Four years of calculations lead to new ideas about the muon abnormality

Two decades ago, an experiment at the Brookhaven National Laboratory of the US Department of Energy. USA (DOE) detected a mysterious mismatch between established theory of particle physics and actual laboratory measurements. When the researchers evaluated the behavior of a subatomic particle called a muon, the results did not match theoretical calculations, posing a potential challenge to the Standard Model: our current understanding of how the universe works.

Since then, scientists around the world have been trying to verify this discrepancy and determine its importance. The answer could defend the Standard Model, which defines all known subatomic particles and how they interact, or introduce the possibility of completely unknown physics. A multi-agency research team (including Brookhaven, Columbia University and Connecticut, Nagoya and Regensburg Universities, RIKEN) has used the Mira supercomputer from the Argonne National Laboratory to help reduce possible explanations for the discrepancy, delivering a new theoretical calculation I need to refine a piece of this very complex puzzle. The work, funded in part by the DOE Office of Science through its Office of High-Energy Physics and Advanced Scientific Computing Research programs, has been published in the journal Physical Review Letters.

A muon is a heavier version of the electron and has the same electrical charge. The measurement in question is the muon’s magnetic moment, which defines how the particle wobbles when it interacts with an external magnetic field. The previous Brookhaven experiment, known as Muon g-2, examined the muons while interacting with a 50-foot diameter electromagnet storage ring. The experimental results diverged from the value predicted by the theory by an extremely small amount measured in parts per million, but within the scope of the Standard Model, such a difference is large enough to be noticeable.

“If uncertainties are taken into account in both calculations and measurements, we cannot know if this is a real discrepancy or just a statistical fluctuation,” said Thomas Blum, a physicist at the University of Connecticut, who co-authored the article. “So both experimenters and theorists are trying to improve the sharpness of their results.”

As Taku Izubuchi, a physicist at Brookhaven Lab, co-author of the paper, noted, “Physicists have been trying to understand the muon’s anomalous magnetic moment by comparing precise theoretical calculations and precise experiments since the 1940s. This sequence of The Work has led many discoveries in particle physics and continues to expand the limits of our knowledge and capabilities in both theory and experiments. “

If the discrepancy between the experimental results and the theoretical predictions is really real, that would mean that some other factor, perhaps some yet undiscovered particle, is causing the muon to behave differently than expected, and the Standard Model would need to be reviewed.

The team’s work focused on a notoriously difficult aspect of the anomaly involving the strong force, which is one of the four basic forces in nature that govern how particles interact, along with the weak, electromagnetic, and gravitational force. The greatest uncertainties in muon calculations come from particles that interact through the strong force, known as hadronic contributions. These hadronic contributions are defined by a theory called quantum chromodynamics (QCD).

The researchers used a method called lattice QCD to analyze one type of hadronic contribution, light scattering by light. “To do the calculation, we simulated the quantum field in a small cubic box containing the light-to-light scattering process that interests us,” said Luchang Jin, a physicist at the University of Connecticut and co-author of the paper. “We can easily end up with millions of points in time and space in the simulation.”

That’s where Mira came in. The team used the supercomputer, located at the Argonne Leadership Computing Facility (ALCF), to solve QCD’s complex mathematical equations, which code for all possible strong interactions with the muon. The ALCF, a user facility from the DOE Office of Science, recently pulled Mira to make room for the most powerful Aurora supercomputer, an exascale system slated to arrive in 2021.

“Mira was ideal for this job,” said James Osborn, a computer scientist with the Argonne Division of Computer Science and ALCF. “With almost 50,000 nodes connected by a very fast network, our massive parallel system allowed the team to run large simulations very efficiently.”

After four years of performing calculations at Mira, the researchers produced the first result for the contribution of hadronic scattering of light by light to the muon’s anomalous magnetic moment, controlling for all errors.

“For a long time, many people thought that this contribution, because it was so challenging, would explain the discrepancy,” Blum said. “But we found that previous estimates were not far behind, and that the actual value cannot explain the discrepancy.”

Meanwhile, a new version of the Muon g-2 experiment is underway at the Fermi National Accelerator Laboratory, with the goal of reducing uncertainty on the experimental side by a factor of four. Those results will add more information to the theoretical work being done now.

“As far as we know, the discrepancy still stands,” Blum said. “We are waiting to see if the results together point to a new physics, or if the current Standard Model is still the best theory we have to explain nature.”