A TINY discrepancy in the magnetism of the muon, a rare subatomic particle, may signal a crack in the standard model of particle physics. It could turn out to be the first piece of hard evidence for a more complete theory called supersymmetry, say experimenters at Brookhaven National Laboratory in New York.
During its 30-year life, the cobbled-together standard model has described the world of subatomic particles with astounding accuracy. Yet physicists are dying to find a more satisfying description of the microscopic world. 鈥淲e did this experiment to confront the standard model,鈥� says Gerry Bunce of Brookhaven. 鈥淭hat鈥檚 the only reason for doing it.鈥�
The muon is similar to the electron, but 207 times as massive, and behaves like a tiny bar magnet. Based on the standard model, theoretical physicists have calculated the strength of the muon鈥檚 magnetism, known as its magnetic moment, to extremely high precision. For decades, measurements have agreed with these calculations.
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Now the Brookhaven team has squeezed the experimental uncertainty down to just under 1.5 parts in a billion (see Diagram). At this precision, calculation and measurement appear to part company. The researchers found the magnetic moment to be roughly 1 part in 4 billion bigger than the standard model says it should be.
Although tiny, the deviation means that there could be a slew of new particles waiting to be discovered. This is because part of the magnetic moment comes from 鈥渧irtual鈥� particles, fleeting apparitions that emerge briefly from the muon then disappear back into it before they can be detected.
The standard model predicts that virtual versions of the 16 known fundamental particles can only contribute so much to the muon鈥檚 magnetic moment. If there is more magnetism, then other particles, so far unknown, must also be buzzing around the muon. Possible culprits include heavier mirror-image partners of the known particles predicted by supersymmetry. 鈥淚f these particles exist, they could also contribute,鈥� says Lee Roberts of the Brookhaven team.
Physicists are intrigued by ideas like supersymmetry because they might help with some of the big unanswered questions 鈥� for example, why particles have the masses they do. 鈥淢any people are not satisfied with the standard model,鈥� says theoretician Toichiro Kinoshita of Cornell University in New York state. 鈥淚t鈥檚 kind of a patch-up job.鈥�
Despite its shortcomings, physicists have till now never managed to trip up the standard model, after decades of trying. 鈥淓very time you think that you鈥檝e shown the standard model is wrong, it turns out that the experiment is wrong,鈥� says Persis Drell of Cornell. 鈥淲hat could be more frustrating?鈥�
The muon measurement could also fade away as more data accumulates, says theoretician Marcela Carena of Fermilab near Chicago. But if it holds up, it could point the way to a more complete theory, she says. 鈥淚f there is physics beyond the standard model, it is probably going to appear in a subtle way like this.鈥�
