The oscillation of a tiny particle could upset known laws of physics

Written by Dennis Overbye

Evidence is mounting that a tiny subatomic particle appears to disobey known laws of physics, scientists said on Wednesday, a discovery that would open a vast and tantalizing hole in our understanding of the universe.

The result, the 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.

“It’s time for our rover to land on Mars,” said Chris Polly, a physicist at the National Fermi Accelerator Laboratory, or Fermilab, in Batavia, Illinois, who worked on this discovery for most of his time. career.

The famous particle is the muon, which is similar to an electron but much heavier and is an integral part of the cosmos. Polly and his colleagues – an international team of 200 physicists from seven countries – discovered that muons did not behave as expected when pulled through a strong magnetic field at Fermilab.

The outlier behavior poses a significant challenge to the Standard Model, the series of equations that lists 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 Muon g-2 electromagnet as it is transported to Fermilab’s new campus in Batavia, Ill., In 2013 (Cindy Arnold / Fermilab / US Department of Energy via The New York Times)

The results, the first from an experiment called Muon g-2, dovetailed with similar experiments at Brookhaven National Laboratory in 2001 that have since teased physicists.

In a virtual seminar and press conference on Wednesday, Polly pointed to a graph showing white space where the Fermilab results deviated from the theoretical prediction. “We can say with pretty high confidence, there has to be something contributing to this white space,” he said. “What monsters could be hiding there?”

“Today is an extraordinary day, long awaited not only by us but by the entire international physics community,” said Graziano Venanzoni, spokesperson for the collaboration and physicist at the Italian National Institute of Nuclear Physics, in a press release published by Fermilab. The results are also published in a series of articles submitted to several peer-reviewed journals.

The measurements have about a one in 40,000 chance of being a fluke, scientists reported, well below the gold standard needed to claim an official discovery by physics standards. Promising signals are disappearing in science all the time, but more data is on the way. Wednesday’s results represent just 6% of the total data the muon experiment is expected to collect in the coming years.

For decades, physicists have relied on and been bound by the Standard Model, which successfully explains the results of high-energy particle experiments at places like CERN’s Large Hadron Collider. But the model leaves many deep questions about the universe unanswered.

Most physicists believe that a rich mine of physics news is waiting to be found, if only they could see farther and farther. The additional data from the Fermilab experiment could provide a major boost to scientists keen 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 am very excited. I have the impression that this tiny oscillation can shake the foundations of what we thought we knew.

Muons are unlikely to take center stage in physics. Sometimes called “fat electrons,” they look like the familiar elementary particles that power our batteries, lights, and computers and orbit the nuclei of atoms; they have a negative electric charge and they have a property called spin, which makes them behave like tiny magnets. But they are 207 times more massive than their better-known cousins. They are also unstable, radioactively decaying into electrons and super light particles called neutrinos in 2.2 millionths of a second.

The role that muons play in the general scheme of the cosmos remains a puzzle.

Muons owe their current fame to a quirk of quantum mechanics, the non-intuitive rules that underlie the atomic realm.

Among other things, quantum theory holds that empty space is not really empty but actually bubbles up with “virtual” particles coming 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 the equations by a factor called g. According to a formula derived in 1928 by Paul Dirac, an English theoretical physicist and founder of quantum theory, the factor g of a solitary muon should be 2.

But muons are not alone, so the formula must be corrected for quantum hum coming from all the other potential particles in the universe. This causes the muon’s g-factor to be greater than 2, hence the name of the experiment: Muon g-2.

The extent to which g-2 deviates from theoretical predictions is an indication that the universe is still unknown – how many monsters, as Polly said, are lurking in the dark for physicists to find out.

In 1998, physicists at Brookhaven, including then graduate student Polly, 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 through a 50-foot-wide storage ring, a giant racing track controlled by superconducting magnets.

The value of g they obtained was at odds with the Standard Model’s prediction enough to excite the imagination of physicists – but without enough certainty to claim a solid discovery. Moreover, experts could not agree on the exact prediction of the Standard Model, which further clouded hopes.

Running out of money to redo the experiment, Brookhaven removed the 50-foot muon storage ring in 2001. The universe was left hanging.

The Muon ring g-2, at the National Fermilab Accelerator Laboratory in Batavia, Ill., August 28, 2017. The ring operates at minus 450 degrees Fahrenheit and studies the oscillation of muons as they pass through the magnetic field. (Reidar Hahn / Fermilab / US Department of Energy via The New York Times)

The big move

At Fermilab, a new campus dedicated to the study of muons was under construction.

“It opened up a world of possibilities,” Polly recalled in her biopic. At that 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 magnetic racetrack at Brookhaven. And so in 2013, the magnet traveled a 3,200-mile odyssey, mostly by barge, up the east coast, around Florida, and down the Mississippi River, then by truck through Illinois to Batavia, home of the Fermilab.

The experiment started in 2018 with a more intense muon beam and the goal of compiling 20 times more data than the Brookhaven version.

Meanwhile, in 2020, a group of 170 experts known as the Muon Theory Initiative g-2 released a new consensus value for the theoretical value of the muon’s magnetic moment, based on three years of workshops and calculations using the standard model. This response reinforced the initial divergence reported by Brookhaven.

In the dark

The team had to adapt to another wrinkle. To avoid human prejudice – and to avoid any rigging – the experimenters engaged in a practice, called blindness, which is common to great experiments. In this case, the master clock that keeps track of muon oscillation had been set at a rate unknown to researchers. The figurine was sealed in envelopes locked in the offices of Fermilab and the University of Washington in Seattle.

In a ceremony on February 25 that was videotaped and watched worldwide on Zoom, Polly opened the envelope for Fermilab and David Hertzog of the University of Washington opened the envelope for Seattle. The number inside was entered into a spreadsheet, providing a key to all of the data, and the result came out in a chorus of wows.

“It really led to a really exciting time, as no one in the collaboration knew the answer until that same moment,” said Saskia Charity, a Fermilab postdoctoral fellow who was working remotely from Liverpool, England during the pandemic. .

There was pride in making such a difficult measurement, and then joy that the results matched Brookhaven’s.

“It seems like confirmation that Brookhaven was no accident,” said Carena, the theorist. “They have a real chance to break the standard model.”

Physicists say the anomaly gave them ideas on how to search for new particles. Among them are particles light enough to be within reach of the Large Hadron Collider or its projected successor. Indeed, some may have already been recorded but are so rare that they have not yet emerged from the blizzard of data recorded by the instrument.

Another candidate called the Z-prime could shed light on some Big Bang puzzles, according to Gordan Krnjaic, a cosmologist at Fermilab.

The g-2 outcome, he said in an email, could set the physics agenda for the next generation. “If the central value of the observed anomaly remains fixed, the new particles cannot hide forever,” he said. “We will learn a lot more about fundamental physics in the future.”