This article was written as part of a course in fall of 2018. Some details about the experiment’s timeline (marked with an *) have changed since then. For example, the first results of the Fermilab experiment did not come out in 2019; they are being released tomorrow, April 7, 2021.
Beneath Midwestern prairieland and herds of grazing bison just west of Chicago, an unassuming experiment is attempting to uncover new physics. Here, at Fermi National Accelerator Laboratory (Fermilab, for short), upcoming results from the Muon g-2 experiment may point to the existence of a mysterious, never-before seen particle.
Muons are particles very similar to electrons, but about 200-times heavier. The goal of Muon g-2 (pronounced “gee-minus-two”) is to make one very, very accurate measurement of a property of the muon called the magnetic moment, or “wobble,” which describes how it acts in a magnetic field. This measurement has the potential to upend our current understanding of physics.
Since the 1970s, particle physicists have been developing a theory called the Standard Model. It is a remarkably accurate and elegant description of how the Universe works at its most fundamental levels. But it is incomplete.
Muon g-2 may reveal some of what we don’t know. The experiment initially ran from 1999 to 2001 at Brookhaven National Laboratory in Long Island, New York. Theoretical physicists had already calculated exactly how strong the wobble should be using the Standard Model. If the theory was right, their prediction ought to have agreed with the result from the Muon g-2 experiment.
But the prediction and the measurement did not agree. Brookhaven’s results from the early 2000s revealed a 0.1 percent discrepancy between the theoretical value and the measured value. And while this may sound small, it’s actually very significant in the realm of particles.
So where could this difference be coming from? What’s wrong with the Standard Model?
Maybe nothing’s wrong. Perhaps the anomaly was the result of an error in the experiment. The Brookhaven team measured the muon’s wobble to a precision of 3 to 4 sigma, meaning that there is less than a 0.13 percent (or 1 in 741) chance that their result was simply random. But the criterion for a “discovery” is 5 sigma—a 0.000028 percent (1 in 3,486,914) chance that the result was a false positive. To achieve this level of precision, physicists have to run the experiment again, this time with more muons. A lot more muons.
That’s where Fermilab can help. Its underground accelerator complex—a network of magnets and tunnels tangled like a city subway system—can generate beams of muons at the high energies necessary for a more accurate measurement. Once the muons have made their way through the accelerator complex, they are routed to the Muon g-2 storage ring, a giant superconducting magnet where scientists study the muons’ wobbles.
As muons move through a magnetic field, like the field flowing through the storage ring, they interact with particles that blink in and out of existence. This violates the conservation of energy, but quantum mechanics allows it. “Every particle we’ve ever discovered in the universe we know, with some frequency, will pop in and out of nowhere, just appear and disappear,” says Chris Polly, a scientist at Fermilab and co-spokesperson for the Muon g-2 experiment. “You can think of them as little tiny dance partners for the muons that appear every once in a while and grab the hand of the muon and suddenly change the way it interacts with the magnetic field.”
Theorists have to take these dance partners into account when making their calculations. So the other possible explanation for the muon’s anomalous wobble—the one that has physicists most excited—is that some unknown dance partner, an as-yet unknown particle, is grabbing the muon’s hand and causing it to wobble in an unpredictable way.
As of this past summer,* Muon g-2 at Fermilab has already collected twice as much data as the Brookhaven run, and the collaboration has begun analysis; their first results will come out in the summer of 2019.* But they aim to take more than 20 times as much data as Brookhaven to cross the coveted 5 sigma threshold, so an unequivocal discovery might still be a few years away.
In July of 2018—around the same time Muon g-2 began taking data at Fermilab—a team of Brookhaven scientists calculated the most precise prediction yet of the anomaly. This makes the upcoming results from Muon g-2 even more exciting. “If we make both the theory and the experiment more precise, and then we compare them again…and we’re confident in our errors—both theory and experiment—and they disagree, then we say we have discovered new physics,” says Thomas Blum, a professor of physics at the University of Connecticut and member of the Brookhaven team. In other words, he says, if there is a discrepancy, “our most precise theory, the thing we call the Standard Model, cannot explain this new observation.”
Of course, there is always a chance that the new wobble measurement will match the most precise calculated value, causing the anomaly to disappear. But the researchers maintain that the experiment is still interesting in the negative. “If you don’t see anything, then you’ve proven that, at this level of precision, everything you know about the Universe seems to work the way you expected it to,” says Polly.
Physicists around the world are working on new theories of particle physics, ones that go “beyond the Standard Model.” Many of these take into account the anomalous magnetic moment of the muon. If Muon g-2 shows there is no anomaly, physicists will know definitively that these theories are incomplete.
Regardless of the result, Muon g-2 is sure to help broaden our understanding of physics. “I think the main reason we do this is because people fundamentally want to understand who they are, what our place in the universe is,” says Polly. “We all want to understand the world we live in. That’s why we do particle physics.”
