The mantle and dagger story behind this year’s most anticipated particle physics result | Science

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

In 1986, television journalist Dan Rather was assaulted in New York City. A deranged assailant hit him as he demanded a lot: “Kenneth, what is the frequency?” The query became a pop culture meme, and rock band REM even based a hit song on it. Now, that could be the motto of the team poised 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 and shorter cousin of the electron. The effort is to measure a single frequency with exquisite precision. In enticing results from 2001, g-2 found that the muon is slightly more magnetic than theory predicts. If confirmed, this excess would signal, for the first time in decades, the existence of new massive particles that an atom breaker might be able to produce, explains Aida El-Khadra, theorist at the University of Illinois at Urbana-Champaign. “It would be a very clear sign of a new physics, so it would be a huge deal.”

The steps the g-2 experimenters are taking to ensure they aren’t mistaken in claiming a false discovery are the fruit of spy novels, involving locked cabinets, sealed envelopes, and a second known secret frequency of only two people, both outside. the g-2 team. “My wife won’t choose me for responsible jobs like this, so I’m not sure why an important experience made it,” says Joseph Lykken, director of research at Fermilab, 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 requires that the muon be enveloped in particles and antiparticles floating in and out of a vacuum too quickly to be observed directly. These “virtual particles” increase the muon’s magnetism by about 0.001%, an excess denoted by g-2. Theorists can predict the excess very accurately, assuming the vacuum fizzles with only the particles in their dominant theory. But these predictions will not square with the measured value if the vacuum is also hiding new massive particles. (The electron exhibits similar effects, but is less sensitive to new particles than the muon because it is much less massive.)

To measure telltale magnetism, g-2 researchers shoot a beam of muons (or, to be more precise, their antimatter counterparts) through a 15-meter-wide circular particle accelerator. Thousands of muons enter the ring with their axis of rotation pointing in the direction they are moving, like a soccer ball thrown by a right-handed quarterback. A vertical magnetic field bends their paths around the ring and also twists their axis of rotation, or precession, like a wobbling gyroscope.

Without the additional magnetism of the virtual particles, muons would precede at the same speed as they orbit the ring and, thus, always rotate in their direction of travel. However, the additional magnetism makes the muons preload faster than they orbit, about 30 times every 29 orbits – an effect that, in principle, simplifies the measurement of the excess.

Excess of magnetism

In orbit, each muon decays to produce a positron which flies through one of the detectors lining the ring. Positrons have higher energy when muons rotate in the direction they are flowing, and lower energy when rotating in the opposite direction. So as the muons circulate, the flow of high-energy positrons oscillates at a frequency that reveals how much additional magnetism the virtual particles create.

To measure this frequency accurately enough to search for new particles, physicists need to closely monitor every aspect of the experiment, says Chris Polly, a Fermilab physicist and co-spokesperson for the 200-member g-2 team. . For example, to standardize the magnetic field of the ring to 25 parts in 1 million, the researchers adorned the poles of its electromagnets with more than 9,000 strips of steel thinner than a sheet of paper, explains Polly, who worked on the g-2. experience since its inception in 1989 at Brookhaven National Laboratory in Upton, New York. Each sheet acts as a magnetic “wedge” that makes a tiny adjustment in the field.

At Brookhaven, the experiment collected data from 1997 to 2001. Finally, the researchers measured muon magnetism with an accuracy of 0.6 parts in 1 billion, arriving at a value of about 2.4 parts per billion. greater than the theoretical value at the time. In 2013, they transported the 700-ton ring 5,000 kilometers by barge to the Fermilab in Batavia, Illinois. By using a purer and more intense muon beam, the redesigned g-2 ultimately aims to reduce experimental uncertainty to a quarter of its current value. The result announced this spring will not meet that target, says Lee Roberts, a g-2 physicist at Boston University. But if it matches Brookhaven’s result, it would strengthen the case for new particles lurking in a vacuum.

However, g-2 researchers need to be careful not to make a mistake while making the more than 100 tiny corrections necessary to different aspects of the experiment. To avoid subconsciously directing the frequency to the desired value, experimenters blind themselves to the actual frequency until they have finalized their analysis.

The blind has several layers, but the last one is the most important. To mask the true frequency at which the positron flow oscillates, the experiment runs on a clock that does not run in actual nanoseconds, but at an unknown frequency, chosen at random. At the start of each month, Greg Bock, of Lykken and Fermilab, enters an eight-digit value into a locked-up frequency generator. The last step in the measurement is to open the sealed envelope containing the unknown frequency, the key to converting the clock readings to real time. “It’s like the Oscars,” Lykken says.

Any allusion to new physics will emerge from the gap between the measured result and the theorists’ prediction. This 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 experience is now greater than ever. The gap between theorists’ consensus value for muon magnetism and Brookhaven’s value is now 3.7 times the total uncertainty, El-Khadra says, not too far from the five times required to claim a discovery.

Still, the gap is perhaps less exciting than it was 20 years ago, says William Marciano, theorist at Brookhaven. At that time, many physicists believed that this could be a hint of supersymmetry, a theory that predicts a heavier partner for each standard model particle. But if such partners were lurking in a vacuum, the world’s largest atom smasher, Europe’s Large Hadron Collider, would likely have destroyed them by now, Marciano says. “It’s not impossible to explain [the muon’s magnetism] with supersymmetry, ”says Marciano,“ but you have to stand up to do it. “

Still, physicists are eagerly awaiting the new measurement because, if the deviation is real, Something new must be the cause. The team is still deciding when to roll back the data, says Roberts, who has worked on g-2 since its inception. “At Brookhaven, I was always sitting on the edge of my chair [during unblinding], and I think I’ll be there too.