Muons Don't Abide by Expectations

Stymied. Muon researchers aren't sure just how oddly their particles are behaving.

Scientists at Brookhaven National Laboratory have confirmed earlier evidence that the Standard Model of particle physics fails to predict how certain particles swoop through a magnetic field. However, physicists are still debating just how significant the discrepancy is.

The new result, presented 30 July at a seminar at Brookhaven National Laboratory in New York, is twice as precise as results presented early last year (ScienceNOW, 8 February 2001). In both cases, physicists used a 14-meter-wide superconducting magnet to induce muons--heavier siblings of the electron--to curve around in a circle. In so doing, they measured the muon's propensity to twist in a magnetic field, known as the muon's magnetic moment. The Standard Model of particle physics, the theoretical framework that explains how particles interact, predicts what the magnetic moment should be. But the new experimental value, like last year's measure, doesn't match the theoretical predictions.

Unfortunately, theoretical predictions disagree, too. Physicists use two different methods to calculate the magnetic moment of the muon. One--the more reliable--yields a value far below the experimental value, whereas the other gives one relatively close to it. So it's hard to tell just how far the experimental result is from the Standard Model at this point.

"My first statement would be not to be in a hurry" to jump to a conclusion, says Simon Eidelman, a physicist at the Budker Institute of Nuclear Physics in Novosibirsk, Russia. It's too early to tell whether there's a problem with the theory or with experiments that feed into the theory, or whether the Brookhaven experiment heralds new physics beyond the Standard Model, Eidelman says. "When and where all this will converge, I can't tell."

Related sites
Brookhaven's muon project
More about the muon project from the University of Minnesota