A new estimate of the strength of the magnetic field of the subatomic particle is consistent with the standard model of particle physics.
A new estimate of the strength of the magnetic field around the muon – a subatomic particle similar to but heavier than an electron – closes the gap between theory and experimental measurements and brings it in line with the standard model that has governed particle physics for decades.
A paper describing the research from an international research group is published today (April 7, 2021) in the journal Nature.
Twenty years ago, in an experiment at the Brookhaven National Laboratory, physicists discovered what appeared to be a discrepancy between the measurements of the muon’s “magnetic moment” – the strength of its magnetic field – and theoretical calculations of what that measurement should be, which increased the tantalizing possibility of physical particles or forces that have not yet been discovered. The new finding reduces this discrepancy, suggesting that muons magnetism is probably not mysterious at all. To achieve this result, instead of relying on experimental data, researchers simulated every aspect of their calculations from scratch – a task that requires massive supercomputer power.
“Most of the phenomena in nature can be explained by what we call the ‘standard model’ of particle physics,” said Zoltan Fodor, professor of physics at Penn State and head of the research team. “We can predict the properties of the particles extremely accurately based on this theory alone, so when theory and experiment do not match, we can get excited that we may have found something new, something beyond the standard model.”
For a discovery of new physics in addition to the standard model, there is agreement among physicists that the disagreement between theory and measurement must reach five sigma – a statistical measure corresponding to a probability of approx. 1 out of 3.5 million.
In the case of muon, measurements of its magnetic field deviated from the existing theoretical predictions by approx. 3.7 sigma. Exciting, but not enough to declare a discovery of a new breach of the rules of physics. So scientists set out to improve both the measurements and the theory in hopes of either reconciling theory and measurement or raising the sigma to a level that would allow the declaration of a discovery of new physics.
“The existing theory for estimating the strength of muon’s magnetic field was based on experimental electron-positron erasure measurements,” Fodor said. “To have a different approach, we used a fully verified theory that was completely independent of dependence on experimental measurements. We started with pretty basic equations and built the whole estimate from scratch. ”
The new calculations required hundreds of millions of CPU hours at several supercomputer centers in Europe and brought the theory back in line with measurement. However, the story is not over yet. New, more accurate experimental measurements of muons magnetic moment are expected soon.
“If our calculations are correct and the new measurements do not change history, it seems that we do not need any new physics to explain the muon’s magnetic moment – it follows the rules of the standard model,” said Fodor. “Although the prospect of new physics is always enticing, it is also exciting to see theory and experiment adjust. It demonstrates the depth of our understanding and opens up new opportunities for exploration. ”
The excitement is far from over.
“Our result needs to be cross-checked by other groups and we expect them to,” Fodor said. “Furthermore, our findings mean that there is a tension between the previous theoretical results and our new ones. This discrepancy must be understood. In addition, the new experimental results may be close to old ones or closer to the previous theoretical calculations. We have many years of excitement ahead of us. ”
Reference: “Leading hadronic contribution to muon magnetic moment from lattice QCD” by Sz. Borsanyi, Z. Fodor, JN Guenther, C. Hoelbling, SD Katz, L. Lellouch, T. Lippert, K. Miura, L. Parato, KK Szabo, F. Stokes, BC Toth, Cs. Torok and L. Varnhorst, April 7, 2021, Nature.
DOI: 10.1038 / s41586-021-03418-1
In addition to Fodor, the research group includes Szabolcs Borsanyi, Jana N. Guenther, Christian Hoelbling, Sandor D. Katz, Laurent Lellouch, Thomas Lippert, Laurent Lellouch, Kohtaroh Miura, Letizia Parato, Kalman K.Szabo, Finn Stokes, Balint C Toth, Csaba Torok, Lukas Varnhorst.
Participating institutions include Penn State, University of Wuppertal, Germany; Jülich Supercomputing Center in Germany; Eötvös University in Budapest, Hungary; University of California, San Diego; the University of Regensburg in Germany; Aix Marseille Univ, University of Toulon in Marseille, France; Helmholtz Institute Mainz in Germany; Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University in Japan.
The research was funded by the German Research Foundation (DFG); the German Federal Ministry of Education and Research (BMBF) the Hungarian National Agency for Research, Development and Innovation and the Excellence Initiative of Aix-Marseille, a French Investissements d’Avenirf program through the Chaire d’Excellence initiative and the Laboratoire d’Excellence.