Muon's Magnetic Mystery

Researchers from JSC computed an important contribution to the muon's magnetic moment, which has recently been published in Nature (DOI: 10.1038/s41586-021-03418-1). The muon is an elementary particle, a short-lived cousin of the electron. For about 20 years the calculation of its magnetic moment has disagreed with its measurement, indicating that a not-yet-known particle or force might be influencing the muon. An experiment at the Fermi National Accelerator Laboratory (FNAL) in the United States has apparently confirmed this puzzling discrepancy, according to an announcement on 7 April 2021. The event has received significant media attention.

Nonetheless, thanks to the computing power of Jülich's JUQUEEN, JURECA, and JUWELS supercomputers, a new ab-initio calculation has challenged previous computations and brought the theoretical prediction closer to the experimental value. According to this result, no new forces or particles are necessary to explain the FNAL and previous measurements. The new calculation has a precision comparable to the experiment and previous theory computations.

As Kalman Szabo, co-author of the paper and leader of the Relativistic Quantum Field Theory research group, points out, “before claiming new physics, we have to understand the differences between our new result and previous theory computations, and new supercomputer calculations are essential for that.” Thomas Lippert, another co-author, emphasizes that “this case shows again that Jülich's world-class HPC infrastructure is essential for doing world-class science in Europe.” Beside Lippert and Szabo, JSC's Finn Stokes and Csaba Török belong to the research team, which also involves partners from Wuppertal, Marseille, and Budapest.

Contact: Prof. Kalman Szabo, k.szabo@fz-juelich.de

from JSC News No. 280, 26 April 2021