Artist’s conception of the mystery of the magnetic moment of the muon – a sub-atomic particle similar to, but heavier than, an electron (represented by the Greek letter mu). A new estimate of the strength of the muon’s magnetic field closes the gap between theory and experimental measurements, bringing it into line with the standard model of particle physics. Credit: Dani Zemba, Penn State
A new estimate of the strength of the sub-atomic particle’s magnetic field corresponds to the standard model of particle physics.
A new estimate of the strength of the magnetic field around the muon – a sub-atomic particle similar to, but heavier than, an electron – narrows the gap between theory and experimental measurements and compares it with the standard model guided by physics decades long.
A paper describing the research by an international team of scientists appears today (April 7, 2021) in the journal Earth.
Twenty years ago, in an experiment at the Brookhaven National Laboratory, physicists found a discrepancy between the measurement of the muon’s magnetic moment ‘- the strength of its magnetic field – and the theoretical calculations of what the measurement should be, what the seductive increases possibility of physical particles or forces not yet discovered. The new finding narrows this gap, suggesting that the muon’s magnetism is probably not mysterious at all. Instead of relying on experimental data, researchers have mimicked every aspect of their calculations from the ground up – a task that requires large amounts of computing power.
“Most of the phenomena in nature can be explained by what we call the ‘standard model’ of particle physics,” said Zoltan Fodor, a professor of physics at Penn State and a leader of the research team. “We can predict the properties of particles extremely accurately based on this theory alone, so if the theory and experiment do not match, we can get excited that we could find something new, something outside the standard model.”
For the discovery of new physics outside the standard model, there is consensus among physicists that the difference of opinion between theory and measurement should reach five sigma – a statistical measure equal to the probability of about 1 in 3.5 million.
In the case of the muon, measurements of the magnetic field deviated by about 3.7 sigma from the existing theoretical predictions. Interesting, but not enough to explain a discovery of a new break in the physical rules. Therefore, researchers decided to improve the measurements as well as the theory in the hope of reconciling theory and measurement, or to raise sigma to a level that allows the discovery of new physics.
“The existing theory for estimating the strength of the muon’s magnetic field was based on experimental electron-positron destruction measurements,” Fodor said. ‘To get a different approach, we used a fully verified theory that was completely independent of reliance on experimental measurements. We started with basic equations and built the whole estimate from the ground up. ”
The new calculations required hundreds of millions of processing hours at various supercomputer centers in Europe and brought the theory back into line with the measurement. However, the story is not over yet. New, more accurate experimental measurements of the muon’s magnetic moment are expected soon.
“If our calculations are correct and the new measurements do not change the story, it seems that we do not need any new physics to explain the magnetic moment of the muon. It follows the rules of the standard model, ”said Fodor. ‘While the prospect of new physics is always enticing, it is also exciting to align theories and experiments. It shows the depth of our understanding and offers new opportunities for exploration. ‘
The excitement is far from over.
“Our results need to be crossed by other groups and we expect that,” Fodor said. ‘Furthermore, our finding means that there is a tension between the previous theoretical results and our new ones. This difference must be understood. In addition, the new experimental results may be close to the old ones or closer to the previous theoretical calculations. We have many years of excitement ahead of us. ”
Reference: “Leading Hadronic Contribution to the Muon Magnetic Moment of Grid 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, 7 April 2021, Earth.
DOI: 10.1038 / s41586-021-03418-1
In addition to Fodor, the research team 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, the University of Wuppertal in Germany; the Jülich Supercomputer Center in Germany; Eötvös University in Budapest, Hungary; the University of California, San Diego; the University of Regensburg in Germany; Aix Marseille Univ, University of Toulon in Marseille, France; Helmholtz Mainz Institute in Germany; the 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 Office for National 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.