Big news recently struck the physics world: researchers at the Fermi National Accelerator Laboratory (FNAL), in the midst of their Muon g-2 experiment, published mounting evidence that those muon particles might defy the predictions of the Standard Model of Physics, casting some small doubt on core tenets of modern physics that have otherwise survived more or less unscathed for half a century. Now, a team of German, French and Hungarian researchers have used supercomputers to conduct their own muon calculations, reconciling the FNAL experiment with the Standard Model. 

The muon discrepancy dates back 20 years to an observation made at Brookhaven National Laboratory (BNL). In the intervening two decades, researchers around the world have sought to understand why the particle exhibited a larger magnetic moment (the strength and arrangement of a magnetic object) than the Standard Model would predict. Then, just this month, FNAL released the first results of its muon-centric experiment, showing key variables that did, indeed, appear to be out of alignment with the Standard Model.

“[Our estimate] reflects the interactions of the muon with everything else in the universe. But when the theorists calculate the same quantity, using all of the known forces and particles in the Standard Model, we don’t get the same answer,” explained Renee Fatemi, the University of Kentucky physicist who managed the simulations for the Muon g-2 experiment, in an interview with Eric Gedenk at the Gauss Centre for Supercomputing (GCS). “This is strong evidence that the muon is sensitive to something that is not in our best theory.”

Meanwhile, in Europe, a multi-institution collaboration was running ultra-high resolution quantum chromodynamics calculations to understand muons in magnetic fields. To do this, they turned to the Juwels supercomputer (pictured in the header) at the Jülich Supercomputing Centre (JSC). Juwels’ two modules are, respectively, capable of 6.2 Linpack petaflops and 44.1 Linpack petaflops, making its “booster module” alone the seventh-most powerful publicly ranked supercomputer in the world. 

The result: a new Standard Model-driven estimate for those same key values of the muon in a magnetic field – an estimate that agrees with the results of the FNAL experiment. “For the first time, lattice results have a precision comparable to these experiments. Interestingly our result is consistent with the new FNAL experiment, as opposed to previous theory results, that are in strong disagreement with it,” said Kalman Szabo, co-author of the paper. 

“Though our results on the muon g-2 are new, and have to be thoroughly scrutinized by other groups, we have a long record of computing various physical phenomena in quantum chromodynamics.” added Zoltan Fodor, another of the paper’s authors. “Our previous major achievements were computing the mass of the proton, the proton-neutron mass difference, the phase diagram of the early universe and a possible solution for the dark matter problem. These paved the way to our most recent result.”