The story of the cloak and dagger behind this year’s most anticipated outcome in particle physics science

While muons race around a ring in the Fermi National Accelerator Laboratory, their axes of rotation rotate and reflect the influence of invisible particles.


By Adrian ChoJan. 27, 2021, 1:30 p.m.

In 1986 the television journalist Dan Rather was attacked in New York City. A troubled attacker hit him while cryptically demanding, “Kenneth, what is the frequency?” The query became a pop culture meme, and rock band REM even based a hit on it. Now it might be the motto for the team to deliver the most anticipated result of the year in particle physics.

As early as March, the muon g-2 experiment at the 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 is to measure a single frequency with exquisite precision. In tantalizing results from 2001, G-2 found that the muon is slightly more magnetic than theory predicts. If confirmed, the excess would signal the existence of novel high-mass particles that a nuclear destroyer could potentially produce for the first time in decades, 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 big deal.”

The steps G-2 experimenters take to ensure that they are not mistaken into making a false discovery are spy novels, which include locked cabinets, sealed envelopes, and a second, secret frequency known only to two people, both outside the g-2 team. “My wife is not going to choose me for such responsible jobs, so I don’t know why an important experiment was conducted,” said Joseph Lykken, chief research officer of Fermilab, one of the keepers of the secret.

Like the electron, the muon rotates like a point, and its spin fills it with magnetism. Quantum theory also requires that the muon be enveloped by particles and antiparticles that are flitting in and out of the vacuum too quickly to be directly observed. These “virtual particles” increase the muon’s magnetism by about 0.001%, an excess that is referred to as g-2. Theorists can predict the excess very accurately, provided the vacuum is just bubbling with the particles in their prevailing theory. However, these predictions will not agree with the measured value if the vacuum also hides massive new particles. (The electron shows similar effects, but is less sensitive to new particles than the muon because it is much less massive.)

To measure the tell-tale magnetism, G-2 researchers fire a beam of muons (or, more precisely, their antimatter counterparts) into a 15-meter-wide circular particle accelerator. Thousands of muons enter the ring with their axis of rotation pointing in the direction they’re moving, like a soccer ball thrown by a right-handed quarterback. A vertical magnetic field bends their trajectories around the ring, causing their spin axis to rotate or move like a wobbling gyroscope.

Without the additional magnetism of the virtual particles, the muons would move at the same speed with which they orbit the ring and therefore always rotate in their direction of movement. However, due to the additional magnetism, the muons move faster than they orbit, about 30 times per 29 orbits – an effect that, in principle, makes it easy to measure the excess.

Excessive magnetism

As theorists improved their calculations, the gap between the expected magnetism of the muon and a measurement from 2005 remained.

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As they orbit, each muon decays to create a positron that flies into one of the detectors that line the ring. The positrons have higher energy when the muons rotate in the direction they are circulating and lower energy when they rotate in the opposite direction. As the muons move, the flow of high-energy positrons oscillates at a frequency that shows how much additional magnetism the virtual particles create.

To measure this frequency accurately enough to look for new particles, physicists need to closely control every aspect of the experiment, says Chris Polly, physicist at Fermilab and co-spokesman for the 200-strong g-2 team. For example, to bring the ring’s magnetic field down to 25 parts in 1 million, researchers adorned the poles of its electromagnets with more than 9,000 strips of steel that are thinner than a sheet of paper, says Polly, who has worked on the G-2 since his experiment Founded in 1989 at Brookhaven National Laboratory in Upton, New York. Each leaf acts as a magnetic “washer” that makes a tiny adjustment on site.

In Brookhaven, the experiment collected data from 1997 to 2001. Ultimately, the researchers measured the magnetism of the muon with an accuracy of 0.6 parts in 1 billion and reached a value that was about 2.4 parts per billion above the theoretical value at the time. In 2013, they hauled the 700-ton ring 5,000 kilometers by barge to Fermilab in Batavia, Illinois. With a cleaner, more intense muon beam, the revised g-2 ultimately aims to reduce the experimental uncertainty to a quarter of its current value. The result, which will be announced this spring, won’t achieve that goal, says Lee Roberts, a G-2 physicist at Boston University. However, if it is in line with the Brookhaven result, it would strengthen the case for new particles lurking in the vacuum.

However, G-2 researchers need to make sure they aren’t mistaken as they make the 100+ tiny corrections that the various aspects of the experiment require. In order to avoid that the frequency is unconsciously directed to the desired value, the experimenters blind themselves to the true frequency until they have completed their analysis.

The blinding has several layers, but the last one is the most important. In order to hide the true frequency with which the positron flux oscillates, the experiment runs with a clock that does not tick in real nanoseconds, but with an unknown frequency that is selected at random. At the beginning of each month-long run, Greg Bock of Lykken and Fermilab enter an eight-digit value into a frequency generator that is kept under lock and key. The last step in the measurement is to open the sealed envelope with the unknown frequency, the key to converting the clock values ​​in real time. “It’s like the Academy Awards,” says Lykken.

Indications of new physics arise from the gap between the measured result and the prediction of the theorists. This prediction has its own uncertainties, but over the past 15 years the calculations have become more precise and consistent, and the differences of opinion between theory and experiment are now greater than ever. The gap between the theorists’ consensus value for muon magnetism and the Brookhaven value is now 3.7 times the total uncertainty, says El-Khadra, not too far from the five times it takes to claim a discovery.

Even so, the discrepancy may be less exciting than it was 20 years ago, says William Marciano, a theorist at Brookhaven. At the time, many physicists thought it might be an indication of supersymmetry, a theory that predicts a heavier partner for each Standard Model particle. But if such partners lurk in a vacuum, the world’s largest nuclear destroyer, Europe’s Large Hadron Collider, would probably have blown them up by now, says Marciano. “It’s not impossible to explain [the muon’s magnetism] with super symmetry, “says Marciano,” but you have to be upside down to do it. “

Still, the physicists are eagerly awaiting the new measurement, because if the discrepancy is real, it must be caused by something new. The team still decides when to delete the data, says Roberts, who has worked on G-2 since the beginning. “In Brookhaven I always sat on the edge of a chair [during unblinding]and i think i will be here too. “

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