Fermilab’s new measurement of subatomic muon particle challenges standard physics theories
Imagine trying to prove that 1+1=2, but when you do the calculations, it turns out that the result is off by 0.1%. That scenario is similar to the riddle that’s facing physicists worldwide as they try to explain how the universe works in relation to a tiny particle called a muon.
Now, after years of excruciatingly exact experiments, researchers at Fermilab in Batavia have made new measurements of the muon that challenge the generally accepted laws of physics. The results, announced Thursday, potentially could lead to the discovery of new particles and expand the boundaries of physics.
The outgoing spokesman for the project, Fermilab senior scientist Brendan Casey, addressed the possibilities raised by the latest results.
“We can tell that something’s missing,” he said. “We can’t tell what it is. … It’s wide open right now.”
The riddle that physicists are trying to solve arose in 2001 when Brookhaven National Laboratory on Long Island, New York, first announced measurements of the muon’s motion that differed from what physicists predicted.
The muon is a particle in atoms that is 200 times more massive than the electron, which makes it a good target for measurements of its interactions with other particles, detected by the spin and wobble of its internal magnet. How it behaves alerts observers to every force acting on it, known or unknown.
A simple estimate based on the expected strength of the magnet, known as “g,” is 2. But Brookhaven found that the actual measurement was off by about 0.1%. Physicists have struggled to explain that difference ever since. Theories include supersymmetry, in which each particle has a partner; something called lattice gauge theory; or that there could be other unseen particles.
To zero in on the measurements of the muon, the giant 50-foot-around magnet that Brookhaven used was shipped to Fermilab in 2013. Fermi National Accelerator Laboratory, as it is formally known, generates muons traveling at nearly the speed of light, and then runs the particles some 1,000 times around a circular magnetic obstacle course to see how they interact with other particles.
After six years of experiments, Fermilab finished its measurements last month and announced the latest results Thursday to great international interest from physicists: g-2 = 0.00233184110 +/- 0.00000000043 (stat.) +/- 0.00000000019 (syst.).
The results, while incomprehensible to the average person, are twice as accurate as preliminary findings disclosed in 2021, and give further evidence of a disturbance in the force, so to speak, or an unexplained fluctuation in reality.
“This measurement is an incredible experimental achievement,” said Peter Winter, co-spokesperson for the Muon g-2 Theory Initiative collaboration. “Getting the systematic uncertainty down to this level is a big deal and is something we didn’t expect to achieve so soon.”
University of Illinois at Urbana-Champaign Professor Aida X El-Khadra said the results set up a future “showdown” between theory and experimentation.
El-Khadra is the chair of the Muon g-2 Theory Initiative, which uses the Standard Model to predict the value of the muon “magnetic anomaly,” as it is known.
The group made a prediction in 2020, but as new evidence has arisen since then, it will make a new refined prediction next year. That will be compared to the final Fermilab measurement when it’s released in 2025, to see if there is still a discrepancy that would suggest unknown new particles or forces.
“None of us want to be close to the experimental value,” El-Khadra said. “We want it to be completely incompatible, because then we could say, ha ha, we finally have put a crack in Standard Model of particle physics. We know there have to be new particles or forces out there.”
The Muon g-2 team — made up of 182 collaborators from 33 institutions in seven countries — submitted its results in a paper to Physical Review Letters.
But there’s more work to do to be absolutely certain of the results. The latest announcement is based on just the first three years of measurements. Researchers still have three more years of data to crunch before they make a final pronouncement.
The research is supported by multiple funding agencies in seven countries. The primary sponsor is the U.S. Department of Energy Office of Science, which invested $47 million into the main construction project, plus several other infrastructure projects.
Such basic research is unlikely to have immediate practical applications, but an understanding of how subatomic particles work has led to advancements such as transistors, lasers and MRIs.
University of Chicago Professor Dan Hooper said the findings could signal a “new physics” beyond current theories, but it’s too early to tell.
He worked on a theory involving a force called Z prime, which if true, would cause muons to interact, and would change our understanding of the early stages of the universe.
“We live for this stuff,” he said. “If you told me we’re not going to discover anything beyond the Standard Model for the next 30 years, I’d stop doing particle physics.”
Asked why the average person should care, Fermilab researchers said such research can lead to the development of useful technologies, but also try to answer humans’ basic questions about why the universe is the way it is.
