Theoretical Calculations Offer New Insights into Muon Anomalous Magnetic Moment

Twenty years ago, researchers from Brookhaven National Laboratory at the U.S. Department of Energy (DOE) performed an experiment that identified a mysterious discrepancy between actual laboratory measurements and the established particle physics theory.

A typical diagrammatic representation of the hadronic light-by-light scattering contribution with Argonne’s Mira supercomputer in the background. Image Credit: Photo courtesy of Luchang Jin, University of Connecticut.

When the scientists assessed the behavior of a subatomic particle known as the muon, the outcomes did not match with the theoretical calculations, representing an eventual challenge to the Standard Model—that is, one’s prevalent interpretation of the workings of the Universe.

Since that time, researchers across the world have been attempting to substantiate this mismatch and find out its relevance. The solution is either to introduce the prospect of an entirely unknown physics, or to support the Standard Model, which characterizes all the identified subatomic particles and the way they communicate.

A multi-institutional group of researchers has utilized Mira supercomputer, which belongs to Argonne National Laboratory, to help scale down the potential explanations for the mysterious mismatch. This ultimately delivered a new, accurate theoretical calculation that improves one part of this highly complicated riddle.

The multi-institutional team included Columbia University, Brookhaven National Laboratory, RIKEN, and the universities of Nagoya, Connecticut, and Regensburg.

The study was published in the Physical Review Letters journal and was partly funded by the DOE’s Office of Science via its Office of High Energy Physics and Advanced Scientific Computing Research programs.

A muon has the same electric charge as that of the electron, despite being a heavier version of the electron. The magnetic moment of the muon is the measurement in question. This magnetic moment defines how the particle vibrates upon interacting with an external magnetic field.

Muon g-2, a previous experiment conducted at Brookhaven National Laboratory, analyzed muons when they were interacting with an electromagnet storage ring that had a diameter of 50 feet. The experimental outcomes deviated from the theoretically predicted value, by a very small amount quantified in parts per million; however, in the context of the Standard Model, a difference like that is sufficiently big to be perceptible.

If you account for uncertainties in both the calculations and the measurements, we can’t tell if this is a real discrepancy or just a statistical fluctuation. So both experimentalists and theorists are trying to improve the sharpness of their results.

Thomas Blum, Study Co-Author and Physicist, University of Connecticut

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

If the mismatch between theoretical predictions and experimental results is indeed tangible, the same would point to some other factor—probably some kind of particle that is yet to be identified—is driving the muon to act in a different way than predicted, and the Standard Model would also have to be upgraded.

The researchers’ work focused on an extremely challenging aspect of the muon anomaly involving the powerful force, which is one of the four fundamental forces in nature that controls the interaction of particles, together with weak, gravitational, and electromagnetic force.

With regard to the muon calculations, the biggest reservations come from particles that communicate via the powerful force, called hadronic contributions. A theory, known as quantum chromodynamics (QCD), defines these hadronic contributions.

The scientists employed a technique known as lattice QCD to examine light-by-light scattering—a type of hadronic contribution.

To do the calculation, we simulate the quantum field in a small cubic box that contains the light-by-light scattering process we are interested in. We can easily end up with millions of points in time and space in the simulation.

Luchang Jin, Study Co-Author and Physicist, University of Connecticut

That is where the Mira supercomputer came in. The scientists utilized this supercomputer, which was installed at the Argonne Leadership Computing Facility (ALCF), to solve the QCD’s intricate mathematical equations. These QCD can encode all potential robust interactions with the muon.

The DOE Office of Science User Facility—ALCF—had now retired the Mira supercomputer to make space for the more robust Aurora supercomputer, which is an exascale system set to be launched in 2021.

Mira was ideally suited for this work,” stated James Osborn, a computational scientist with the ALCF and Argonne National Laboratory’s Computational Science division. “With nearly 50,000 nodes connected by a very fast network, our massively parallel system enabled the team to run large simulations very efficiently.”

After four years of running the theoretical calculations on the Mira supercomputer, the scientists created the world’s first result for the hadronic light-by-light scattering contribution to the anomalous magnetic moment of the muon, governing for all the errors.

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

Thomas Blum, Study Co-Author and Physicist, University of Connecticut

In the meantime, the latest version of the Muon g-2 experiment is being conducted at Fermi National Accelerator Laboratory, in an attempt to minimize the ambiguity on the experimental side by a factor of four. Such results will provide a better understanding of the theoretical work that is currently being performed.

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

Journal References:

Blum, T., et al. (2020) Hadronic Light-by-Light Scattering Contribution to the Muon Anomalous Magnetic Moment from Lattice QCD. Physical Review Letters. doi.org/10.1103/PhysRevLett.124.132002.

Source: https://www.bnl.gov/

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