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'This is the only computation on the market, so some people are uncomfortable,' he says.

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His team's result is free of such uncertainties. For key data, it depends on results mainly from two colliders, and the two data sets disagree to a worrisome extent, Fodor says. He also raises questions about the consensus value. Given the uncertainties, that's too close to claim a discrepancy, Fodor says. And the new result is only one part per billion below the experimental value, the team reported in Nature.

Now, using hundreds of millions of processor hours at the Jülich Research Center in Germany, Fodor's group has produced a lattice calculation of the hadronic vacuum polarization and a value for the muon's magnetism that rivals the consensus standard model value in precision. Several groups have also applied the lattice to the muon's magnetism, albeit with sizable uncertainties. Twelve years ago, theorists showed this 'lattice QCD' technique could calculate the masses of the proton and the neutron, which are both hadrons. Theorists can attempt brute-force QCD calculations on supercomputers, if they model the continuum of space and time as a lattice of discrete points occupied by quarks and particles called gluons, which convey the strong force. ALTOUNIAN/ SCIENCE DATA: ABI ET AL., PHYS. Never mind the gap? If a new 'lattice' value for the magnetism of the muon is correct, a mysterious gap between other predictions and a recent measurement would all but disappear. Instead, they have to rely on data from accelerators that create hadrons by colliding electrons and positrons.

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The theory of quarks and the strong nuclear force that binds them, quantum chromodynamics (QCD), is so unwieldy that theorists cannot calculate the effects through the usual series of ever smaller approximations. In it, the muon emits and reabsorbs particles known as hadrons, which consist of other particles called quarks. One family of processes, known as hadronic vacuum polarization, is especially challenging and limits the precision of the entire calculation. To make that prediction, the theorists had to account for the thousands of ways standard model particles can flit about the muon and affect its behavior. That's why physicists were so excited when the Muon g-2 experiment at Fermi National Accelerator Laboratory confirmed a 20-year-old hint that the muon is about 2.5 parts per billion more magnetic than the standard model predicts, according to the consensus value, hammered out last year by the 132-member Muon g-2 Theory Initiative. Such particles might someday be blasted into existence at an atom smasher. Standard model particles should increase its magnetism by about 0.1%, and as-yet-unknown particles would add their own boost. These 'virtual' particles cannot be observed directly, but they can affect the muon's properties, including magnetism.

Thanks to quantum uncertainty, particles and antiparticles also constantly flit in and out of existence around the muon. Quantum mechanics and relativity demand that the muon have a certain basic magnetism. 'We can't immediately ignore everything we know and switch over to a single new result of a new method,' says Christoph Lehner, a theorist at the University of Regensburg.Ī heavier, unstable cousin of the electron, the muon acts like a tiny bar magnet, and its magnetism provides a means to dowse for hints of new particles. However, others say it's too early to throw out the previous calculation, which is the product of decades of painstaking effort. 'The standard model is just fine, according to our calculation,' says Zoltan Fodor, a theorist at Pennsylvania State University, University Park, and the leader of the Budapest-Marseille-Wuppertal (BMW) collaboration, which produced the new theoretical result. Their value sits closer to the experimental result and makes the tantalizing discrepancy nearly vanish. But on the same day, 14 theorists published a paper that suggests the consensus theoretical prediction is wrong. On 7 April, a collaboration of more than 200 experimenters announced to great fanfare that a particle called the muon is slightly more magnetic than predicted by physicists' standard model, a discrepancy that could signal new particles waiting to be discovered. Talk about raining on your colleagues' parade.