New Particles or an Extra Minus Sign?

A math error may have produced a tiny effect that many physicists hailed as evidence of new particles.

Last February, scientists reported that the muon, the heavier cousin of the electron, is slightly more magnetic than the Standard Model of Elementary Particles predicts (ScienceNOW, 8 February). The measured magnetism, which depends on other particles that flit in and out of existence around the muon, is 4 parts per billion bigger than predicted, physicists at Brookhaven National Laboratory in Upton, New York, discovered. The slight discrepancy suggests the presence of a new type of particle, the researchers said, perhaps one predicted by a theory called supersymmetry, which assumes that every type of particle possesses a much heavier, still undiscovered partner.

But the calculation for the predicted value may contain a mistake, say Marc Knecht and Andreas Nyffeler of the Center for Theoretical Physics in Marseilles, France, who repeated a key part of it. Finding the theoretical value is no picnic: Physicists must add up the results from a series of hugely complicated calculations, one for each combination of particles emitted and reabsorbed by the muon. When two groups independently calculated the result for a combo known as "hadronic light-by-light scattering" in 1995, they both got the sign of the answer wrong, Knecht and Nyffeler argue in a paper posted online on 6 November on the Los Alamos preprint server. If the negative results are made positive, the theoretical prediction goes up enough to cut the discrepancy in half, weakening the case for supersymmetry.

But the error could be in the new calculation, says Toichiro Kinoshita, a physicist at Cornell University in Ithaca, New York, who did the calculation with colleagues at the KEK laboratory in Tsukuba, Japan. "I went through our paper and didn't find anything wrong," Kinoshita says. "Now I'm going through their paper to see if something isn't right." Chasing down the errant minus sign may not be easy, says Johan Bijnens of Lund University in Sweden, who did the other 1995 calculation with colleagues in Spain and Italy. "This is a detailed and error-prone business that one has to work through slowly," Bijnens says.

Related sites

The paper by Knecht and Nyffeler
Brookhaven muon experiment