23 April 2026
An international team of physicists has achieved unprecedented accuracy in computing the magnetic properties of the muon using several supercomputers including Europe's first exascale machine JUPITER. The result, published in Nature, resolves long-standing uncertainty between theory and experiment.
The muon is an elementary particle - a short-lived, heavier cousin of the electron. For more than 20 years, physicists have been puzzled by a small but persistent mismatch between theoretical predictions and extremely precise measurements of how the muon behaves like a tiny magnet, hinting at the possible existence of new physics beyond the Standard Model of particle physics.
A major turning point came in 2021, when a landmark calculation by an international team involving Jülich researchers brought the theoretical prediction significantly closer to the experimental value, challenging earlier interpretations of the discrepancy. Building on this work, the team has now further refined the calculations, achieving an even higher level of precision - and bringing theory and experiment into near-perfect agreement.
The new result reduces the uncertainty by a factor of 1.6, making it almost twice as precise as previous calculations. The updated prediction agrees with the latest experimental measurement within just 0.5 standard deviations, providing a remarkable validation of the Standard Model to 11 digits.
The agreement between theory and experiment is a highly non-trivial check of the Standard Model.
"The agreement between theory and experiment is a highly non-trivial check of the Standard Model", says Prof. Kálmán Szabó from the Jülich Supercomputing Centre, who coordinated the Jülich contribution. "On the theory side, the calculations incorporate all fundamental interactions - electromagnetic, weak, and strong - which require very different and often highly complex computational methods."
Combining these contributions into a single value that matches the experimental precision is a significant computing challenge. To put it into perspective: the latest experimental accuracy is comparable to measuring a person's body weight with an uncertainty as small as a weight of a single eye-lash - setting an exceptionally high bar for computation.
New calculations close the gap between theory and experiment
At the heart of the advance is a highly precise calculation of the most uncertain part of the theoretical prediction, related to the strong force of the Standard Model. The computations were carried out largely on Jülich supercomputers, including JUWELS, JURECA and JUPITER - Europe's first exascale supercomputer - which are all operated at the Jülich Supercomputing Centre, Forschungszentrum Jülich.
"This contribution is extremely challenging to calculate with high precision, because the strong interaction must be controlled across all energy scales," says Prof. Zoltan Fodor, whose ERC Advanced Grant also supported the Jülich team.
To tackle this challenge, the researchers used a hybrid approach, combining state-of-the-art lattice quantum chromodynamics (QCD) simulations with carefully selected experimental data from electron-positron collisions across different energy ranges. This strategy enables a level of precision that neither method could reach on its own.
As a result, the muon's behaviour can now be fully described within the Standard Model of particle physics - with no indication of previously unknown physics.
Original publication
A. Cotellucci, Z. Fodor, D. Giusti, A. Kotov, T. Lippert, K. Szabo mit Forscher:innen der Universitäten Adelaide, Budapest, Marseille und Wuppertal
Hybrid calculation of hadronic vacuum polarization in muon g-2 to 0.48%
Nature (2026), DOI: 10.1038/s41586-026-10449-z
