Since the 1970s, the Standard Model of Physics has served as the basis from which particle physics are investigated. Both experimentalists and theoretical physicists have tested the Standard Model's accuracy, and it has remained the law of the land when it comes to understanding how the subatomic world behaves.
This week, cracks formed in that foundational set of assumptions. Researchers of the "Muon g-2" collaboration from the Fermi National Accelerator Laboratory (FNAL) in the United States published further experimental findings that show that muons -- heavy subatomic relatives of electrons -- may have a larger "magnetic moment" than earlier Standard Model estimates had predicted, indicating that an unknown particle or force might be influencing the muon. The work builds on anomalous results first uncovered 20 years ago at Brookhaven National Laboratory (BNL), and calls into question whether the Standard Model needs to be rewritten.
Meanwhile, researchers in Germany have used Europe's most powerful high-performance computing (HPC) infrastructure to run new and more precise lattice quantum chromodynamics (lattice QCD) calculations of muons in a magnetic field. The team found a different value for the Standard Model prediction of muon behaviour than what was previously accepted. The new theoretical value is in agreement with the FNAL experiment, suggesting that a revision of the Standard Model is not needed. The results are now published in Nature.
The team primarily used the supercomputer JUWELS at the Jülich Supercomputing Centre (JSC), with the computational time provided by the Gauss Centre for Supercomputing (GCS) as well at JSC's JURECA system, along with extensive computations performed at the other two GCS sites -- on Hawk at the High-Performance Computing Center Stuttgart (HLRS) and on SuperMUC-NG at the Leibniz Supercomputing Centre (LRZ).
Both the experimentalists and theoretical physicists agreed that further research must be done to verify the results published this week. One thing is clear, however: the HPC resources provided by GCS were essential for the scientists to achieve the precision necessary to get these groundbreaking results.
"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 Prof. Kalman Szabo, leader of the Helmholtz research group, "Relativistic Quantum Field Theory" at JSC and co-author of the Nature publication. "Before deciding the fate of the Standard Model, one has to understand the theoretical differences, and new lattice QCD computations are inevitable for that."
Minor discrepancies, major implications
When BNL researchers recorded unexplained muon behaviour in 2001, the finding left physicists at a loss -- the muon, a subatomic particle 200 times heavier than an electron, showed stronger magnetic properties than predicted by the Standard Model of Physics. While the initial finding suggested that muons may be interacting with previously unknown subatomic particles, the results were still not accurate enough to definitely claim a new finding.
Over the next 20 years, heavy investments in new, hyper-sensitive experiments done at particle accelerator facilities as well as increasingly sophisticated approaches based in theory have sought to confirm or refute the BNL group's findings. During this time, a research group led by the University of Wuppertal's Prof. Zoltan Fodor, another co-author of the Nature paper, was progressing with big steps in lattice QCD simulations on the supercomputers provided by GCS. "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." said Prof. Fodor. "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."
Lattice QCD calculations allow researchers to accurately plot subatomic particle movements and interactions with extremely fine time resolution. However, they are only as precise as computational power allows -- in order to perform these calculations in a timely manner, researchers have had to limit some combination of simulation size, resolution, or time. As computational resources have gotten more powerful, researchers have been able to do more precise simulations.
"This foundational work shows that Germany's world-class HPC infrastructure is essential for doing world-class science in Europe," said Prof. Thomas Lippert, Director of the Jülich Supercomputing Centre, Professor for Quantum Computing and Modular Supercomputing at Goethe University Frankfurt, current Chairman of the GCS Board of Directors, and also co-author of the Nature paper. "The computational resources of GCS not only play a central role in deepening the discourse on muon measurements, but they help European scientists and engineers become leaders in many scientific, industrial, and societal research areas."
While Fodor, Lippert, Szabo, and the team who published the Nature paper currently use their calculations to cool the claims of physics beyond the Standard Model, the researchers are also excited to continue working with international colleagues to definitively solve the mystery surrounding muon magnetism. The team anticipates that even more powerful HPC systems will be necessary to prove the existence of physics beyond the Standard Model. "The FNAL experiment will increase the precision by a factor of four in two years. We theorists have to keep up with this pace if we want to fully exploit the new physics discovery potential of muons." Szabo said.
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