Muon Discovery brings physicists a step closer to theoretical confrontation

On July 24, a large team of researchers gathered in Liverpool to reveal a single number related to the behavior of the muon, a subatomic particle that may open a gateway to new physics for our universe.

All eyes were on the computer screen when someone wrote a secret code to release the results. The first number that came out was met with indignation: too many unsettling shots, oh my god and what have we done wrong. “There was a collective exhalation across multiple continents,” said Kevin Bates, a Virginia Tech physicist who was five hours away and almost present at the meeting, after making the final calculation. The new measurement was almost identical to what the physicists had calculated two years earlier – now with twice the accuracy.

So comes the latest finding from the Muon g-2 Collaboration, which is running an experiment at the Fermi National Accelerator Laboratory, or Fermilab, in Batavia, Illinois, to study the deflecting motion of the muon. measurements, Announce to the public And It has been submitted to Physical Review Letters On Thursday morning, physicists were one step closer to discovering whether there are more types of matter and energy that make up the universe than have been calculated.

“It’s all really down to that single digit,” said Hannah Penney, a physicist at MIT’s Lincoln Laboratory who worked on muon measurements as a graduate student.

Scientists are testing the Standard Model, a grand theory that includes all known particles and forces in nature. Although the Standard Model has successfully predicted the outcome of countless experiments, physicists have long had a hunch that its framework is incomplete. The theory fails to explain gravity, nor can it explain dark matter (the glue that holds our universe together), or dark energy (the force that holds it apart).

One of the many ways researchers are looking for physics beyond the Standard Model is by studying muons. As heavier cousins ​​of the electron, muons are unstable, surviving for only two millionths of a second before decaying into lighter particles. They also act like little bar magnets: put the muon in a magnetic field, and it will swing like a top. The speed of this motion depends on a property of the muon called the magnetic moment, which physicists abbreviate with the symbol g.

In theory, g should be exactly equal to 2. But physicists know that this value is faltered by the “quantum foam” of virtual particles scurrying out of existence and preventing empty space from ever being truly empty. These passing particles change the rate of oscillation of the muon. By taking an inventory of all the forces and particles in the Standard Model, physicists can predict how much g will be compensated for. They call this deviation g-2.

But if there are unknown particles in play, experimental measurements of g will not match this prediction. “And that’s what makes the muon so exciting to study,” said Dr. Penney. “It’s sensitive to all the particles out there, even those we don’t know about yet.” She added that any difference between theory and experiment means that new physics is on the horizon.

To measure g-2, researchers at Fermilab generated a beam of muons and directed them at a 50-foot-diameter donut-shaped magnet, the interior of which was filled with virtual particles that have come to life. As the muons raced around the ring, detectors along its edge recorded how fast they wobbled.

Using 40 billion muons — five times as much data as the researchers had in 2021 — the team measured g-2 to be 0.00233184110, a deviation of one-tenth of a percent from 2. The result was an accuracy of 0.2 parts per million. Dr. Bates said this is like measuring the distance between New York City and Chicago with an uncertainty of only 10 inches.

“It’s an amazing achievement,” said Alex Keshavarzi, a University of Manchester physicist and member of the Muon g-2 Collaboration. “This is the world’s most accurate measurement ever made in a particle accelerator.”

But whether the measured g-2 matches the Standard Model prediction has yet to be determined. That’s because theoretical physicists have two ways of calculating g-2, based on different ways of calculating the strong force, which holds protons and neutrons together inside the nucleus.

The traditional calculation is based on 40 years of intense force measurements made by experiments around the world. With this approach, the g-2 prediction is only as good as the data used, said Aida Khadra, a theoretical physicist at the University of Illinois Urbana-Champaign and chair of the moon-g-2 theory initiative. She said empirical limitations in that data could make that prediction less accurate.

A newer technology called grid computation, which uses supercomputers to model the universe as a four-dimensional grid of points in time and space, has also emerged. Al-Khadhra confirmed that this method does not benefit from data at all. There’s just one problem: it generates a g-2 prediction that differs from the traditional method.

“Nobody knows why these two people are so different,” said Dr. Keshavarzi. “They should be exactly the same.”

Compared to the traditional prediction, the latest g-2 measurement has a variance of more than 5 sigma, which corresponds to a 1 in 3.5 million chance that the result is a fluke, Dr. Keshavarzi said, adding that this degree of certainty was beyond the level required to claim the discovery. . (This is an improvement from 4.2-Sigma result in 2021and a 3.7-sigma measurement made at Brookhaven National Laboratory near the turn of the century.)

But when they compared it to the retinal prediction, Dr. Keshavarzi said, there was no discrepancy at all.

Experiments rarely go beyond theory in physics, said Dr. Bates, but this is one of those times. “The interest is in the theoretical community,” he added. “The lights are now on them.”

Dr. Penney said, “We’re on the edge of our seats to see how this discussion of theory plays out.” Physicists expect to better understand g-2 predictions by 2025.

While the two theoretical camps work on it, the experimentalists will hone the g-2 measurement further. They have more than twice the amount of data left to scrutinize, and once that’s included, their accuracy will improve by another factor.

The latest finding moves physicists a step closer to confronting the Standard Model. But even if new physics is confirmed, more work will be needed to figure out what that actually is. Dr. Keshavarzi said the discovery of the incompleteness of known laws of nature will lay the foundation for a new generation of experiments, as it will tell physicists where to look.

For Dr. Bates, who has spent nearly 30 years pushing the boundaries of the Standard Model, proving the existence of new physics will be a celebratory event and a reminder of all that remains to be done. “On the one hand, you’ll have a toast and celebrate the success, a real breakthrough,” he said. “But then you get back to work. What are the next ideas we can work on?”