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The Mighty Muon

How a tiny particle and a giant magnetic donut are helping solve the mysteries of the universe

A tiny particle is making big news—and changing the way physicists understand the universe. That particle? A muon (pronounced mew-on). Roughly 200 times the size of an electron, muons are being used in an experiment that challenges the Standard Model of physics, which captures scientists’ current best understanding of the particles that make up the universe. Accurately explaining most forces and how they interact with matter, the Standard Model has been widely accepted since the 1970s and is the foundation of most modern high school science textbooks. However, there are some large gaps that are still unexplained. Most notably, the Standard Model doesn’t account for the force of gravity, or for the dark energy and dark matter that make up 95% of our universe.

That’s where the Fermi National Accelerator Laboratory’s (aka Fermilab) Muon g-2 experiment comes in. To understand more about muons, over 200 scientists from around the world have been injecting them into a giant donut-shaped magnetic tube to observe how they move. By making predictions about the speed of the muon’s wobble based on the Standard Model and then comparing those predictions to experimental results, physicists can see whether the current theory is complete.

Critical to the veracity of this experiment is the work of UMass researcher and professor of physics David Kawall, whose team has had a leading role in measuring the strength of the magnetic field within the muon storage and calibrating the magnet itself to ensure the highest level of accuracy.

A side image of four circular magnets with a purple hue

Some of Fermilab’s hardworking magnets

The scientists’ original theory, based on the Standard Model, hypothesized that muons’ spins would precess (think wobble) at a certain speed due to their interactions with other particles. But after the first experiment (completed in 2021), physicists saw that the muons’ interaction with the storage ring magnetic field is stronger (basically, they wobbled faster) than expected based on calculations using the standard model. This initial result was some of the first solid evidence showing that there are particles and forces at play that the Standard Model doesn’t explain. So, they fine-tuned the experiment and ran it again. In August 2023, Fermilab announced the results, which validated the evidence from 2021 with even more precision. Another iteration is already underway, expected to be completed in 2025.

What does this all mean? For theoretical physicists, this could mean creating a new paradigm within which they will need to investigate the inner workings of the universe. “New theories of physics will have to be consistent with the measurement results,” explains Kawall. “Some theories won’t survive, others will. This winnowing process leads to better, more accurate theories of nature.”

And over time, the results and the understanding that come from these experiments may, in fact, have a ripple effect on fields outside of physics. “In the long term, these more accurate and complete theories of nature might have practical implications,” says Kawall. For example, students and younger researchers who have been working on the Muon g-2 have already found ways to apply their knowledge from building the machinery and running the experiment to solve other current-day, real-world problems. “Several of the young scientists on the experiment are now developing new technologies for medical imaging,” shares Kawall. “So this will certainly have an impact. We just need to be patient and support these kinds of efforts that often have long time horizons.”

When the researchers finalize their findings of their experiment, the world will be closer to understanding the creation, composition, and expansion of the universe.

Hear more about the Muon g-2 experiment from Professor Kawall: