Particle physicists at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have released the third and final measurements of a mysterious “wobbling” of a subatomic particle called a muon (pronounced mew-on), which is similar to an electron but with 200 times more mass.
The results offer the most precise measurement to date of this anomalous wiggle that has not yet been explained by modern physics.
Virginia Tech physicists Kevin Pitts and Esra Barlas Yucel are members of the global research team working on this experiment, which is called Muon g-2 (“g minus two”).
What exactly did physicists measure?
“When muons are sent into a magnetic field, they act like tiny bar magnets, and spin around and around. But their spin isn’t perfect; it wobbles a little bit,” said Pitts, dean of Virginia Tech’s College of Science and a professor in the Department of Physics. “Think of a spinning top just before it’s about to fall over.”
“We’ve been measuring the frequency, or the number of cycles per second, of this wobble to a ridiculous precision,” said Pitts.
How precise are we talking?
The final results are precise to the tune of 127 parts per billion.
“If you could measure the width of the United States with this level of precision, you’d be able to detect a difference of just over half a millimeter. That’s the thickness of a single grain of sand,” said Barlas Yucel, a senior research associate in the Virginia Tech Department of Physics.
Why do we care about this measurement?
The muon’s wobbling doesn’t sync up with the predictions from the most important theory in particle physics: the Standard Model.
“The Standard Model is our best description of what the universe is made of, what kind of particles are contained within, and how they interact with each other. But it isn’t complete. There are things that it doesn’t explain yet, such as gravity, dark matter, or why there is more matter than antimatter,” said Barlas Yucel. “And this is why scientists look closely at situations that don’t conform. They are looking for cracks between theories or perhaps new physics.”
Do the results show new physics?
Even as the muon g-2 results were becoming more defined, new theoretical calculations were recasting the Standard Model itself, bringing its predictions ever closer to the experimental results, and reducing the evidence of new physics.
“Theorists are still deliberating if there’s a true discrepancy between theory and measurement,” Pitts said. “But what we know about the standard model is that it’s right a heck of a lot more often than it’s wrong. New physics or not, the measurements announced today provide a clear benchmark for a new extension of the Standard Model of particle physics.”
Barlas Yucel added: “We’re also extracting entirely different physics from the same data. Two independent analyses are still to come, one of which is searching for signs of dark matter. It shows the versatility of this amazing experiment.”
About Pitts
Kevin Pitts is the dean of the Virginia Tech College of Science and a professor in the Department of Physics. Previously, he was chief research officer at Fermilab National Accelerator Laboratory and a professor of physics at the University of Illinois at Urbana-Champaign.
About Barlas Yucel
Esra Barlas Yucel is a Senior Research Associate in the Virginia Tech Department of Physics. She serves as the analysis co-coordinator for the Muon g-2 experiment.
By Kelly Izlar