Scientists find mysterious particle that wobbles is still acting weird

Independent

03-06-2025

Final results from a long-running U.S. experiment have revealed that a tiny particle continues to act strangely, which scientists say is good news for our current understanding of the laws of physics.
The experiment focused on muons, mysterious particles considered heavier cousins to electrons. Muons wobble like a top when inside a magnetic field, and scientists have been studying that motion to see if it lines up with the Standard Model, the foundational rulebook of physics.
Tova Holmes, an experimental physicist at the University of Tennessee Knoxville, who is not part of the collaboration, said the experiment is "a huge feat in precision."
Experiments in the 1960s and 1970s seemed to indicate all was well. However, tests at Brookhaven National Laboratory in the late 1990s and early 2000s produced something unexpected: the muons weren't behaving as they should.
Decades later, an international collaboration of scientists decided to rerun the experiments with an even higher degree of precision. The team raced muons around a magnetic, ring-shaped track — the same one used in Brookhaven's experiment — and studied their signature wiggle at the Fermi National Accelerator Laboratory near Chicago.
The first two sets of results — unveiled in 2021 and 2023 — seemed to confirm the muons' weird behavior, prompting theoretical physicists to try to reconcile the new measurements with the Standard Model.
Now, the group has completed the experiment and released a measurement of the muon's wobble that agrees with what they found before, using more than double the amount of data compared to 2023. They submitted their results to the journal Physical Review Letters.
That said, it's not yet closing time for our most basic understanding of what's holding the universe together. While the muons raced around their track, other scientists found a way to more closely reconcile their behavior with the Standard Model with the help of supercomputers.
There's still more work to be done as researchers continue to put their heads together and future experiments take a stab at measuring the muon wobble — including one at the Japan Proton Accelerator Research Complex that's expected to start near the end of the decade. Scientists also are still analyzing the final muon data to see if they can glean information about other mysterious entities like dark matter.
'This measurement will remain a benchmark ... for many years to come,' said Marco Incagli with the National Institute for Nuclear Physics in Italy.
By wrangling muons, scientists are striving to answer fundamental questions that have long puzzled humanity, said Peter Winter with Argonne National Laboratory.
'Aren't we all curious to understand how the universe works?' said Winter.
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