Picture an 11,000-pound elephant standing on a bathroom scale. Now imagine that scale is so precise, it can tell that you placed a couple of sunflower seeds on the elephant's back.
That is the level of precision achieved by an international collaboration, including Cornell researchers, hosted by the U.S. Department of Energy's Fermi National Accelerator Laboratory. The group set out to measure the magnetic anomaly of the muon - a tiny, elusive particle that could have very big implications for understanding the subatomic world.
On June 3, the collaboration - which consists of 176 scientists from 34 institutions in seven countries - announced that the third and final round of data, collected between 2021 and 2023, has been analyzed, and the researchers have increased the precision of their measurement by more than a factor of four, to 127 parts-per-billion.
That level of precession was made possible, in part, by a team led by Lawrence Gibbons, professor of physics in the College of Arts and Sciences, which built hardware to measure hundreds of billions of muons and then helped parse the data.
A quadrupole magnet leads to the muon g-2 particle storage ring at the U.S. Department of Energy's Fermi National Accelerator Laboratory, where an international collaboration has been measuring the muon's magnetic anomaly.
"We're all feeling incredibly proud and happy with the final result, in terms of what we set out to do, and how beautiful the data was," said Gibbons, who serves an analysis coordinator on the project. "A group of amazing young colleagues really worked on this data to understand it at such an amazing level."
While the scientists are celebrating the conclusion of the decade-plus effort, there is one group of researchers who may be less enthusiastic about the result: the physics theorists who rely on the Standard Model to explain the laws that govern fundamental particles. The model has incredible predictive power. But it's a work in progress, and the tiny muon might make that work more difficult.
"People do extensions of the Standard Model and introduce new symmetries, which give you new particles and forces that can give a natural explanation for some of these things," Gibbons said. "But along the way, all those new things actually will affect the strength of this magnetic field for the muon that we measure. So we've just made the theorists' lives a lot harder, because if they come up with a new model, they have a lot less wiggle room on what they predict for the anomaly."
308.5 billion muons and a lot of froth
Muons are essentially tiny magnets with their own magnetic field, just like electrons, but more than 200 times more massive and far more unstable. They decay in a few millionths of a second and are difficult to observe at the quantum mechanical level because the vacuum in which they exist is not an empty cavity, but a big bubbling froth.
The muon g-2 collaboration - named for the value of the magnet's strength caused by its intrinsic spin - was a response to a historic 1998 experiment at Brookhaven National Laboratory that stunned the physics community by indicating that muons' magnetic field deviates significantly from the Standard Model's prediction.
To measure the muon's magnetic field more precisely, in 2013, the Brookhaven magnetic ring was transported to the Fermilab facility in Batavia, Illinois, where it was coupled with an even stronger particle accelerator that could produce more than 20 times as many muons. In 2018, the group launched the first of six experiments, ultimately firing a total of 308.5 billion muons into a 14-meter-diameter magnetic ring at nearly the speed of light.
Gibbons's team created the high-speed digitizers for the calorimeters that captured the muon data.
"Before g-2, I really had not had a chance to get my hands dirty building hardware and electronics for the experiments that I was involved in the past," he said. "So this has given me a lot of expertise in that end of things."
The team included two Cornell doctoral students and a postdoctoral researcher, all of whom Gibbons credited with devising a new way of modeling the muon's effects - and there was a lot to model.
"This last round constituted our largest data set. It was about two and a half times larger than the combination of the previous data," Gibbons said. "So there was just a lot of work, but the nice side of that is it let us explore, with really high precision, a lot of subtle effects that we needed to understand in the data."
'It is so incredibly stressful'
Initially, many scientists hoped the muon g-2 experiment would indicate the existence of some kind of new fundamental particle or force in the universe that has yet to be accounted for. But judging from the current theory predictions, that seems less and less likely, according to Gibbons.
"It looks like that window is closing," he said.
The g-2 collaboration did confirm the Brookhaven result, and that has pushed the theorists to try to understand their Standard Model predictions much more deeply and examine the different methods they have been using.
"A little bit has to happen in terms of finalizing what happens with the Standard Model calculation itself, but that's going to come," Gibbons said. "I think within a couple of years, they'll also be at the 100-part-per-billion level. And then we'll be able to rule out whole classes of people's favorite extensions of the Standard Model, just because they can no longer fit with the measurement that we've done."
A Cornell team led by Lawrence Gibbons, professor of physics, built digitizer modules that were installed around the muon g-2 ring.
The muon's magnetic anomaly may have a new definitive measurement, but there are plenty of other particles awaiting the same level of scrutiny. Gibbons is using his newfound expertise in building hardware for a project that will measure another finicky subatomic particle: the pion.
The pion project will also include two of his fellow g-2 researchers. The collaborative nature of the muon g-2 experiment was one of the things Gibbons loved most about it.
"The team wasn't just particle physicists. There are high-energy physicists, nuclear physicists, atomic physicists and accelerator physicists, plus, of course, really talented engineers and technicians, kind of under the hood, that helped us build and run things over the years," he said. "None of those people were superfluous. You needed each one of those different groups to take part in this experiment, to reach the level of precision and understanding that that precision took."
One part of the project he won't miss: the unblinding of the results from the seven independent analysis groups.
"It is so incredibly stressful to unblind. And it only gets more stressful each successive time," he said. "This was the most precise result, and it's the last one. You're just like, what if it comes out way off from everything else? It was 16 years ago that I launched on this journey. So that final unblinding was to see, how did my last 16 years of really intense effort pan out?"
And the verdict?
"It was great," Gibbons said.
