A long-running experiment finds a tiny particle is still acting weird

Results from a U.S. experiment show muons behaving strangely, challenging the laws of physics, sparking further research.

Final results from a long-running U.S.-based experiment announced Tuesday show a tiny particle continues to act strangely — but that's still good news for the laws of physics as we know them.

“This experiment is a huge feat in precision,” said Tova Holmes, an experimental physicist at the University of Tennessee, Knoxville who is not part of the collaboration.

The mysterious particles called muons are considered heavier cousins to electrons. They wobble like a top when inside a magnetic field, and scientists are studying that motion to see if it lines up with the foundational rulebook of physics called the Standard Model.

Experiments in the 1960s and 1970s seemed to indicate all was well. But tests at Brookhaven National Laboratory in the late 1990s and early 2000s produced something unexpected: the muons weren't behaving like 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.