The Cloak-and-Dagger Tale Behind an Anticipated Result in Particle Physics

As muons race around a ring at the Fermi National Accelerator Laboratory, their spin axes twirl, reflecting the influence of unseen particles.

FERMI NATIONAL ACCELERATOR LABORATORY

The cloak-and-dagger tale behind this year’s most anticipated result in particle physics

By Adrian ChoJan. 27, 2021 , 1:30 PM

In 1986, the TV journalist Dan Rather was attacked in New York City. A deranged assailant pummeled him while cryptically demanding, “Kenneth, what’s the frequency?” The query became a pop culture meme, and the rock band R.E.M. even based a hit song on it. Now, it could be the motto for the team about to deliver the year’s most anticipated result in particle physics.

As early as March, the Muon g-2 experiment at Fermi National Accelerator Laboratory (Fermilab) will report a new measurement of the magnetism of the muon, a heavier, short-lived cousin of the electron. The effort entails measuring a single frequency with exquisite precision. In tantalizing results dating back to 2001, g-2 found that the muon is slightly more magnetic than theory predicts. If confirmed, the excess would signal, for the first time in decades, the existence of novel massive particles that an atom smasher might be able to produce, says Aida El-Khadra, a theorist at the University of Illinois, Urbana-Champaign. “This would be a very clear sign of new physics, so it would be a huge deal.”

The measures that g-2 experimenters are taking to ensure they don’t fool themselves into claiming a false discovery are the stuff of spy novels, involving locked cabinets, sealed envelopes, and a second, secret frequency known to just two people, both outside the g-2 team. “My wife won’t pick me for responsible jobs like this, so I don’t know why an important experiment did,” says Joseph Lykken, Fermilab’s chief research officer, one of the keepers of the secret.

Like the electron, the muon spins like a top, and its spin imbues it with magnetism. Quantum theory also demands that the muon is enshrouded by particles and antiparticles flitting in and out of the vacuum too quickly to be observed directly. Those “virtual particles” increase the muon’s magnetism by about 0.001%, an excess denoted as g-2. Theorists can predict the excess very precisely, assuming the vacuum fizzes with only the particles in their prevailing theory. But those predictions won’t jibe with the measured value if the vacuum also hides massive new particles. (The electron exhibits similar effects, but is less sensitive to new particles than the muon because it is much less massive.)

To measure the telltale magnetism, g-2 researchers fire a beam of muons (or, to be more precise, their antimatter counterparts) into a 15-meter-wide circular particle accelerator. Thousands of muons enter the ring with their spin axis pointing in the direction they travel, like a football thrown by a right-handed quarterback. A vertical magnetic field bends their trajectories around the ring and also makes their spin axis twirl, or precess, like a wobbling gyroscope.

Were it not for the extra magnetism from the virtual particles, the muons would precess at the same rate that they orbit the ring and, thus, always spin in their direction of travel. However, the extra magnetism makes the muons precess faster than they orbit, roughly 30 times for every 29 orbits—an effect that, in principle, makes it simple to measure the excess.

Excess magnetism

As theorists have improved their calculations, the gap between the expected magnetism of the muon and a 2005 measurement has persisted.

P. A. ZYLA ET AL. (PARTICLE DATA GROUP), PROG. THEOR. EXP. PHYS. 2020, 083C01, ADAPTED BY V. ALTOUNIAN/SCIENCE

As they orbit, each muon decays to produce a positron, which flies into one of the detectors lining the ring. The positrons have higher energy when the muons are spinning in the direction they are circulating and lower energy when they are spinning the opposite way. So as the muons go around and around, the flux of high-energy positrons oscillates at a frequency that reveals how much extra magnetism the virtual particles create.

To measure that frequency with enough precision to search for new particles, physicists must tightly control every aspect of the experiment, says Chris Polly, a physicist at Fermilab and co-spokesperson for the 200-member g-2 team. For example, to make the ring’s magnetic field uniform to 25 parts in 1 million, researchers have adorned the poles of its electromagnets with more than 9000 strips of steel thinner than a sheet of paper, says Polly, who has worked on the g-2 experiment since its inception in 1989 at Brookhaven National Laboratory in Upton, New York. Each sheet acts as a magnetic “shim” that makes a minuscule adjustment in the field.

At Brookhaven, the experiment collected data from 1997 to 2001. Ultimately, researchers measured the muon’s magnetism to a precision of 0.6 parts in 1 billion, arriving at a value about 2.4 parts per billion bigger than the theoretical value at the time. In 2013, they hauled the 700-ton ring 5000 kilometers by barge to Fermilab in Batavia, Illinois. Using a purer, more intense muon beam, the revamped g-2 ultimately aims to reduce the experimental uncertainty to one-quarter of its current value. The result to be announced this spring won’t reach that goal, says Lee Roberts, a g-2 physicist at Boston University. But if it matches the Brookhaven result, it would strengthen the case for new particles lurking in the vacuum.

However, g-2 researchers must ensure they don’t fool themselves while making the more than 100 tiny corrections that the various aspects of the experiment require. To avoid subconsciously steering the frequency toward the value they want, the experimenters blind themselves to the true frequency until they’ve finalized their analysis.

The blinding has multiple layers, but the last is the most important. To hide the true frequency at which the flux of positrons oscillates, the experiment runs on a clock that ticks not in real nanoseconds, but at an unknown frequency, chosen at random. At the start of each monthslong run, Lykken and Fermilab’s Greg Bock punch an eight-digit value into a frequency generator that’s kept under lock and key. The last step in the measurement is to open the sealed envelope containing the unknown frequency, the key to converting the clock readings into real time. “It’s like the Academy Awards,” Lykken says.

Any hints of new physics will emerge from the gap between the measured result and theorists’ prediction. That prediction has its own uncertainties, but over the past 15 years, the calculations have become more precise and consistent, and the disagreement between theory and experiment is now bigger than ever. The gap between theorists’ consensus value for the muon’s magnetism and the Brookhaven value is now 3.7 times the total uncertainty, El-Khadra says, not too far from the five times needed to claim a discovery.

Nevertheless, the discrepancy may be less exciting than it was 20 years ago, says William Marciano, a theorist at Brookhaven. At that time, many physicists thought it could be a hint of supersymmetry, a theory that predicts a heavier partner for each standard model particle. But if such partners lurk in the vacuum, the world’s largest atom smasher, Europe’s Large Hadron Collider, probably would have blasted them out by now, Marciano says. “It’s not impossible to explain [the muon’s magnetism] with supersymmetry,” Marciano says, “but you have to stand on your head to do it.”

Still, physicists eagerly await the new measurement because, if the discrepancy is real, something new must be causing it. The team is still deciding when it will unblind the data, says Roberts, who has worked on g-2 since it began. “At Brookhaven, I was always sitting on the edge of my chair [during unblinding], and I think I will be here, too.”

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