Evidence is mounting that a tiny subatomic particle seems to be disobeying the known laws of physics, scientists announced Wednesday, 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. The new work, they said, could eventually lead to breakthroughs more dramatic than the heralded discovery in 2012 of the Higgs boson, a particle that imbues other particles with mass.
“This is our Mars rover landing moment,” said Chris Polly, a physicist at the Fermi National Accelerator Laboratory, or Fermilab, in Batavia, Illinois, where the research is being conducted. He has been working on the project for most of his career.
Polly is part of an international team of 200 physicists from 35 institutions and seven countries who have been operating an experiment at Fermilab involving muons, subatomic particles that are akin to electrons but far heavier. When muons were shot through an intense magnetic field, they did not behave quite as expected, according to precise theoretical predictions.
“This quantity we measure reflects the interactions of the muon with everything else in the universe,” said Renee Fatemi, a physicist at the University of Kentucky. “This is strong evidence that the muon is sensitive to something that is not in our best theory.”
The results agreed with similar experiments at the Brookhaven National Laboratory in 2001 that have teased physicists ever since.
“After 20 years of people wondering about this mystery from Brookhaven, the headline of any news here is that we confirmed the Brookhaven experimental results,” Polly said at a news conference Wednesday.
He pointed to a graph displaying white space between the theoretical prediction for the muons’ behavior and the new findings from Fermilab. “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 researchers announced their first findings from the experiment, called Muon g-2, in a virtual seminar and news conference Wednesday. The results are also being published in a set of papers submitted to the Physical Review Letters, Physical Review A, Physical Review D and Physical Review Accelerators and Beams.
“Today is an extraordinary day, long awaited not only by us but by the whole international physics community,” Graziano Venanzoni, a spokesperson for the collaboration and a physicist at the Italian National Institute for Nuclear Physics, said in a statement issued by Fermilab.
The measurements have about one chance in 40,000 of being a fluke, the scientists reported, a statistical status called “4.2 sigma.” That is still short of the gold standard — “5 sigma,” or about 3 parts in 10 million — needed to claim an official discovery by physics standards. Promising signals disappear all the time in science, but more data are on the way that could put their study over the top. Wednesday’s results represent only 6% of the total data the muon experiment is expected to garner in the coming years.
The additional data could provide a major boost to scientists eager to build the next generation of expensive particle accelerators.
For decades, physicists have relied on and have been bound by a theory called the Standard Model, a suite of equations that enumerates the fundamental particles in the universe (17 by last count) and the ways they interact. It successfully explains the results of high-energy particle experiments in places like CERN’s Large Hadron Collider. But the model leaves deep questions about the universe unanswered, and most physicists believe that a rich trove of new physics waits to be found, if only they could see deeper and further.
It might also lead to explanations for the kinds of cosmic and human mysteries that occupy the restless nights of a lonely species locked down by an implacable virus. What exactly is dark matter, the unseen stuff that astronomers say makes up one-quarter of the universe by mass? Indeed, why is there matter in the universe at all?
On Twitter, physicists responded with a mixture of enthusiasm and caution. “Of course the possibility exists that it’s new physics,” Sabine Hossenfelder, a physicist at the Frankfurt Institute for Advanced Study, said. “But I wouldn’t bet on it.”
Fabiola Gianotti, director-general of CERN, sent her congratulations and called the results “intriguing.”
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.”
‘Who Ordered That?’
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 they have a property called spin, which makes them behave like tiny magnets. But they are 207 times as massive as their better-known 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 creation is still a puzzle. “Who ordered that?” Columbia University physicist I.I. Rabi said when they were first discovered in 1936. Nowadays, muons are produced copiously at places like the Large Hadron Collider when more ordinary particles are crashed together at high energies.
Muons recently slipped onto center stage because of 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.
“You might think that it’s possible for a particle to be alone in the world,” Polly said in a biographical statement posted by Fermilab. “But in fact, it’s not lonely at all. Because of the quantum world, we know every particle is surrounded by an entourage of other particles.”
This entourage influences the behavior of existing particles, including a property of the muon called its magnetic moment, characterized 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 the formula must be corrected for the quantum buzz arising from all the 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 set out to explore this cosmic ignorance by actually measuring g-2 and comparing it to predictions. The group included Polly, then a graduate student; he made his mark, when things were not going well, by discovering that some delicate detectors had been smeared with fingerprints.
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.
At the time, redoing the experiment would not have increased the precision enough to justify the cost, Carena said, and in 2001 Brookhaven retired the 50-foot muon storage ring. The universe was left hanging.
The Big Move
Enter Fermilab, where 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 and his colleagues could redo the g-2 experiment there, with more precision. He became the project manager for the experiment.
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 magnet resembled a flying saucer, and it drew attention as it was driven south across Long Island at 10 mph. “I walked along and talked to people about the science we were doing,” Polly wrote. “Moving it through the Chicago suburbs to Fermilab offered another chance for outreach. It stayed over one night in a Costco parking lot. Well over a thousand people came out to see it and hear about the science.”
The experiment started up in 2018 with a more intense muon beam and the goal of compiling 20 times as much data as the Brookhaven version.
Meanwhile, 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 answer reinforced the original discrepancy reported by Brookhaven.
Reached by phone Monday, Aida X. El-Khadra, a physicist at the University of Illinois and a co-chair of the Muon g-2 Theory Initiative, said they had been waiting for this result for a long time.
“I have not had the feeling of sitting on hot coals before,” El-Khadra said.
On the day of the Fermilab announcement, another group, using a different technique known as a lattice calculation to compute the muon’s magnetic moment, got a different answer than El-Khadra’s group.
“Yes, we claim that there is no discrepancy between the Standard Model and the Brookhaven result, no new physics,” Zoltan Fodor of Pennsylvania State University, one of the authors of a report published in Nature on Wednesday, said in an interview, adding a note of uncertainty to the proceedings.
El-Khadra called it an “amazing calculation.”
The new lattice calculations, she said, needed to be checked against independent work from other groups to eliminate the possibility of systematic errors.
Into the Dark
The Fermilab had to accommodate another wrinkle. To avoid human bias — and to prevent any fudging — the experimenters engaged in a practice, called blinding, that is common to big experiments. In this case, the master clock that keeps track of the muons’ wobble had been set to a rate unknown to the researchers. The figure was sealed in a pair of envelopes locked in the office of Joe Lykken, deputy director of research at Fermilab, and at the University of Washington in Seattle.
In a ceremony Feb. 25 that was recorded on video and watched around the world on Zoom, Polly opened the Fermilab envelope, and David Hertzog from the University of Washington opened the Seattle envelope. The number inside was entered into a spreadsheet, providing a key to all the data, and the result popped out to a chorus of wows.
“That really led to a really exciting moment, because nobody on the collaboration knew the answer until the same moment,” said Saskia Charity, a Fermilab postdoctoral fellow who has been working remotely from Liverpool, England, during the pandemic.
The first reaction, she recalled, was pride that they had managed to perform such a hard measurement.
The second was that the results from Fermilab matched the previous results from Brookhaven. This was great news to the physicists who had worried that the Brookhaven result was an anomaly that would evaporate with more data.
“This seems to be a confirmation that Brookhaven was not a fluke,” Carena, the theorist, said. “They have a real chance to break the Standard Model.”
And what will they find when they break it?
The muon anomaly, physicists said, has now given them ideas for how to search for new particles. Lykken and Arkani-Hamed noted that among the prospective candidates were particles lightweight enough to be within the grasp of the Large Hadron Collider or its projected successor. Indeed, some might already have been recorded but are so rare that they have not yet emerged from the blizzard of data recorded by the instrument.
Another possibility, championed by Dan Hooper and Gordan Krnjaic, both of Fermilab, is a lightweight particle called Z-prime; its existence could also explain why the cosmos appears to be expanding slightly faster than the standard cosmological models predict. Any Z-primes would have decayed into lighter particles called neutrinos early in the Big Bang, pumping extra energy into the cosmic expansion and giving it a boost before disappearing.
Krnjaic said the g-2 result could set the agenda for particle physics for the next generation. “If the central value of the observed anomaly stays fixed, the new particles can’t hide forever,” he said. “We will learn a great deal more about fundamental physics going forward.”
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