Written by Dennis Overbye
There is growing evidence that a tiny subatomic particle does not seem to obey the known laws of physics, scientists announced on Wednesday. This realization would open a huge and enticing hole in our understanding of the universe.
The result, say physicists, suggests that there are forms of matter and energy that are of crucial importance for the nature and development of the cosmos and are not yet known to science.
“This is the moment our Mars rover lands,” said Chris Polly, a physicist at the Fermi National Accelerator Laboratory or Fermilab in Batavia, Illinois, who has spent most of his career working towards this finding.
The famous particle is the muon, which resembles an electron but is much heavier and is an integral part of the cosmos. Polly and his colleagues – an international team of 200 physicists from seven countries – found that muons did not behave as predicted when they were shot by an intense magnetic field in the Fermilab.
The deviating behavior poses a major challenge for the standard model, the suite of equations that enumerate the fundamental particles in the universe (17, counted last) and how they interact.
“This is strong evidence that the muon is sensitive to something that doesn’t match our best theory,” said Renee Fatemi, a physicist at the University of Kentucky.
The results, the first from an experiment called Muon g-2, were consistent with similar experiments at Brookhaven National Laboratory in 2001 that have annoyed physicists since then.
At a virtual seminar and press conference on Wednesday, Polly pointed to a graph with a white space where the Fermilab results deviated from the theoretical prediction. “We can say with pretty high confidence that there has to be something that adds to this white space,” he said. “What monsters might be lurking there?”
“Today is an extraordinary day that not only we, but the entire international physics community has long been waiting for,” said Graziano Venanzoni, collaboration spokesman and physicist at the Italian National Institute for Nuclear Physics, in a statement from Fermilab. The results are also published in a series of articles submitted to several peer-reviewed journals.
The measurements have about a 40,000 chance of being a coincidence, the scientists reported, well below the gold standard required to claim an official discovery by physical standards. Promising signals keep disappearing in science, but more data is on its way. Wednesday’s results account for only 6% of the total data the muon experiment is expected to collect in the coming years.
For decades, physicists have relied on the Standard Model, which successfully explains the results of high-energy particle experiments at places like CERN’s Large Hadron Collider. However, the model leaves many deep questions about the universe unanswered.
Most physicists believe that a rich treasure trove of new physics is waiting to be found, if only they could see deeper and further. The additional data from the Fermilab experiment could give an important boost to scientists looking to build the next generation of expensive particle accelerators.
Marcela Carena, head of theoretical physics at Fermilab, who was not part of the experiment, said: “I’m very excited. I have a feeling that tiny wobble could shake the very foundations of what we thought we knew. “
Muons are an unlikely particle that is at the fore in physics. Sometimes referred to as “fat electrons”, they resemble the familiar elementary particles that power our batteries, lights, and computers and whiz around atomic nuclei. They have a negative electrical charge and a property called spin that makes them behave like tiny magnets. But they’re 207 times as massive as their better-known cousins. They are also unstable and decay radioactively into electrons and super-light particles called neutrinos in 2.2 millionths of a second.
What role muons play in the overall pattern of the cosmos is still a mystery.
Muons owe their current fame to a quirk of quantum mechanics, the non-intuitive rules that underlie the atomic domain.
The quantum theory states, among other things, that empty space is not really empty, but actually cooks with “virtual” particles that whiz in and out of existence.
This wake affects the behavior of existing particles, including a property of the muon called magnetic moment, which is represented in equations by a factor called g. According to a formula derived in 1928 by Paul Dirac, the English theoretical physicist and founder of quantum theory, the g-factor of a lone muon should be 2.
But muons are not alone, so the formula for the quantum sum that comes from all other potential particles in the universe needs to be corrected. As a result, the factor g for the muon is more than 2, hence the name of the experiment: muon g-2.
How far g-2 deviates from theoretical predictions is an indication of how much is still unknown about the universe – how many monsters, as Polly put it, lurk in the dark for physicists to spot.
In 1998, Brookhaven physicists, including Polly, who was then a PhD student, set out to explore this cosmic ignorance by actually measuring g-2 and comparing it to predictions.
In the experiment, an accelerator called Alternating Gradient Synchrotron generated muon beams and sent them into a 50-foot-wide storage ring, a giant race track controlled by superconducting magnets.
The value of g they received didn’t agree enough with the Standard Model’s prediction to stimulate the physicists’ imagination – but with insufficient confidence to claim a solid discovery. In addition, experts could not agree on the exact prediction of the Standard Model, which further clouded the hopeful waters.
Brookhaven ran out of money to repeat the experiment in 2001 and withdrew the 50-foot muon storage ring. The universe got stuck.
The big step
A new campus for studying muons has been built in Fermilab.
“That opened up a world of possibilities,” recalled Polly in his biographical article. At the time, Polly was working at the Fermilab. He asked the laboratory to repeat the G-2 experiment there. You blamed him.
However, they needed the Brookhaven 50-foot magnetic separation line to run the experiment. And so in 2013 the Magnet embarked on a 3,200-mile odyssey, mostly by barge, along the east coast, around Florida and up the Mississippi, then by truck via Illinois to Batavia, the home of Fermilab.
The experiment began in 2018 with a more intense muon beam and the goal of compiling 20 times as much data as in the Brookhaven version.
In 2020, a group of 170 experts known as the Muon g-2 Theory Initiative published a new consensus value for the theoretical value of the magnetic moment of muon, based on three-year workshops and calculations using the Standard Model. That answer compounded the original discrepancy that Brookhaven reported.
Into the dark
The team had to pick up another fold. To prevent human bias – and to prevent any fudging – the experimenters engaged in a practice known as blinding, which is common in large-scale experiments. In this case, the master clock, which tracks the shaking of the muons, was set at a rate unknown to the researchers. The figure was sealed in envelopes that were sealed in the Fermilab and University of Washington offices in Seattle.
In a ceremony in February, Polly opened the Fermilab envelope and David Hertzog of the University of Washington opened the Seattle envelope. The number it contained was entered into a spreadsheet that provided a key to all of the data, and the result became a chorus of wows.
“It really turned into a really exciting moment, as until that very moment no one in the collaboration knew the answer,” said Saskia Charity, a Fermilab postdoctoral fellow who worked remotely from Liverpool, England during the pandemic.
It was proud that they had managed to take such a tough measurement, and then joy that the results matched those from Brookhaven.
“This seems to confirm that Brookhaven was no accident,” said Carena, the theorist. “You have a real chance of breaking the standard model.”
Physicists say the anomaly gave them ideas for finding new particles. Among them are particles that are light enough to be captured by the Large Hadron Collider or its planned successor. Some may have already been recorded, but are so rare that they have not yet emerged from the snowstorm of the data recorded by the instrument.
Another candidate named Z-Prime could shed some light on some of the riddles in the Big Bang, according to Gordan Krnjaic, a cosmologist at Fermilab.
The G-2 outcome, he said in an email, could set the agenda for physics in the next generation. “If the central value of the observed anomaly remains fixed, the new particles cannot hide forever,” he said. “We’ll learn a lot more about basic physics in the future.”