Results of Muon Experiment Offer More Precision But No Added Clarity

Science|Muon Experiment Was ‘Hugely Successful’ but Clarified Little

It has been 12 years since physicists transported a giant magnetic ring down the Atlantic coast, around Florida, up the Mississippi River and across two interstates to Batavia, Ill. On Tuesday, the team behind that ring unveiled their final result: the most precise value yet recorded for the tiny wobble of a subatomic particle called the muon.

Physicists hoped that the measurement, submitted to the journal Physical Review Letters, would open a window to new types of energy and matter that so far have only been theorized.

“We want to know how our universe formed, what it’s made out of and how it interacts,” said Peter Winter, a physicist at Argonne National Laboratory and a spokesman for the Muon g-2 Collaboration, which ran the experiment at Fermi National Accelerator Laboratory, or Fermilab. The new result, he said, “will stand as a benchmark for years to come.”

But a glaring problem remains. Physicists have predicted two distinct values for the muon’s wobble but aren’t sure which is correct. The new result matches one prediction, but until the other prediction can be satisfyingly explained away, scientists won’t know if they have uncovered evidence of new physics.

“The Fermilab experiment is hugely successful, they did their job,” said Aida El-Khadra, a physicist at the University of Illinois Urbana-Champaign who leads the Muon g-2 Theory Initiative. “We theorists, we still need to follow up.”

Until the dust settles, Dr. El-Khadra added, “the jury is still out.”

Muons are similar to electrons but far heavier and unstable in nature. When placed in a magnetic field, they precess, or wobble, like a spinning top. The speed of that wobble depends on a property of the muon related to its internal magnetism, known to physicists as g.