A particle known as the muon is ever-so-slightly more magnetic than predicted, physicists have reported. This apparently tiny discrepancy could knock a large hole in the current best theory of fundamental particles, the Standard Model.
The magnetism of the muon is about four parts in a billion greater than predicted, according to experimenters working at Brookhaven National Laboratory in New York.
That suggests that the Standard Model is incomplete and that physicists may be on the verge of discovering a collection of new particles, such as those predicted by a theory called supersymmetry.
"The most natural explanation for such a deviation is that you're seeing supersymmetry," says Bill Marciano a theoretical physicist at Brookhaven.
The muon is a particle similar to the electron, but roughly 207 times as massive. Like the electron, the muon acts like a tiny bar magnet. Theoretical physicists have used the Standard Model to calculate the strength of the muon's magnetism, a quantity known as its magnetic moment, to extremely high precision.
For decades the results of these calculations have always agreed with the measured values to within the experimental uncertainties. But the Brookhaven researchers have pushed the uncertainties down to just under one and a half parts per billion. At that level the predicted and measured values appear to diverge.
"It's a 99% certainty and a 1% chance that there's no difference," says Priscilla Cushman, a team member and a physicist from the University of Minnesota, in Minneapolis.
The researchers studied a beam of positively charged muons travelling at near light speed as it circulated in a vertical magnetic field. Because muons have electric charge, the beam travelled in a horizontal circle around the field lines.
At the same time, each muon's magnetic moment interacted with the field, turning in the horizontal plane much as a horizontal gyroscope turns under the pull of gravity.
The researchers measured the number of times each muon made it around the circle and the number of times its magnetic moment turned around. They compared these numbers to precisely determine muon's magnetic moment.
Thanks to the Heisenberg Uncertainty Principle, the magnetic moment of the muon depends on the properties of many other particles, such as the photon or the W boson. That's because the uncertainty principle allows a muon to emit a "virtual" particle and to gobble it back up before it can be directly detected. The virtual particle can interact with the magnetic field and affect the moment of the muon.
But the calculations show that the six quarks, six leptons, and four bosons in the Standard Model cannot account for all of the muons's magnetism, says Lee Roberts, a physicist from Boston University, who was involved in the research. That suggests that the muon is emitting and reabsorbing other types of particles, perhaps some of the supersymmetric partners.
Many physicists would like to see the Standard Model unravel because it cannot answer some basic conceptual questions, such as why the particles have the masses they do. If it holds up, the tiny difference could give researchers the start they need.
"These little hints in physics often go a long way," Marciano says. "It doesn't take much to get people started along the right track."
More at: BNL Muon g-2
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