‘Wobble’ shaking the core of physics
Subatomic muon may be disruptive, evidence shows
Evidence is mounting that a subatomic particle seems to be disobeying the known laws of physics, scientists have announced, a finding that would open a vast and tantalizing hole in our understanding of the universe.
The result, physicists say, suggests that there are forms of matter and energy vital to the nature and evolution of the cosmos that are not yet known to science.
“This is our Mars rover landing moment,” said Chris Polly, a physicist at the Fermi National Accelerator Laboratory, or Fermilab, in Batavia, Illinois, who has been working toward this finding for most of his career.
The particle celebre is the muon, which is akin to an electron but far heavier and is an integral element of the cosmos. Polly and his colleagues — a team of 200 physicists from seven countries — found that muons did not behave as predicted when shot through an intense magnetic field.
The aberrant behavior poses a firm challenge to the Standard Model, the suite of equations that enumerates the fundamental particles in the universe (17, at last count) and how they interact.
“This is strong evidence that the muon is sensitive to something that is not in our best theory,” said Renee Fatemi, a physicist at the University of Kentucky.
The results, the first from an experiment called “Muon g-2,” agreed with similar experiments at the Brookhaven National Laboratory in 2001 that have teased physicists ever since.
At a virtual seminar and news conference Wednesday, Polly pointed to a graph displaying white space
where the Fermilab findings deviated from the theoretical prediction.
“We can say with fairly high confidence, there must be something contributing to this white space,” he said. “What monsters might be lurking there?”
The results are also being published in a set of papers submitted to several peer-reviewed journals.
The measurements have about one chance in 40,000 of being a fluke, the scientists reported, well short of the gold standard needed to claim an official discovery by physics standards. Promising signals disappear all the time in science, but more data are on the way.
Wednesday’s results represent only 6% of the total data the muon experiment is expected to garner in the coming years.
For decades, physicists have relied on and have been bound by the Standard Model, which successfully
explains the results of high-energy particle experiments in places like CERN’s Large Hadron Collider. But the model leaves many deep questions about the universe unanswered.
Most physicists believe that a rich trove of new physics waits to be found, if only they could see deeper and further. The additional data from the Fermilab experiment could provide a major boost to scientists.
Marcela Carena, head of theoretical physics at Fermilab, who was not part of the experiment, said, “I’m very excited. I feel like this tiny wobble may shake the foundations of what we thought we knew.”
Muons are an unlikely particle to hold center stage in physics. Sometimes called “fat electrons,” they resemble the familiar elementary particles that power our batteries, lights and computers and whiz around the nuclei of atoms; they have
a negative electrical charge and a property called “spin,” which makes them behave like tiny magnets.
But they are 207 times as massive as their betterknown cousins. They are also unstable, decaying radioactively into electrons and superlightweight particles called “neutrinos” in 2.2 millionths of a second.
What part muons play in the overall pattern of the cosmos is still a puzzle. Muons owe their current fame to a quirk of quantum mechanics, the nonintuitive rules that underlie the atomic realm.
Among other things, quantum theory holds that empty space is not really empty but is in fact boiling with “virtual” particles that flit in and out of existence.
This entourage influences the behavior of existing particles, including a property of the muon called its magnetic moment, represented in equations by a
factor called “g.” According to a formula derived in 1928 by Paul Dirac, the English theoretical physicist and a founder of quantum theory, the g factor of a lone muon should be 2.
But muons are not alone, so the formula must be corrected for the quantum buzz from other potential particles in the universe. That leads the factor g for the muon to be more than 2, hence the name of the experiment: Muon g-2.
The extent to which g-2 deviates from theoretical predictions is one indication of how much is still unknown about the universe — how many monsters, as Polly put it, are lurking in the dark for physicists to discover.
In 1998 physicists at Brookhaven, including Polly, who was then a graduate student, set out to explore this cosmic ignorance by actually measuring g-2 and comparing it to predictions.
In the experiment, an accelerator called the “Alternating Gradient Synchrotron” created beams of muons and sent them into a 50-foot-wide storage ring, a giant racetrack controlled by superconducting magnets.
The value of g they obtained disagreed with the Standard Model’s prediction by enough to excite the imaginations of physicists, but without enough certainty to claim a solid discovery. Moreover, experts could not agree on the Standard Model’s exact prediction, further muddying hopeful waters.
Lacking money to redo the experiment, Brookhaven retired the 50-foot muon storage ring in 2001. The universe was left hanging.
Meanwhile, at Fermilab, a new campus devoted to studying muons was being built.
“That opened up a world of possibility,” Polly recalled in his biographical article.
By this time, Polly was working at Fermilab; he urged the lab to redo the g-2 experiment there. They put him in charge.
To conduct the experiment, however, they needed the 50-foot magnet racetrack from Brookhaven. And so in 2013 the magnet went on a 3,200-mile odyssey, mostly by barge, down the Eastern Seaboard, around Florida and up the Mississippi River, then by truck across Illinois to Batavia, home of Fermilab.
The experiment started in 2018 with a more intense muon beam and the goal of compiling 20 times as much data as the Brookhaven version.
In 2020 a group of 170 experts known as the “Muon g-2 Theory Initiative” published a new consensus value of the theoretical value of muon’s magnetic moment, based on three years of workshops and calculations using the Standard Model. That reinforced the original discrepancy reported by Brookhaven.