he overall theory of particle physics, the standard model, explains the zoo of subatomic particles. Thirty years of rigorous experimental challenge have failed to dent it, until now. The standard model incorporates quantum electrodynamics with the strong and weak nuclear forces. In physics, once a standard model of anything is firmly embedded, practitioners come forward with expensive experiments to disprove the model. That’s how modern physics advances. So the standard model is now being subjected to weapons-grade justification, which requires extremely demanding experiments. Our newcomer #6 is all about the latest challenge to the standard model, which looks at the magnetic properties of the muon.

To understand the importance of looking at the magnetic properties of muons let’s go back to the 1960s: Richard Feynman and the origins of quantum electrodynamics (QED). Feynman looked anew at how electrons interact with photons, because certain problems in quantum theory were then plagued by infinities in the solution to the equations. Feynman dealt with the paradoxes by taking into account all possible interactions between an electron and the sea of virtual particles hiding in the universal vacuum. In QED he tamed the catastrophes of earlier theory, and QED would become an aspect of the standard model.

The electron is magnetic, and the standard model predicts the size of its magnetic moment. These calculations take into account interactions with virtual particles in the vacuum. Measuring the electron magnetic moment confronts the standard model with observations. In practice using electrons is not recommended because the physics scales with the square of the mass, so using a heavy electron, the muon (207 heavier than the electron) is the way to go. That’s what scientists at Brookhaven National Laboratory have done (#6) in an experiment that directly challenges the standard model.

In 1999 the Brookhaven collaboration (68 physicists, 11 institutions) measured the anomalous magnetic moment of the muon, finding it slightly larger than expected from the standard model. They used an intense beam of positively charged muons going at close to the speed of light in the world’s largest superconducting magnet. The muons behave like tiny bar magnets, so they turn around in the intense magnetic field. What’s important is how much faster the spin precesses compared to the angular momentum; the frequency difference is a direct measure of the magnetic anomaly. The researchers looked at 10⁷ decays of muons into positrons (which thereby reveal the muon spin angles) to uncover the inherent magnetic properties of the muon. In #6 they announce a minuscule discrepancy between the standard model prediction, and their result is just 4 in a billion (43 ± 16 x 10^-10) from the theory, and yet is sufficient to undermine confidence in the standard model.

What new physics is implied? One view is that the standard model is inadequate, and most particle physicists do agree it is not the final answer. But only now are we down to the tenth significant figure of testing it to destruction. Another possibility is that the data in #6 are victim to a statistical quirk. However, team member Priscilla Cushman, University of Minnesota, says, "It’s a 99% certainty and a 1% chance that there’s no difference." The data for runs in 2000 will refine the result by a factor of 2. For now though, #6 hints at a world in which 6 quarks, 6 leptons, and 4 bosons cannot account for the muon’s magnetism. Team member Lee Roberts of Boston University suggests that the muon may be emitting and reabsorbing other types of particle. Roberts tells Science Watch, "Many people believe that the discovery of supersymmetry—a theory that predicts the existence of companion particles for all known particles—may be just around the corner. We may have opened the first tiny window on the supersymmetric world."