtual particles on the muon's magnetic strength. Indeed, in the late 1970s, experimenters found a value of the muon's magnetic strength close to the theoretical prediction. The Brookhaven team has redone the measurement with six times the precision. The result "is not the value that we think it would be based on the particles we know and the forces as we understand them," Miller says.

So far, the researchers have evaluated magnetic measurements of 1 billion muons. Until they can analyze the remaining 4 billion muon measurements in their raw data, they can't rule out that the deviation is a statistical fluke, they say. What's more, there's a slim chance that new data from experiments elsewhere will shift the standard model's theoretical prediction so that it actually matches the Brookhaven result.

"A certain amount of caution is certainly in order. It's a difficult experiment," comments Frank Wilczek of the Massachusetts Institute of Technology. Still, he says, the results are "exciting to everyone who's thinking about fundamental physics."

The experiment hints that something hitherto unknown may be out there. Many physicists regard the new finding as a possible sign of a theory known as supersymmetry, which predicts the existence of a partner particle for every known particle. Virtual forms of these particles could arise from the muons and explain the Brookhaven findings.

Gordon L. Kane of the University of Michigan in Ann Arbor, and his colleagues have used the new results to calculate maximum masses for supersymmetric particles and have found some to be comparable to the masses of known particles. The size of the discrepancy in the Brookhaven experiments suggests that any new influence that may have shown itself has a mass similar to the masses of already known particles, he says.

Supersymmetry, however, is only one possible explanation for the discrepancy, Hughes cautions. In another, the muon may be composed of smaller subunits rather than being a fundamental particle, as now believed.